Another term which is often used when describing light spectra is ‘white light’. It is important to note that the term ‘white light’ is based on human visual perception, so this is the light that we see as being white, and that there is actually a range of spectra which all qualify as white light. For instance, both the sunlight spectrum and that of an incandescent lightbulb are officially called ‘white light’. However, if you look at the spectral composition you will see that they are actually very different: sunlight, for example, is cool white light, while incandescent light is warm white light which is rich in red light and only contains a small amount of blue light.
How does the human eye perceive ‘white light’?
Why are sunlight and incandescent lights both considered ‘white light’ when they are so different? This has to do to with the ability of our eyes for adaptation. The human eye can adapt very well to very different lighting conditions, both in brightness and in color. If you are outside on a sunny day the light level might be 100,000 lux and you can see everything perfectly. If you are indoors in the evening, where the light level is typically 100 lux or less, you can still see everything around you very well. This shows that our eyes can adapt over a very large range.
We are also able to adapt to different light colors, and our brains have the ability to actually automatically ‘balance out’ the color white. Let’s explain via this example: when you look at a white piece of paper in the sun, it looks white. If you take the same piece of paper inside and look at it under an incandescent lamp, which has a different spectrum than sunlight, the paper will still look white. This is basically due to our brains making sense of the world around us. Another thing to mention is that we see colors through the light that is being reflected by the object. So, if we have full spectrum sunlight shining on a leaf then the leaf will look green to us because it reflects relatively more of the green light than the other colors.
What happens in a horticultural environment?
When you mix the colors red and blue, you get purple. If we enter a space lit with a light spectrum with only red and blue light, and no other color in the spectrum, this environment will appear purple to us. A good example of this in a horticultural environment is a climate room with a red/blue light spectrum. When walking into this room the light appears purple. Also, in this environment the plants will actually not look green to us, but they will appear to be grey or black. This is because there is no green light in the light spectrum to be reflected to our eyes, to make the leaves appear green.
On the other hand, when applying a red/blue spectrum in a greenhouse you will often actually still be able to see the colors, like the green of the leaves, sufficiently. As long as there is a little bit of sunlight coming in from outside through the glass of the greenhouse, there is enough green light to reflect the color of these leaves. In a closed environment without daylight, like the climate room, we can achieve this same effect by adding a little bit of green to the light spectrum. Our eyes will then adapt - the automatic white balance will come into action - and within a minute the environment will not look purple anymore and we can see colors well. In our GrowWise Research Center we have found that we can work comfortably in an environment with a light spectrum containing deep red/white, because this contains just enough of the different light colors to see the crops well.
Keep in mind that the most common conventional light fixture used in horticulture, the HPS lamp, is also not a white light source, yet we can work under it very well.