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LED Light Therapy Research
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Summary and Introduction
Red and near-infrared light immediately relieves pain and heals tissue. It does this by increasing the amount of ATP energy that cells need to maintain and heal themselves. About 30% of the energy in sunlight is in the 630 nm to 900 nm range (red to near-infrared). These wavelengths are able to pass through the water and blood in tissue and are absorbed by a "proton pump" in mitochondria that helps "kick-start" the conversion of food energy into the cell's ATP energy. Wavelengths shorter than about 630 (yellow, green, blue, UV) are blocked by the hemoglobin in blood. Wavelengths longer than about 900 nm are blocked by water in skin and tissue.
There are 4 clues that indicate the benefit of red and near-infrared light is not an accident, but a highly "intelligent" and natural result of evolution. The clues are: 1) The proton pump is the last in a series of 3 pumps which places it in the best location to pull the food conversion process along. It may also reduce free radicals by preventing electrons from leaking from the electron transport chain. 2) The pump absorbs only the red and near-infrared light. 3) The pump is the primary absorber of these wavelengths in the body, around 30%. 4) Blood has a very sharp decline in its ability to absorb red and near-infrared which indicates blood evolved specifically to allow these wavelengths to pass through. The pump has a longer evolutionary history than blood because it was inherited from bacteria that formed the symbiotic relationship of the first cells. Descendents of these bacteria still exist as purple bacteria and still use very similar proton pumps. Studying the effect of red and near-infrared light on these bacteria may provide clues as to how it affects animal cells.
In hindsight, we can say "people have always known sunlight is good for you". It seems intuitively clear to most people that sunlight helps sick people and enables people to be more active. Now we know from a chemical and biological viewpoint why. Injured cells need the extra ATP to repair themselves. Healthy cells generate enough ATP from the red and near infrared of sunlight to enable more activity. If the ATP is not used (as occurs when resting in bright sunlight) it causes an increase in available glucose for a slight "glucose high". If the glucose is not used, glycogen stores increase (as a result of higher ATP) so that the subject finds it easier to be more active even after exposure. In summary, food energy is turned into ATP for immediate energy and the excess is turned into glycogen for later activity. In regard to the rest of the sun's spectrum: visible light is absorbed by hemoglobin which may have unknown benefits like promoting the release of oxygen or just another form of warmth by absorption as in far-infrared. We know UV creates vitamin D that prevents colon, prostate, and breast cancer and greatly improves the immune system and bone strength. Skin cancer from UV is not a significant problem compared to the benefits of UV. 15 minutes a day of strong UV from summer sunlight is safe and sufficient for vitamin D. Far-infrared provides warmth.
LEDs light arrays are a means to provide then needed wavelengths without heating the tissue or exposing the skin to UV. Companies will argue that lasers, frequencies, and pulse rates are important, but there is a very strong evolutionary argument that indicates full-spectrum is best. Simply put: we evolved to use the energy from sunlight. The absorption of the proton pump is nearly the same for all wavelengths in the red to near-infrared range so one wavelength in this range may not be any more beneficial than other wavelengths. Certainly no company should make a claim otherwise unless they have good publicly-available data to back it up.
Halogen lights emit a spectrum of light that is very similar to sunlight but without as much UV and without as much heat as a heat lamp. Water can be used to block the far infrared from a halogen light that normally heats the water in skin. This can be made much more powerful than LED devices so that application time can be reduced to 5 minutes instead of 30 minutes, but the set up is tricky. A bed of ten 100 Watt halogen lights can reduce many pains that are suffered from not receiving enough sunlight. I have made beds of both LEDs and halogens.
Heat lamps have long been used in medical offices to reduce pain. It was usually believed that merely the penetrating heat of the lamps was the source of the benefit. Now we know the near-infrared portion of these heat lamps may provide the greatest benefit, despite the problems caused by the heat of the far-infrared portion of the heat lamps.
Effects of LED Light Therapy - Highlights of Journal Articles
"But if the rats were treated with LED light with a wavelength of 670 nm for 105 seconds at 5, 25 and 50 hours after being dosed with methanol, they recovered 95 per cent of their sight. Remarkably, the retinas of these rats looked indistinguishable from those of normal rats. 'There was some tissue regeneration, and neurons, axons and dendrites may also be reconnecting,' says Whelan."
"We believe that the use of NASA Light-Emitting Diodes (LED) for light therapy will greatly enhance the natural wound healing process, and more quickly return the soldiers to a pre-injury/ illness level of activity. The use of LED in combat with self-healing patches in future may enable the soldiers even after they are wounded to persist in combat better and longer."
"LED produced improvement of greater than 40% in musculoskeletal training injuries in Navy SEAL team members, and decreased wound healing time in crew members aboard a U.S. Naval submarine. LED produced a 47% reduction in pain of children suffering from oral mucositis. CONCLUSION: We believe that the use of NASA LED for light therapy alone, and in conjunction with hyperbaric oxygen, will greatly enhance the natural wound healing process, and more quickly return the patient to a preinjury/illness level of activity. " "ATS treatments improve sensation in the feet of subjects with DPN, improve balance, and reduce pain."
"This technology may be the answer for problem wounds that are slow to heal....diabetic skin ulcers and other wounds in mice healed much faster when exposed to the special LEDs in the lab. Laboratory research has shown that the LEDs also grow human muscle and skin cells up to five times faster than normal...."
"Light close to and in the near-infrared range has documented benefits for promoting wound healing in human and animals. "
"ATS treatments improve sensation in the feet of subjects with diabetic peripheral neuropathy, improve balance, and reduce pain."
"Near-infrared irradiation potentially enhances the wound healing process, presumably by its biostimulatory effects."
" It was found that laser exposure resulted in more pronounced restoration of functional state of nervous fibers than conventional therapy. Application of laser irradiation of low intensiveness was effective while in combined therapy of distal diabetic polyneuropathy as well as monotherapy."
"exposure of volunteers to visible and infrared polarized (VIP) light leads to a fast increase in the growth promoting (GP) activity of the entire circulating blood for human KCs in vitro, which is a consequence of the transcutaneous photomodification of blood and its effect on the rest of the circulating blood volume."
"The method of monochromatic near infrared stimulation can be used for selective stimulation of several regions of the external auditory canal,.."
LED and LLL irradiation resulted in an increased fibroblast proliferation in vitro. This study therefore postulates possible stimulatory effects on wound healing in vivo at the applied dosimetric parameters.
Wound healing was significantly more rapid with than without FIR. Skin blood flow and skin temperature did not change significantly before or during far-infrared irradiation.
Although more studies are needed, LED therapy appears useful in the prevention of OM in pediatric BMT patients.
News articles on the NASA Study:
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A wound-healing device was placed on the USS Salt Lake City submarine, and doctors reported 50 percent faster healing of crewmember's lacerations when exposed to the LED light. Injuries treated with the LEDs healed in seven days, while untreated injuries took 14 days.
