Understanding light therapy requires understanding light. For those who have already sold or lost their high-school science books, we offer quick introduction shedding light on the two crucial measures that describe a light therapy device.

Why do plants look green?

Rather than jumping feet first into photometry, let’s start with colors. Why are almost all plants green? The answer could be quite trivial: they reflect green light and absorb red and blue. But wouldn’t it be better for them to be black and absorb all the available light instead of reflecting the green energy back to the atmosphere?

Indeed, one of the main jobs of plants is photosynthesis: absorbing light and getting energy they need to live and grow. At the same time, they cannot absorb all electromagnetic radiation because that much energy would burn them. Different pigments and photo-sensitive molecules are tuned to different parts of the light spectrum. Other radiation would cause different biophysical changes, leading to excessive heat or undesired molecular changes. Therefore, it appears efficient to only specialize in processing the energy of a narrow spectral fragment and reflect the rest away. But why did they choose the fragments they did?

Imagine light as a beam of endless sequences of waves of different lengths. With such an idea in mind, scientists measured that the Sun shines with the highest intensity in waves 400 to 700 nanometres long. The light we see as blue has some 450 nm waves, green light about 500-550 and red light 650-700 nm. It is not coincidence that it is these 400-700 nm long waves which our eyes perceive: Rather, it is evolution that tuned our eyes to this most intense radiation.

Would you like to read more about the light perception in our eyes and how it influences our bodily rhythms? Check our articles on the role of the eyes in depression and transmission of light into signals for our internal clock mechanism!

light photons lux lumens illumination

Similar to the process of our eyes, evolution developed and preserved the molecules responding to the most intense light in plants. At the same time, they developed the ability to reflect ultraviolet and infrared light that could harm them. Plus, they “learned” to also reflect green. The Sun shine is especially intense in the green part of the spectrum, peaking at some 560 nm. When the early plant-like organisms were developing, the atmosphere was not preserving them as it preserves us today. Therefore, plants developed mechanisms to only absorb two narrow spectral windows, blue and red, and to reflect everything else, including green, which might have been too intense during the bright days millions of years ago.

What is light?

Electromagnetic radiation, physicists say between argues whether we may describe it rather as a wave of energy or a stream of photons. In the case of light therapy, we accept both descriptions. Generally, light is a stream of photons to us: a flux of tiny particles hitting our retinas that activate photosensitive molecules within our eyes. But when we describe the quality of light, we talk about waves anyway.

You don’t need to be a physicist to understand the specifics of therapy devices – but it helps to understand two crucial features of light: (1) illuminance, which describes the intensity of the light beam we are exposed to, and (2) color, describing the wavelength and temperature.

Intensity of visible light describes how strong the flux of light is. As a measure of energy, it would normally be enumerated in Watts or joules – but such a measure would also include the energy of the invisible parts of the spectrum. Since we don’t care about those parts outside of our spectrum, we measure intensity in lumens: roughly said, units of energy emanated in the visible spectrum. With regard to therapeutic lamps, however, we disregard the flux of light itself and instead we focus on how much light reaches our retina – and that is what is meant by illuminance.

The values describing light therapy: illuminance and color

Illuminance describes quantity of light reaching our eyes (or any other surface). The unit of illuminance is one lux which is equal to one lumen of light hitting one square meter of a surface. Of course, it is not only the intensity of the light beam that determines illuminance: it is also your distance from the light source, the angle between your retina and the lamp and the direction of the light.

The standard illuminance in bright light therapy is 10.000 lux. It is important to notice that while every lamp can claim to be intense enough, it is only true at a certain distance from the lamp. Normally, this is around 15-25 inches or 40-60 cm – but imagining that you are spending 30 mins every morning sitting this far from the lamp, it might be smart to opt for the higher standards or even more, 25-30 inches, to have comfortable and productive time there. (Of course, this measure is different for light glasses where the distance is set – but if you see a lightbox which does not say how far from the lamp to be for the 10.000 lux illuminance, stay miles away, because chances are you are being fooled.)

The wavelength is measured in nanometres, but in light therapy, we normally talk about temperature which is measured in Kelvins. Longer waves mean higher temperatures. Paradoxically, the cold colors like blue correspond to high temperature, while warm colors such as red or orange correspond to low temperature.

It is the prehistoric man to be blamed for the confusion, together with some scientists who died centuries ago. When the prehistoric humans saw the Sun and the fire, they found it warm – and naturally started calling their orange and red-ish light warm. Millenia later, scientists were describing colors of the light, and they grouped them by temperature an object needs in order to emanate light of such color. The Sun is yellow, because it burns predominantly around 5500 kelvins. If you would, however, warm it up even more, it would get blue. Therefore, blue is warmer than yellow, even though we don’t have the personal experience with such high temperatures, which makes us perceive yellow and red as warm and leave the actual hotness of blue and green unnoticed.

But not to worry about physics too much, we generally only distinguish two different types of light in light therapy: white versus blue. White, or “whole-spectrum” light is polychromatic light that includes all visible frequencies. It is typically “colder” than the Sun or a candle, in order to be white rather than yellow, but its frequencies cover the whole visible spectrum. This is the more natural solution, it has a longer tradition in light therapy and is more thoroughly researched. The intensity standard of 10.000 lux is related to this white light.

Following the discovery of melanopsin in 1999, novel, “blue-enriched” devices became popular. Melanopsin is a particle densely distributed in our retinas, which reacts specifically to the blue light (frequency about 470-490nm) and directly informs the “inner clock” in the suprachiasmatic nucleus of the time of day, based on the light exposure. Lamps emanating monochromatic, blue-only light, or blue-enriched white light, can therefore achieve the same effect on the brain with much lower light intensity, since they radiate these frequencies that our inner clock machine is sensitive to. Nevertheless, it is not only melanopsin that matters, and we currently have no precise equation that could determine the dosage of blue light needed for a blue monochromatic lamp to be comparable to the standard of 10.000 lux of white light. Therefore, the evidence regarding beneficial effects is slightly less convincing in the case of blue-enriched (or monochromatic blue) devices, compared to the white lamps.


Understanding the light itself, you can move on to the second article from our series on the science behind light therapy, which describes our perception of light and evolutionary processes that made our brain activation sensitive to the Sun radiation – and then we can continue unveiling the interconnection between our perception of light and sleep problems, depression, or the Parkinson’s: