Everything You Need to Know about Red Light Theraphy
We do not have the hardware to perceive near infrared light (as used in infrared light therapy), just as we do not perceive other wavelengths of EM radiation such as UV, Microwaves, etc. It has recently been proven that the eye can detect a single photon. Like elsewhere on the body, eyes are made up of cells, specialised cells, all performing unique functions. We have rod cells to detect light intensity, cone cells to detect colour, various epithelial cells, humor producing cells, collagen producing cells, etc. Some of these cells (and tissues) are vulnerable to some types of light. All of the cells receive benefits from some other types of light. Research in the area has increased significantly in the last 10 years.
Beneficial light for eyes
Most of the studies that point to beneficial effects use LEDs as the light source with the vast majority around the wavelength of 670nm (red). Wavelength and light type/source are not the only important factors though, as the light intensity and exposure time affect the results.
How does red light help the eyes?
Given that our eyes are the primary light-sensitive tissue in our body, one might think that the absorption of red light by our red cones has something to do with the effects seen in the research. This is not entirely the case.
The primary theory explaining the effects of red and near infrared light therapy, anywhere in the body, involves the interactions between light and the mitochondria. The core function of mitochondria is to produce energy for its cell – light therapy improves its ability to make energy.
The eyes of humans, and specifically the cells of the retina, have the highest metabolic requirements of any tissue in the entire body – they require a lot of energy. The only way to meet this high demand is for the cells to house many mitochondria – and so it is no surprise that cells in the eyes have the highest concentration of mitochondria anywhere in the body.
Seeing as light therapy works via interactions with the mitochondria, and the eyes have the richest source of mitochondria in the body, it is a reasonable assumption to hypothesise that the light will also have the most profound effects in the eyes compared to the rest of the body. On top of that, recent research has shown that degeneration of the eye and retina is directly linked to mitochondrial dysfunction. So a therapy that can potentially restore the mitochondria, of which there are many, in the eye is the perfect approach.
Best wavelength of light
670nm light, a deep red visible type of light, is by far the most studied for all eye conditions. Other wavelengths with positive results include 630nm, 780nm, 810nm & 830nm. Laser vs. LEDs – a note Red light from either lasers or LEDs can be used anywhere on the body, although there is one exception for lasers specifically – the eyes. Lasers are NOT suitable for light therapy of the eyes.
This is due to the parallel/coherent beam property of laser light, which can be focused by the lens of the eye to a tiny point. The entire beam of laser light can enter the eye and all of that energy is concentrated into an intense tiny spot on the retina, giving an extreme power density, and potentially burning/damaging after just a few seconds. LED light projects out at an angle and so does not have this issue.
Power density & dose
Red light passes through the eye with over 95% transmission. This is true for near infrared light and similar for other visible light such as blue/green/yellow. Given this high penetration of red light, the eyes only require a similar treatment modality to the skin. Studies use around 50mW/cm2 power density, with quite low doses of 10J/cm2 or less. For more information on light therapy dosing, see this post.
Harmful light for eyes
Blue, violet and UV light wavelengths (200nm-480nm) are bad for the eyes, being linked to either retinal damage or damage in the cornea, humour, lens and optical nerve. This includes direct blue light, but also blue light as part of white lights such as household/street LED bulbs or computer/phone screens. Bright white lights, especially those with a high colour temperature (3000k+), have a large percentage of blue light and are not healthy for the eyes. Sunlight, especially midday sunlight being reflected off water, also contains a high percentage of blue, leading to eye damage over time. Luckily the earth’s atmosphere filters out (scatters) blue light to some extent – a process termed ‘rayleigh scattering’ – but midday sunlight still has a lot, as does sunlight in space seen by astronauts. Water absorbs red light more so than blue light, so the reflection of sunlight off lakes/oceans/etc is just a more concentrated source of blue. It’s not just reflected sunlight that can do harm though, as ‘surfer’s eye’ is a common issue related to UV light eye damage. Hikers, hunters and other outdoorsmen can develop this. Traditional sailors such as old navy officers and pirates would almost always develop vision issues after a few years, mainly due to sea-sunlight reflections, exacerbated by the nutritional issues. Far infrared wavelengths (and just heat in general) can be harmful for the eyes, as like with other cells of the body, functional damage occurs once the cells get too warm (46°C+ / 115°F+). Workers in old furnace related jobs such as engine management and glass blowing always developed eye issues (as the heat radiating from fires/furnaces is far infrared). Laser light is potentially harmful for the eyes, as mentioned above. Something like a blue or UV laser would be the most destructive, but green, yellow, red and near infrared lasers can still potentially cause harm.