Second daily infrared (JR) laser (820 nm, 25mW) and visible red laser (670 nm, 10 mW) at 1 J/cm2 and 5 J/cm2 on chronic pain. ...five treatment sessions over a two-week period. ...significant reductions in pain over the duration of the study with those groups which received infrared (820nm) laser a 1 J/cm2 and 5 J/cm2 904 nm three times weekly for 2 weeks, ......tendonitis of the shoulder 3.5-inch by 4.5-inch (89-millimeter by 114-millimeter), portable flat array of LEDs, arranged in rows on the top of a small box. ......places the box of LEDs on the outside of the patient's cheek about one minute each day. The red light penetrates to the inside of the mouth, where it seems to promote wound healing and prevent further sores in the patient's mouth.
All 176 patients received six treatments during a period of 3-4 weeks. ..GAAs laser therapy for tendonitis and myofascial pain
A 40 year-old woman presented at the Abe Orthopedic Clinic with a 2-year history of lower back pain and pain in the left hip and leg diagnosed as a ruptured disc between the 5th lumbar/1st sacral vertebrae. .....The gallium aluminum arsenide (GaAlAs) diode laser (830 nm, 60 mW) was used in outpatient therapy, and after 7 months, the patient's condition had dramatically improved, demonstrated by motility exercises. This improvement was confirmed by further MRI scans, which showed clearly the normal condition of the previously herniated L5/SI disc.
Influence of low-level (810nm, GaAlAs semiconductor) laser on bone and cartilage during joint immobilization was examined with rats' knee model. .......The hind limbs of 42 young Wistar rats were operated on in order to immobilize the knee joint. One week after operation they were assigned to three groups: irradiance 3.9W/cm2, 5.8W/cm2, and sham treatment. After 6 times of treatment for another 2 weeks both hind legs were myofascial pain in the cervical region. The patients were submitted to 12 sessions on alternate days to a total energy dose of 5 J each.
RA:From July 1988 to June 1990, 170 patients with a total of 411 affected joints were treated using a GaAlAs diode laser system (830 nm, 60 Mw C/W). Patients mean age was 61 years. 890 nanometer (nm)....Venous ulcers, diabetic ulcers, and post-amputation wounds....It recently has been demonstrated that application of this particular MIRE device to the skin for 30 minutes increases plasma NO in nondiabetic subject volunteers, as measured with a Sievers Instrument, Model 280, Nitric Oxide Detector
Ability of Light to Penetrate Tissue
Red and near infrared light penetrate tissue because they are not blocked by blood or water in tissue like other wavelengths. There is a lot of light penetration of water only when the wavelength is <900> 600 nm as shown in the chart below. See Source.
A doubling of the light intensity at any particular wavelength will double the amount of light energy that reaches a particular depth. Also, doubling the time of application will double the amount of light energy. So if you use a device that is half as strong, you simply have to apply it twice as long. Tissue that is 2 times as deep will receive 4 times less light. This is based on an interesting and common mathematical principle: if the first 1/2 inch of tissue allows only 1/30th of the light at the surface through, then the NEXT 1/2 inch of tissue (1 inch from surface) will allow only 1/30th of THAT light through. So 1/30 of 1/30 is 1/900th. Only 1/900th of the light at the surface will reach 1 inch into tissue (these numbers are just an example). Said another way, if 50% of the light energy reaches 2 mm, then 25% reach 4 mm. 12.5% will reach 6 mm. 6% will reach 8 mm. 3% will reach 10 mm (half an inch). So to reach tissue that is 1/2 inch deep, the light has to be applied 30 minutes to equal the amount of light that is applied for only 1 minute for 2 mm deep. To be the same amount at 1 inch, it has to be applied 900 minutes. The 30 minutes can be reduced to 15 minutes if the light source is twice as strong, or if the light source is simply pushed into the tissue 2 or 3 mm closer. Skin, fat, and muscle all have different absorption ratios. Dark skin will allow even less light to come through. Tissue penetration at different frequencies has been studied in detail and the math is complex. Light penetration depends the different effects of absorption, reflection, and scattering...which interact with each other. And the amount of each changes for different frequencies. But visible and infrared light does NOT travel through bone! It APPEARS to travel through the bones in our fingers only because it disperses a lot and goes AROUND bone. The only veins and arteries you will see are those where the light comes out. The light went around all the others.
More comments on absorption: Oxygenated blood (95% of blood) blocks 580 nm yellow with a factor of about 0.06 and just a few nm's away, the factor at 630 nm red is 200 times less, about 0.0003. This is why we see mostly red and not the other colors when we shine white light through our hand. The blocking factor for blood is also low in the infrared. The light blocking effects of water from 430 blue to 930 infrared increases by a factor of about 4 for each 100 nm increase. From 430 blue to 630 red, the light blocking of water increases by about 20, but it's so low to begin with that red penetrates 10 feet of water almost as well as blue. This might be why deep water is blue - our eyes perceive a lack of red in a white spectrum as blue. But that seems to have an effect only for water deeper than say 17 feet. I chose 17 because 1 foot of water will block 880 infrared as much as 17 feet of clean water blocks 630 red. This is because there's an increase in absorption by a factor of 17 from 630 red to 880 nm. So I think 880 penetrates water really well for at least 6 inches. From 880 to 930, absorption increases another factor of 3, so I think 930 nm is still reasonable for penetrating 2 inches of tissue. The math says 8 feet of water blocks as much energy of 630 red as 2 inches of water blocks 930 infrared (a factor of 50). So I believe the opaqueness of tissue blocks much more light than water all of our red and near-infrared frequencies. As a test, I looked at a remote control (~920 nm) through 2 feet of water with the night vision of a camcorder and it was definitely much dimmer. The Beer-Lambert law can be used for water, but for blood and tissue, there is a lot of scattering as well as absorption, so the law does not apply there as well.
Useful Charts:
Water Absorption Factors, 200 nm to 990 nm
Absorption Factor Chart
660 nm verses 880 nm
I don't know if one is better than the other, but 660 nm penetrates blood and water at least a little bit better. 660 nm has 33% more energy per photon than 880 nm, and blood may require a threshold of energy to release the oxygen. It's complicated enough for me to say I don't know which, if either, is best.
660 nm verses 630 nm
Tissue-wise, they probably work very similarly, but good 660 nm LEDs are more efficient at emitting more light energy. No one should use 630 nm for healing. 630 nm red is slightly orange and 660 nm red is a "deeper" red. Since 660 nm is almost infrared, the human eye is not able to see it as well. 630 nm red is used in key rings, traffic lights, and car tail-lights because it's 6 times easier to see than 660 nm (see the photopic response factor - chart ). The eye doesn't suddenly stop sensing light at 700 nm, but it is a gradual decline in sensitivity. You might find some LEDs on key rings that appear brighter than 660 LEDs, but they are not putting out more light energy or having a larger healing effect.
880 nm verses 850 nm
There are some companies that claim 880 is "the best" frequency. I do not think that opinion is based on any scientific evidence. In fact, from the LED data sheets I've seen, it appears 850 nm is able to put out more light energy with less heat compared to 880 nm. 880 LEDs are putting out frequencies in the range of 870 to 890 and are getting blocked 25% more by water absorption than 850.
930 nm and above
From the meager information I have available on the subject, it appears that any wavelength longer than 930 nm will start to have its energy blocked more than we want by the water in tissue. I'm choosing 930 nm as my magical cutoff point. At < 900 nm, the LEDs will have a slight red glow if you look at them closely. The red glow is brighter the closer you get to 700 (remember 630 is a lot brighter red than 660). Even 920 LEDs will have a very slight red glow in a very dark room.
