There are several myths surrounding the specific light spectra used for growing crops and, more specifically, the use of white, green, or broad-spectrum light. This article is the third in a series of interviews with Signify’s Esther de Beer, manager of the team of plant specialists at Philips Horticulture. In this interview, she explains why you do not need full white light for crop inspection.
Various LED lighting solutions for horticultural environments consist of a particular mix of red and blue light. With only red/blue light, we cannot see other colors within the greenhouse or vertical farm, and everything looks purple, but on the other hand, in most cases, we also do not need full white light from the lighting installation to see other colors. In a greenhouse where there is a little bit of daylight coming in, for example, you can still see colors well, even with only red/blue lighting. In situations where there is no daylight, in most cases, you will be able to do crop inspection fine if at least 5% green light is added to the spectra. This means that the deep red/white spectral range is well suited for working and inspection in a greenhouse or vertical farm.
What should an optimal light spectrum provide?
The light spectra used in horticulture LED lighting solutions must provide the ideal balance between:
- The optimal growth of the crop.
- Optimal energy efficiency.
- It must contain just enough ‘white’ light for proper crop inspection.
What do the different terms mean?
There are a number of terms used to describe light spectra. Some talk about ‘white’ light, and some about ‘broad spectrum’ light. Although ‘broad spectrum’ is not a term that is officially defined, everyone will agree that the sunlight spectrum is an obvious example of a broad-spectrum light. The sunlight spectrum varies somewhat depending on the time of day and weather conditions, but it contains roughly equal amounts of red, blue, green, and far-red light.
When looking at the spectra of LED suppliers offering ‘broad spectrum’ light, we see that these spectra typically contain all colors over the visible range and, therefore, in that respect, are broad-spectrum lights. Yet typically, these spectra do not include far-red, which is necessary for some crops. At Signify, we tailor the light spectra of our LED grow lights to the needs of the crop and to optimal energy efficiency. As a result, most of our spectra contain relatively little green light, and we also add far-red light where we see a benefit for certain crops. Our products with deep red/white and deep red/white/far red are essentially energy-efficient broad-spectrum lights.
Figure 1. Sunlight spectrum vs the Signify spectrum, which contains far-red
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 that 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 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.
Figure 2. How the human eye adapts within 1 minute to different light spectra after entering a climate room
In figure 2, the top three images represent a climate room with only red/blue lighting. When you go into the room, it looks purple. If you stay in there a while longer, the surroundings will continue to seem purple. This is due to the fact that there is no green light in the room, and therefore, the automatic white balance effect does not work. If you look at the three lower images in figure 2, here we have a spectrum containing red/white, so a small percentage of green has been added. When you enter the room, it will still look purple at first, but when you stay in there for approximately one minute your eyes will adapt, and you are able to see the green of the plants.
Therefore, to conclude: you do not need full ‘white’ light for proper crop inspection in a greenhouse or vertical farm, you only need just enough to see the colors and crops sufficiently.