Laser Light verses LEDs
There has been a lot of interest and money in red laser light, but I don't have any reason to believe the "phased" light from a laser will be any better for healing than the more typical light of LEDs. Laser light disperses just as quickly once it enters the skin and I don't think cells can tell the difference between the polarized or phased light of lasers and the "incoherent" light of LEDs. Like infrared, lasers have a superior marketing appeal for companies because it sounds interesting, mysterious, and cool.
Blue, Yellow, and Green
Blue is about 430 to 485 nm. Green is 510 to 565 nm. Yellow is 570 to 590. Red is 620 nm all the way until you can't see it anymore, roughly 740 nm and beyond. There are some companies that claim yellow helps remove wrinkles. I haven't found any research that's not funded and conducted by the people who profit from it, so the jury is still out for me. I don't know if there is an FDA approved study to support them. But there is some good research that shows strong blue light will kill bacteria that cause some forms of acne. I have heard of green to help cancer, but I don't believe it and it certainly can't penetrate more than the skin (as with blue light which can penetrate only 2 mm - see chart below). At least laser red has been used to help halt wet macular degeneration. Infrared 880 nm has FDA approval for diabetic peripheral neuropathy, and 660 nm red has FDA approval for mouth ulcers in children on a type of chemo, although I thought the researched looked funny.
Design info: Comparing LEDs
Designers trying to select LEDs or arrays of LEDs for their devices will have trouble comparing LED brightness from different manufacturers. The plastic encasings can focus the light and make mcd ratings much higher, but the amount of light coming out is the same. A 100 mcd LED at +/- 10 degrees (20 degrees angle of output) has the same total amount of light output as a 2,000 mcd LED at +/- 5 degrees (10 degrees). The equation needed is: Milliwatt output of an LED = mcd / (683 x P) x 2 x pi x (1-cos(1/2 Angle of output)). Companies are not exactly precise in how they measure mcd (millicandela) and the angle output. P is the "photopic response factor" ( graph ) that depends on the wavelength. mcd and P are only meaningful for visible wavelengths (not infrared). P=1 for 555 nm and P=0.061 for 660 nm. For infrared, the measurement has to be mW/SR where SR=steridians. SR is to a sphere as radians are to a circle. 4π of SR units is equal to the total surface area of a sphere. In any event, replace mcd/(683 x P) with mW/SR for infrared LEDs. The right side of the equation is converting the beam angle that the LED datasheets provide to SR units. You often have to guess if the datasheet is stating the full beam angle (sometimes indicated by 2É∆) or just 1/2 of it. All this figuring is only a little useful. You usually just have to buy the LEDs and compare them for your needs. As a very rough estimate, you can say the light output energy of an LED is 30% of the energy input. But some LEDs are much more efficient at light output. Strong LEDs use 50 to 100 mA continuously. But 20 mA red LEDs can put out enough light and are very common. A good and strong 850 nm LED will use 50 mA continuously, but the device will get too hot if you pack the LEDs closely (22 LEDs per square inch for 5 mm packages) and run them anywhere near their max. 0.8 watts per square inch is the maximum energy you can apply to an array that touches skin unless a fan or heat sink is used. So at a typical 12 LEDs per square inch you can apply 70 mW per LED. That 45 mA at 1.5 V for the common 850 nm lamp (all tested companies were identical in efficiency) and 35 mA at 1.9 V for a good 660 nm.
Safety Concerns
Heat generation is the primary concern. I found an article that said skin temperature should never be more than 41 C (105.8F) to meet FDA regs. There is no way to know how hot a design will get until you make it and wrap it as snuggly as possible with ace bandage and stick a thermometer in for 2 hours. No matter how "cool" a heat-producing device operates, if you wrap it good enough and leave it there long enough, it can get hot. It's not just how much energy goes in, but also how much goes out. I've found around 0.8 Watts per square inch to be the maximum energy that can be put into a device that touches the skin with a fan or special heat sinking.
Eye Safety
Strong blue LED's are dangerous to your eyes! White LEDs have been studied for safety, but I wouldn't stare at them because they have blue frequencies in the white. Strong green LEDs have 1/15th the risk of blue. Strong and focused Red and yellow LEDs appear safe, but I would not stare directly at them for more than a minute. If you find a 10,000 mcd 660 nm Red, be careful.
The ACGIH does not seem to have a safety factor based on time of exposure (TLV) for simple LEDs, but it has two categories that can apply to them. One is the TLV for laser light, but lasers are different because they really focus the light in one spot which is much more likely to cause harm. The link at the bottom of this paragraph is a well-researched article that strongly claims you don't need to treat LEDs as lasers when it comes to safety. The other TLV is for light at a range of wavelengths and time exposures, which is good. Blue LEDs may harm the eyes from a photochemical injury called the "blue light hazard". I have personally been harmed by a blue LED key ring (blue photons have much more energy per photon - that's why red light does not affect night vision very much). I had a spot in my vision for months after a 2 second exposure. Distance from such a narrow-beam, strong blue LED only makes the AREA of damage on your retina smaller, not that damage is less likely to occur. Red, yellow, and green also have photochemical risks, but for LEDs, only green has the remote possibility of causing harm (if it's high power with a narrow emission angle). Bright visible light may also harm the retina from thermal activity. Blue is thermally 10 times more dangerous than the others. Reasonably powerful LEDs in red, yellow, and green are also thermally safe. But they are brighter at narrow wavelengths that we have evolved to cope with, so I still consider staring at them for more than a minute to be risky. Infrared light > 770 nm may harm the retina and lens from thermal activity, but has more risk for the lens. Damage to the lens may take the form mainly of cataracts. Infrared should be less than 10 mW/cm^2 if it's applied for greater than 15 minutes. For less than 15 minutes, mW/cm^2 should be < 1800 t^(-0.75). This means 83 mW/cm^2 is safe to the lens for up to one minute. Suppose you're using an excellent 50% efficient LED at its maximum power dissipation of 100 mW. Anything stronger will have difficulty dissipating heat and it's difficult to find more powerful 5 mm LEDs in red and infrared. That's 100x0.50=50 mW light output, but if you place it directly on the lens, it's an exposure in an area of only about 5 mm in diameter, or 0.20 cm^2, so 50/0.2= 250 mW/cm^2 light intensity. So by using the TLV equation above, it appears up to 15 seconds is safe when applying one of the most powerful types of 5 mm infrared LEDs directly to the eye. So, it's possible to increase the risk of cataracts when treating macular degeneration with red and infrared LEDs. From another source: "Near-infrared thermal hazards to the lens (associated with wavelengths of approximately 800 nm to 3,000 nm) with potential for industrial heat cataract. The average corneal exposure to infrared radiation in sunlight is of the order of 1 mW/m^2. By comparison, glass and steel workers exposed to infrared irradiances of the order of 80 to 400 mW/cm^2 daily for 10 to 15 years have reportedly developed lenticular opacities (Sliney and Wolbarsht 1980). These spectral bands include IRA and IRB (see figure 49.1). The American Conference of Governmental Industrial Hygienists (ACGIH) guideline for IRA exposure of the anterior of the eye is a time-weighted total irradiance of 10 mW/cm^2 for exposure durations exceeding 1,000 s (16.7 min) (ACGIH 1992 and 1995). Thermal injury of the cornea and conjunctiva (at wavelengths of approximately 1,400 nm to 1 mm). This type of injury is almost exclusively limited to exposure to laser radiation. " Note: For visible LEDs, Use L= 1000 x mcd/(683 x P) in place of L x (change in wavelength) in the TLV equations. For infrared, use mW/(SR x 1000). The distance from the LED does not change the danger for equations with L in them. The reason for this is because the ACGIH values each and every rod and cone in the retina, and light from a further distance has the same strength for each rod and cone it hits - it is weaker only because it affects a smaller number of them. If you want a better article on the retinal eye safety of LEDs and lasers read this (but they don't address lens safety, do not give an mcd example for blue, and don't discuss focusing that new LED plastic cases use.
Halogen lights contain a lot of blue light and are very dangerous to the eyes.
Sunshine
Bright sun at midday in summer, south of the mason-dixon line (let's just say Florida, Texas and California which covers most of us non-New Yorkers anyway) has a lot more energy in the red and near infrared (about 39 mW/cm^2 - see chart below) than most LEDs. But the Sun's light is not pulsed and the energy is spread over the full red and near-infrared healing range (630 to 920 nm or so) instead of being very strong in pulses at a precise wavelength. The wavelengths longer than 920 nm are why the Sun is hot - the water in your skin is absorbing the energy. Strong sunlight is probably as good for you as any LED array....if you avoid getting too much UV. I have used a corrugated plastic sheet like those used in green houses, but the special kind that is clear and blocks UVA and UVB (I tested it to be sure with a UVA/UVB meter). It costs about $40 per sheet at Lowes and I bought just one sheet. One sheet is barely big enough to cover your body. I place two chairs at each end of my lounge chair and use their backs to support the plastic sheet. Otherwise, you can use sunscreen, which may not block the aging and cancer-causing rays very well. But 15 minutes of direct UV sun each day produces a lot of vitamin D which prevents a lot more of the big cancers (prostate, breast, and colon) and it's not long enough to worry about skin cancer. My arthritic fingers feel better for about a week after spending some time on the beach.
Halogen, Incandescent, and Infrared Heat Lamps
The Sun, Halogen lamps, incandescent lamps, and infrared heat lamps all emit light based on the black body radiation principle. This principle determines the amount of energy emitted at each wavelength of light. The chart below shows how the energy changes for the sun verses an incandescent lamp. The sun is much hotter and it's energy shifted to shorter wavelengths (higher energy). Halogen lamps have a curve almost the same as the one shown for incandescent, but shifted a little closer to the Sun's curve. Infrared lights are shifter more away from the Sun's curve. Halogen, incandescent, and infrared heat lamps all heat up a metal filament of tungsten to produce light. The filament "incandesces" which means it produces light by black body radiation. The only difference is that halogen gas can allow the filament to get hotter than regular incandescent bulbs and heat lamps are designed to have a cooler operating tungsten filament. They operate at approx the following temperatures: halogen - 3200 K, incandescent - 2800 K, infrared lamp - 2400 K. The cooler infrared filament means more energy will be in the far infrared range, which heats tissue by heating the water in the skin. Since water absorbs the far infrared very well, the water is "catching" the energy. This is how operating at a lower temperature can produce more heat in tissue.
Halogen lights contain a lot of blue light and are very dangerous to the eyes.
In summary, halogen lamps will produce light like the Sun and it can provide more light energy in the healing (tissue penetration) range of wavelengths than regular incandescent and infrared heat lamps. This will be much more energy than LEDs can provide, but the energy will be spread out over a larger range of wavelengths (see chart above comparing LEDs and Sun). I do not know if this is better or worse, but the halogen is closer to the Sun's natural spectrum. Halogen lamps even have glass covers that block UV light so that desk lamps do not cause sunburn to hands or harm eyes. As with the LEDs, halogen lamps put out about 30% of the energy they use as light energy in the tissue penetration range. So a 50 W Halogen spot-light that concentrates it's light in a 10x10 cm area (at the glass bulb) will produce 50/3/100= 0.16 W/cm^2 = 160 mW/cm^2 in the tissue penetration range. This is about 4 times the best LED array and sunlight, but it's also about 4 times as much heat in the skin as you would experience in the brightest midday sun in Miami. If you try to use a halogen lamp for healing and wonder if you're getting enough light, simply compare the heat you feel to the heat you would feel from bright sun. If it feels like very bright sun, its more healing energy than LEDs can provide, although not concentrated at specific frequencies and not pulsed. Since the peak frequency of halogen is in the tissue penetration range, it is more efficient than any other black-body radiation source at providing energy in the tissue penetration range without too much heat in the far-infrared (Sun is almost as good). Plexiglas may be used to block some of the far-infrared that heats the tissue. Well-designed LEDs will not have the heat problem at all, which is one reason they are being used. LEDs are more powerful over the short range of wavelengths they cover, but not over the full range of healing frequencies. The strongest dose of red and infrared light I have given my arthritic fingers was with a halogen lamp and using Plexiglas to block some (not all) of the far-infrared heat frequencies. It certainly loosened up my joints and felt good (mainly from the slight heat), but they were a little swollen the next morning. The dosage may have been about 2,000 mW/cm^2 for 20 seconds (40 J/cm^2). It was a 60 Watt spot light held 6 inches from the joint. Maybe 10% stayed in the spot, and 33% was in the frequencies of interest so 60 x 0.10 x 0.33 = 2 W = 2,000 mW. BTW the best thing for arthritic joints is intense STRENGTH training, niacinamide, and glucosamine. The best thing for a joint is strong muscles, tendons, and ligaments surrounding it.
Does Pulsing LEDs Help?
Some companies claim pulsing the light is important, but I haven't seen any data to support it. Pulsing increases the light at all depths for a brief period of time. This helps deeper tissue be exposed to more short-pulse light without the LED getting too hot. But a constant light source can provide the same amount of total light energy per minute when operated at lower intensity. Pulsing probably does not provide more light per minute to the deeper tissue. 50 mW/cm^2 applied for half a second during each second of application will not provide more light at 1 cm depth than 25 mW/cm^2 applied for the full 1 second. They also generate the same amount of heat. There are two ways pulsing may help: 1) if there is a "light power threshold effect" in cells rather than just a "total light energy applied" effect. By this I mean there could very well be something in tissue that requires a certain amount of "activation energy" to cause a reaction to occur. With LEDs, maybe a certain number of photons are required to strike a molecule at the same time to have an effect. 2) If there is something interesting in tissue that responds to certain rates of pulsing. But it would take an enourmous amount of clever research to determine which frequencies can do what. In summary, I know of two ways it MAY help, but I do not have any data to support. I have seen 2 web sites claim that pulsing makes the LEDs more efficient but the light output verses current input curves indicate that being constantly on at a given temperature (and therefore averaged energy input) is more efficient.
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Twickle Purple note: This company has the best priced LED products I seen yet. You can check them it here. They are simple, but we don't really need fancy.
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Summary and Introduction
Red and near-infrared light immediately relieves pain and heals tissue. It does this by increasing the amount of ATP energy that cells need to maintain and heal themselves. About 30% of the energy in sunlight is in the 630 nm to 900 nm range (red to near-infrared). These wavelengths are able to pass through the water and blood in tissue and are absorbed by a "proton pump" in mitochondria that helps "kick-start" the conversion of food energy into the cell's ATP energy. Wavelengths shorter than about 630 (yellow, green, blue, UV) are blocked by the hemoglobin in blood. Wavelengths longer than about 900 nm are blocked by water in skin and tissue.
There are 4 clues that indicate the benefit of red and near-infrared light is not an accident, but a highly "intelligent" and natural result of evolution. The clues are: 1) The proton pump is the last in a series of 3 pumps which places it in the best location to pull the food conversion process along. It may also reduce free radicals by preventing electrons from leaking from the electron transport chain. 2) The pump absorbs only the red and near-infrared light. 3) The pump is the primary absorber of these wavelengths in the body, around 30%. 4) Blood has a very sharp decline in its ability to absorb red and near-infrared which indicates blood evolved specifically to allow these wavelengths to pass through. The pump has a longer evolutionary history than blood because it was inherited from bacteria that formed the symbiotic relationship of the first cells. Descendents of these bacteria still exist as purple bacteria and still use very similar proton pumps. Studying the effect of red and near-infrared light on these bacteria may provide clues as to how it affects animal cells.
In hindsight, we can say "people have always known sunlight is good for you". It seems intuitively clear to most people that sunlight helps sick people and enables people to be more active. Now we know from a chemical and biological viewpoint why. Injured cells need the extra ATP to repair themselves. Healthy cells generate enough ATP from the red and near infrared of sunlight to enable more activity. If the ATP is not used (as occurs when resting in bright sunlight) it causes an increase in available glucose for a slight "glucose high". If the glucose is not used, glycogen stores increase (as a result of higher ATP) so that the subject finds it easier to be more active even after exposure. In summary, food energy is turned into ATP for immediate energy and the excess is turned into glycogen for later activity. In regard to the rest of the sun's spectrum: visible light is absorbed by hemoglobin which may have unknown benefits like promoting the release of oxygen or just another form of warmth by absorption as in far-infrared. We know UV creates vitamin D that prevents colon, prostate, and breast cancer and greatly improves the immune system and bone strength. Skin cancer from UV is not a significant problem compared to the benefits of UV. 15 minutes a day of strong UV from summer sunlight is safe and sufficient for vitamin D. Far-infrared provides warmth.
LEDs light arrays are a means to provide then needed wavelengths without heating the tissue or exposing the skin to UV. Companies will argue that lasers, frequencies, and pulse rates are important, but there is a very strong evolutionary argument that indicates full-spectrum is best. Simply put: we evolved to use the energy from sunlight. The absorption of the proton pump is nearly the same for all wavelengths in the red to near-infrared range so one wavelength in this range may not be any more beneficial than other wavelengths. Certainly no company should make a claim otherwise unless they have good publicly-available data to back it up.
Halogen lights emit a spectrum of light that is very similar to sunlight but without as much UV and without as much heat as a heat lamp. Water can be used to block the far infrared from a halogen light that normally heats the water in skin. This can be made much more powerful than LED devices so that application time can be reduced to 5 minutes instead of 30 minutes, but the set up is tricky. A bed of ten 100 Watt halogen lights can reduce many pains that are suffered from not receiving enough sunlight. I have made beds of both LEDs and halogens.
Heat lamps have long been used in medical offices to reduce pain. It was usually believed that merely the penetrating heat of the lamps was the source of the benefit. Now we know the near-infrared portion of these heat lamps may provide the greatest benefit, despite the problems caused by the heat of the far-infrared portion of the heat lamps.
Effects of LED Light Therapy - Highlights of Journal Articles
"But if the rats were treated with LED light with a wavelength of 670 nm for 105 seconds at 5, 25 and 50 hours after being dosed with methanol, they recovered 95 per cent of their sight. Remarkably, the retinas of these rats looked indistinguishable from those of normal rats. 'There was some tissue regeneration, and neurons, axons and dendrites may also be reconnecting,' says Whelan."
"We believe that the use of NASA Light-Emitting Diodes (LED) for light therapy will greatly enhance the natural wound healing process, and more quickly return the soldiers to a pre-injury/ illness level of activity. The use of LED in combat with self-healing patches in future may enable the soldiers even after they are wounded to persist in combat better and longer."
"LED produced improvement of greater than 40% in musculoskeletal training injuries in Navy SEAL team members, and decreased wound healing time in crew members aboard a U.S. Naval submarine. LED produced a 47% reduction in pain of children suffering from oral mucositis. CONCLUSION: We believe that the use of NASA LED for light therapy alone, and in conjunction with hyperbaric oxygen, will greatly enhance the natural wound healing process, and more quickly return the patient to a preinjury/illness level of activity. " "ATS treatments improve sensation in the feet of subjects with DPN, improve balance, and reduce pain."
"This technology may be the answer for problem wounds that are slow to heal....diabetic skin ulcers and other wounds in mice healed much faster when exposed to the special LEDs in the lab. Laboratory research has shown that the LEDs also grow human muscle and skin cells up to five times faster than normal...."
"Light close to and in the near-infrared range has documented benefits for promoting wound healing in human and animals. "
"ATS treatments improve sensation in the feet of subjects with diabetic peripheral neuropathy, improve balance, and reduce pain."
"Near-infrared irradiation potentially enhances the wound healing process, presumably by its biostimulatory effects."
" It was found that laser exposure resulted in more pronounced restoration of functional state of nervous fibers than conventional therapy. Application of laser irradiation of low intensiveness was effective while in combined therapy of distal diabetic polyneuropathy as well as monotherapy."
"exposure of volunteers to visible and infrared polarized (VIP) light leads to a fast increase in the growth promoting (GP) activity of the entire circulating blood for human KCs in vitro, which is a consequence of the transcutaneous photomodification of blood and its effect on the rest of the circulating blood volume."
"The method of monochromatic near infrared stimulation can be used for selective stimulation of several regions of the external auditory canal,.."
LED and LLL irradiation resulted in an increased fibroblast proliferation in vitro. This study therefore postulates possible stimulatory effects on wound healing in vivo at the applied dosimetric parameters.
Wound healing was significantly more rapid with than without FIR. Skin blood flow and skin temperature did not change significantly before or during far-infrared irradiation.
Although more studies are needed, LED therapy appears useful in the prevention of OM in pediatric BMT patients.
News articles on the NASA Study:
A wound-healing device was placed on the USS Salt Lake City submarine, and doctors reported 50 percent faster healing of crewmember's lacerations when exposed to the LED light. Injuries treated with the LEDs healed in seven days, while untreated injuries took 14 days.
Second daily infrared (JR) laser (820 nm, 25mW) and visible red laser (670 nm, 10 mW) at 1 J/cm2 and 5 J/cm2 on chronic pain. ...five treatment sessions over a two-week period. ...significant reductions in pain over the duration of the study with those groups which received infrared (820nm) laser a 1 J/cm2 and 5 J/cm2 904 nm three times weekly for 2 weeks, ......tendonitis of the shoulder 3.5-inch by 4.5-inch (89-millimeter by 114-millimeter), portable flat array of LEDs, arranged in rows on the top of a small box. ......places the box of LEDs on the outside of the patient's cheek about one minute each day. The red light penetrates to the inside of the mouth, where it seems to promote wound healing and prevent further sores in the patient's mouth.
All 176 patients received six treatments during a period of 3-4 weeks. ..GAAs laser therapy for tendonitis and myofascial pain
A 40 year-old woman presented at the Abe Orthopedic Clinic with a 2-year history of lower back pain and pain in the left hip and leg diagnosed as a ruptured disc between the 5th lumbar/1st sacral vertebrae. .....The gallium aluminum arsenide (GaAlAs) diode laser (830 nm, 60 mW) was used in outpatient therapy, and after 7 months, the patient's condition had dramatically improved, demonstrated by motility exercises. This improvement was confirmed by further MRI scans, which showed clearly the normal condition of the previously herniated L5/SI disc.
Influence of low-level (810nm, GaAlAs semiconductor) laser on bone and cartilage during joint immobilization was examined with rats' knee model. .......The hind limbs of 42 young Wistar rats were operated on in order to immobilize the knee joint. One week after operation they were assigned to three groups: irradiance 3.9W/cm2, 5.8W/cm2, and sham treatment. After 6 times of treatment for another 2 weeks both hind legs were myofascial pain in the cervical region. The patients were submitted to 12 sessions on alternate days to a total energy dose of 5 J each.
RA:From July 1988 to June 1990, 170 patients with a total of 411 affected joints were treated using a GaAlAs diode laser system (830 nm, 60 Mw C/W). Patients mean age was 61 years. 890 nanometer (nm)....Venous ulcers, diabetic ulcers, and post-amputation wounds....It recently has been demonstrated that application of this particular MIRE device to the skin for 30 minutes increases plasma NO in nondiabetic subject volunteers, as measured with a Sievers Instrument, Model 280, Nitric Oxide Detector
Ability of Light to Penetrate Tissue
Red and near infrared light penetrate tissue because they are not blocked by blood or water in tissue like other wavelengths. There is a lot of light penetration of water only when the wavelength is <900> 600 nm as shown in the chart below. See Source.
A doubling of the light intensity at any particular wavelength will double the amount of light energy that reaches a particular depth. Also, doubling the time of application will double the amount of light energy. So if you use a device that is half as strong, you simply have to apply it twice as long. Tissue that is 2 times as deep will receive 4 times less light. This is based on an interesting and common mathematical principle: if the first 1/2 inch of tissue allows only 1/30th of the light at the surface through, then the NEXT 1/2 inch of tissue (1 inch from surface) will allow only 1/30th of THAT light through. So 1/30 of 1/30 is 1/900th. Only 1/900th of the light at the surface will reach 1 inch into tissue (these numbers are just an example). Said another way, if 50% of the light energy reaches 2 mm, then 25% reach 4 mm. 12.5% will reach 6 mm. 6% will reach 8 mm. 3% will reach 10 mm (half an inch). So to reach tissue that is 1/2 inch deep, the light has to be applied 30 minutes to equal the amount of light that is applied for only 1 minute for 2 mm deep. To be the same amount at 1 inch, it has to be applied 900 minutes. The 30 minutes can be reduced to 15 minutes if the light source is twice as strong, or if the light source is simply pushed into the tissue 2 or 3 mm closer. Skin, fat, and muscle all have different absorption ratios. Dark skin will allow even less light to come through. Tissue penetration at different frequencies has been studied in detail and the math is complex. Light penetration depends the different effects of absorption, reflection, and scattering...which interact with each other. And the amount of each changes for different frequencies. But visible and infrared light does NOT travel through bone! It APPEARS to travel through the bones in our fingers only because it disperses a lot and goes AROUND bone. The only veins and arteries you will see are those where the light comes out. The light went around all the others.
More comments on absorption: Oxygenated blood (95% of blood) blocks 580 nm yellow with a factor of about 0.06 and just a few nm's away, the factor at 630 nm red is 200 times less, about 0.0003. This is why we see mostly red and not the other colors when we shine white light through our hand. The blocking factor for blood is also low in the infrared. The light blocking effects of water from 430 blue to 930 infrared increases by a factor of about 4 for each 100 nm increase. From 430 blue to 630 red, the light blocking of water increases by about 20, but it's so low to begin with that red penetrates 10 feet of water almost as well as blue. This might be why deep water is blue - our eyes perceive a lack of red in a white spectrum as blue. But that seems to have an effect only for water deeper than say 17 feet. I chose 17 because 1 foot of water will block 880 infrared as much as 17 feet of clean water blocks 630 red. This is because there's an increase in absorption by a factor of 17 from 630 red to 880 nm. So I think 880 penetrates water really well for at least 6 inches. From 880 to 930, absorption increases another factor of 3, so I think 930 nm is still reasonable for penetrating 2 inches of tissue. The math says 8 feet of water blocks as much energy of 630 red as 2 inches of water blocks 930 infrared (a factor of 50). So I believe the opaqueness of tissue blocks much more light than water all of our red and near-infrared frequencies. As a test, I looked at a remote control (~920 nm) through 2 feet of water with the night vision of a camcorder and it was definitely much dimmer. The Beer-Lambert law can be used for water, but for blood and tissue, there is a lot of scattering as well as absorption, so the law does not apply there as well.
Useful Charts:
Water Absorption Factors, 200 nm to 990 nm
Absorption Factor Chart
660 nm verses 880 nm
I don't know if one is better than the other, but 660 nm penetrates blood and water at least a little bit better. 660 nm has 33% more energy per photon than 880 nm, and blood may require a threshold of energy to release the oxygen. It's complicated enough for me to say I don't know which, if either, is best.
660 nm verses 630 nm
Tissue-wise, they probably work very similarly, but good 660 nm LEDs are more efficient at emitting more light energy. No one should use 630 nm for healing. 630 nm red is slightly orange and 660 nm red is a "deeper" red. Since 660 nm is almost infrared, the human eye is not able to see it as well. 630 nm red is used in key rings, traffic lights, and car tail-lights because it's 6 times easier to see than 660 nm (see the photopic response factor - chart ). The eye doesn't suddenly stop sensing light at 700 nm, but it is a gradual decline in sensitivity. You might find some LEDs on key rings that appear brighter than 660 LEDs, but they are not putting out more light energy or having a larger healing effect.
880 nm verses 850 nm
There are some companies that claim 880 is "the best" frequency. I do not think that opinion is based on any scientific evidence. In fact, from the LED data sheets I've seen, it appears 850 nm is able to put out more light energy with less heat compared to 880 nm. 880 LEDs are putting out frequencies in the range of 870 to 890 and are getting blocked 25% more by water absorption than 850.
930 nm and above
From the meager information I have available on the subject, it appears that any wavelength longer than 930 nm will start to have its energy blocked more than we want by the water in tissue. I'm choosing 930 nm as my magical cutoff point. At < 900 nm, the LEDs will have a slight red glow if you look at them closely. The red glow is brighter the closer you get to 700 (remember 630 is a lot brighter red than 660). Even 920 LEDs will have a very slight red glow in a very dark room.
Laser Light verses LEDs
There has been a lot of interest and money in red laser light, but I don't have any reason to believe the "phased" light from a laser will be any better for healing than the more typical light of LEDs. Laser light disperses just as quickly once it enters the skin and I don't think cells can tell the difference between the polarized or phased light of lasers and the "incoherent" light of LEDs. Like infrared, lasers have a superior marketing appeal for companies because it sounds interesting, mysterious, and cool.
Blue, Yellow, and Green
Blue is about 430 to 485 nm. Green is 510 to 565 nm. Yellow is 570 to 590. Red is 620 nm all the way until you can't see it anymore, roughly 740 nm and beyond. There are some companies that claim yellow helps remove wrinkles. I haven't found any research that's not funded and conducted by the people who profit from it, so the jury is still out for me. I don't know if there is an FDA approved study to support them. But there is some good research that shows strong blue light will kill bacteria that cause some forms of acne. I have heard of green to help cancer, but I don't believe it and it certainly can't penetrate more than the skin (as with blue light which can penetrate only 2 mm - see chart below). At least laser red has been used to help halt wet macular degeneration. Infrared 880 nm has FDA approval for diabetic peripheral neuropathy, and 660 nm red has FDA approval for mouth ulcers in children on a type of chemo, although I thought the researched looked funny.
Design info: Comparing LEDs
Designers trying to select LEDs or arrays of LEDs for their devices will have trouble comparing LED brightness from different manufacturers. The plastic encasings can focus the light and make mcd ratings much higher, but the amount of light coming out is the same. A 100 mcd LED at +/- 10 degrees (20 degrees angle of output) has the same total amount of light output as a 2,000 mcd LED at +/- 5 degrees (10 degrees). The equation needed is: Milliwatt output of an LED = mcd / (683 x P) x 2 x pi x (1-cos(1/2 Angle of output)). Companies are not exactly precise in how they measure mcd (millicandela) and the angle output. P is the "photopic response factor" ( graph ) that depends on the wavelength. mcd and P are only meaningful for visible wavelengths (not infrared). P=1 for 555 nm and P=0.061 for 660 nm. For infrared, the measurement has to be mW/SR where SR=steridians. SR is to a sphere as radians are to a circle. 4π of SR units is equal to the total surface area of a sphere. In any event, replace mcd/(683 x P) with mW/SR for infrared LEDs. The right side of the equation is converting the beam angle that the LED datasheets provide to SR units. You often have to guess if the datasheet is stating the full beam angle (sometimes indicated by 2É∆) or just 1/2 of it. All this figuring is only a little useful. You usually just have to buy the LEDs and compare them for your needs. As a very rough estimate, you can say the light output energy of an LED is 30% of the energy input. But some LEDs are much more efficient at light output. Strong LEDs use 50 to 100 mA continuously. But 20 mA red LEDs can put out enough light and are very common. A good and strong 850 nm LED will use 50 mA continuously, but the device will get too hot if you pack the LEDs closely (22 LEDs per square inch for 5 mm packages) and run them anywhere near their max. 0.8 watts per square inch is the maximum energy you can apply to an array that touches skin unless a fan or heat sink is used. So at a typical 12 LEDs per square inch you can apply 70 mW per LED. That 45 mA at 1.5 V for the common 850 nm lamp (all tested companies were identical in efficiency) and 35 mA at 1.9 V for a good 660 nm.
Safety Concerns
Heat generation is the primary concern. I found an article that said skin temperature should never be more than 41 C (105.8F) to meet FDA regs. There is no way to know how hot a design will get until you make it and wrap it as snuggly as possible with ace bandage and stick a thermometer in for 2 hours. No matter how "cool" a heat-producing device operates, if you wrap it good enough and leave it there long enough, it can get hot. It's not just how much energy goes in, but also how much goes out. I've found around 0.8 Watts per square inch to be the maximum energy that can be put into a device that touches the skin with a fan or special heat sinking.
Eye Safety
Strong blue LED's are dangerous to your eyes! White LEDs have been studied for safety, but I wouldn't stare at them because they have blue frequencies in the white. Strong green LEDs have 1/15th the risk of blue. Strong and focused Red and yellow LEDs appear safe, but I would not stare directly at them for more than a minute. If you find a 10,000 mcd 660 nm Red, be careful.
The ACGIH does not seem to have a safety factor based on time of exposure (TLV) for simple LEDs, but it has two categories that can apply to them. One is the TLV for laser light, but lasers are different because they really focus the light in one spot which is much more likely to cause harm. The link at the bottom of this paragraph is a well-researched article that strongly claims you don't need to treat LEDs as lasers when it comes to safety. The other TLV is for light at a range of wavelengths and time exposures, which is good. Blue LEDs may harm the eyes from a photochemical injury called the "blue light hazard". I have personally been harmed by a blue LED key ring (blue photons have much more energy per photon - that's why red light does not affect night vision very much). I had a spot in my vision for months after a 2 second exposure. Distance from such a narrow-beam, strong blue LED only makes the AREA of damage on your retina smaller, not that damage is less likely to occur. Red, yellow, and green also have photochemical risks, but for LEDs, only green has the remote possibility of causing harm (if it's high power with a narrow emission angle). Bright visible light may also harm the retina from thermal activity. Blue is thermally 10 times more dangerous than the others. Reasonably powerful LEDs in red, yellow, and green are also thermally safe. But they are brighter at narrow wavelengths that we have evolved to cope with, so I still consider staring at them for more than a minute to be risky. Infrared light > 770 nm may harm the retina and lens from thermal activity, but has more risk for the lens. Damage to the lens may take the form mainly of cataracts. Infrared should be less than 10 mW/cm^2 if it's applied for greater than 15 minutes. For less than 15 minutes, mW/cm^2 should be < 1800 t^(-0.75). This means 83 mW/cm^2 is safe to the lens for up to one minute. Suppose you're using an excellent 50% efficient LED at its maximum power dissipation of 100 mW. Anything stronger will have difficulty dissipating heat and it's difficult to find more powerful 5 mm LEDs in red and infrared. That's 100x0.50=50 mW light output, but if you place it directly on the lens, it's an exposure in an area of only about 5 mm in diameter, or 0.20 cm^2, so 50/0.2= 250 mW/cm^2 light intensity. So by using the TLV equation above, it appears up to 15 seconds is safe when applying one of the most powerful types of 5 mm infrared LEDs directly to the eye. So, it's possible to increase the risk of cataracts when treating macular degeneration with red and infrared LEDs. From another source: "Near-infrared thermal hazards to the lens (associated with wavelengths of approximately 800 nm to 3,000 nm) with potential for industrial heat cataract. The average corneal exposure to infrared radiation in sunlight is of the order of 1 mW/m^2. By comparison, glass and steel workers exposed to infrared irradiances of the order of 80 to 400 mW/cm^2 daily for 10 to 15 years have reportedly developed lenticular opacities (Sliney and Wolbarsht 1980). These spectral bands include IRA and IRB (see figure 49.1). The American Conference of Governmental Industrial Hygienists (ACGIH) guideline for IRA exposure of the anterior of the eye is a time-weighted total irradiance of 10 mW/cm^2 for exposure durations exceeding 1,000 s (16.7 min) (ACGIH 1992 and 1995). Thermal injury of the cornea and conjunctiva (at wavelengths of approximately 1,400 nm to 1 mm). This type of injury is almost exclusively limited to exposure to laser radiation. " Note: For visible LEDs, Use L= 1000 x mcd/(683 x P) in place of L x (change in wavelength) in the TLV equations. For infrared, use mW/(SR x 1000). The distance from the LED does not change the danger for equations with L in them. The reason for this is because the ACGIH values each and every rod and cone in the retina, and light from a further distance has the same strength for each rod and cone it hits - it is weaker only because it affects a smaller number of them. If you want a better article on the retinal eye safety of LEDs and lasers read this (but they don't address lens safety, do not give an mcd example for blue, and don't discuss focusing that new LED plastic cases use.
Halogen lights contain a lot of blue light and are very dangerous to the eyes.
Sunshine
Bright sun at midday in summer, south of the mason-dixon line (let's just say Florida, Texas and California which covers most of us non-New Yorkers anyway) has a lot more energy in the red and near infrared (about 39 mW/cm^2 - see chart below) than most LEDs. But the Sun's light is not pulsed and the energy is spread over the full red and near-infrared healing range (630 to 920 nm or so) instead of being very strong in pulses at a precise wavelength. The wavelengths longer than 920 nm are why the Sun is hot - the water in your skin is absorbing the energy. Strong sunlight is probably as good for you as any LED array....if you avoid getting too much UV. I have used a corrugated plastic sheet like those used in green houses, but the special kind that is clear and blocks UVA and UVB (I tested it to be sure with a UVA/UVB meter). It costs about $40 per sheet at Lowes and I bought just one sheet. One sheet is barely big enough to cover your body. I place two chairs at each end of my lounge chair and use their backs to support the plastic sheet. Otherwise, you can use sunscreen, which may not block the aging and cancer-causing rays very well. But 15 minutes of direct UV sun each day produces a lot of vitamin D which prevents a lot more of the big cancers (prostate, breast, and colon) and it's not long enough to worry about skin cancer. My arthritic fingers feel better for about a week after spending some time on the beach.
Halogen, Incandescent, and Infrared Heat Lamps
The Sun, Halogen lamps, incandescent lamps, and infrared heat lamps all emit light based on the black body radiation principle. This principle determines the amount of energy emitted at each wavelength of light. The chart below shows how the energy changes for the sun verses an incandescent lamp. The sun is much hotter and it's energy shifted to shorter wavelengths (higher energy). Halogen lamps have a curve almost the same as the one shown for incandescent, but shifted a little closer to the Sun's curve. Infrared lights are shifter more away from the Sun's curve. Halogen, incandescent, and infrared heat lamps all heat up a metal filament of tungsten to produce light. The filament "incandesces" which means it produces light by black body radiation. The only difference is that halogen gas can allow the filament to get hotter than regular incandescent bulbs and heat lamps are designed to have a cooler operating tungsten filament. They operate at approx the following temperatures: halogen - 3200 K, incandescent - 2800 K, infrared lamp - 2400 K. The cooler infrared filament means more energy will be in the far infrared range, which heats tissue by heating the water in the skin. Since water absorbs the far infrared very well, the water is "catching" the energy. This is how operating at a lower temperature can produce more heat in tissue.
Halogen lights contain a lot of blue light and are very dangerous to the eyes.
In summary, halogen lamps will produce light like the Sun and it can provide more light energy in the healing (tissue penetration) range of wavelengths than regular incandescent and infrared heat lamps. This will be much more energy than LEDs can provide, but the energy will be spread out over a larger range of wavelengths (see chart above comparing LEDs and Sun). I do not know if this is better or worse, but the halogen is closer to the Sun's natural spectrum. Halogen lamps even have glass covers that block UV light so that desk lamps do not cause sunburn to hands or harm eyes. As with the LEDs, halogen lamps put out about 30% of the energy they use as light energy in the tissue penetration range. So a 50 W Halogen spot-light that concentrates it's light in a 10x10 cm area (at the glass bulb) will produce 50/3/100= 0.16 W/cm^2 = 160 mW/cm^2 in the tissue penetration range. This is about 4 times the best LED array and sunlight, but it's also about 4 times as much heat in the skin as you would experience in the brightest midday sun in Miami. If you try to use a halogen lamp for healing and wonder if you're getting enough light, simply compare the heat you feel to the heat you would feel from bright sun. If it feels like very bright sun, its more healing energy than LEDs can provide, although not concentrated at specific frequencies and not pulsed. Since the peak frequency of halogen is in the tissue penetration range, it is more efficient than any other black-body radiation source at providing energy in the tissue penetration range without too much heat in the far-infrared (Sun is almost as good). Plexiglas may be used to block some of the far-infrared that heats the tissue. Well-designed LEDs will not have the heat problem at all, which is one reason they are being used. LEDs are more powerful over the short range of wavelengths they cover, but not over the full range of healing frequencies. The strongest dose of red and infrared light I have given my arthritic fingers was with a halogen lamp and using Plexiglas to block some (not all) of the far-infrared heat frequencies. It certainly loosened up my joints and felt good (mainly from the slight heat), but they were a little swollen the next morning. The dosage may have been about 2,000 mW/cm^2 for 20 seconds (40 J/cm^2). It was a 60 Watt spot light held 6 inches from the joint. Maybe 10% stayed in the spot, and 33% was in the frequencies of interest so 60 x 0.10 x 0.33 = 2 W = 2,000 mW. BTW the best thing for arthritic joints is intense STRENGTH training, niacinamide, and glucosamine. The best thing for a joint is strong muscles, tendons, and ligaments surrounding it.
Does Pulsing LEDs Help?
Some companies claim pulsing the light is important, but I haven't seen any data to support it. Pulsing increases the light at all depths for a brief period of time. This helps deeper tissue be exposed to more short-pulse light without the LED getting too hot. But a constant light source can provide the same amount of total light energy per minute when operated at lower intensity. Pulsing probably does not provide more light per minute to the deeper tissue. 50 mW/cm^2 applied for half a second during each second of application will not provide more light at 1 cm depth than 25 mW/cm^2 applied for the full 1 second. They also generate the same amount of heat. There are two ways pulsing may help: 1) if there is a "light power threshold effect" in cells rather than just a "total light energy applied" effect. By this I mean there could very well be something in tissue that requires a certain amount of "activation energy" to cause a reaction to occur. With LEDs, maybe a certain number of photons are required to strike a molecule at the same time to have an effect. 2) If there is something interesting in tissue that responds to certain rates of pulsing. But it would take an enourmous amount of clever research to determine which frequencies can do what. In summary, I know of two ways it MAY help, but I do not have any data to support. I have seen 2 web sites claim that pulsing makes the LEDs more efficient but the light output verses current input curves indicate that being constantly on at a given temperature (and therefore averaged energy input) is more efficient.
© 2006 heelspurs.com LLC
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Twickle Purple note: This company has the best priced LED products I seen yet. You can check them it here. They are simple, but we don't really need fancy.
. This is an extremly well put together piece and I look forward to reading and digesting it (I've only skimmed through at the moment). 
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