When it comes to measuring how "good" a grow light is, or how much light it produces for plants, it can be confusing because a lot of terminology gets tossed around. The truth is, there is no single best way to measure light, but there are several ways, each with its strengths and weaknesses.
Lumens
One of the most common measurements used abroad is lumens, which measure "luminous flux," or how much light a person can see. It measures how "bright" something is perceived by the human eye, and weights it so that light we see is more valuable than light we can't see. The amount of light received at a specific point in space is called lux, which is lumens per square meter (1 lumen/square meter = 1 lux).
When it comes to plants, the types of light we can see and the types of light they can use are pretty close. So even though lumens don't measure express light as precisely as plants see it, it does give us a good general idea of how bright multiple types of grow lights are for plants. Not perfect, but good.
For example, lumens are an excellent way to compare the amount of light put out by fluorescent, CFL, HID, MH, and HPS grow lights. However, when it comes to LED grow lights, lumens are no longer a good method because LEDs generally only put out the light in a very narrow band of light, and a weighted number of lumens is not a good way to predict the amount that will be available, which brings us to another way to measure grow lights.
Photosynthetically Active Radiation (PAR)
PAR does not take into account the amount of light produced by a light source as seen by humans, it actually only takes into account the range of the solar radiation spectrum from 400 to 700nm, which is the range of the spectrum that plants can use for photosynthesis. It is very close to the range of light that humans can see, but it is not exactly the same.
PAR refers to light in the 400-700nm range because this is the range that plants use for photosynthesis. Plants are more efficient at producing energy from light in certain parts of the PAR spectrum. For example, plants are most efficient at photosynthesis when using light in the red and blue ranges. This is why you see graphs like this when people talk about PAR, which is a measure of how well each chlorophyll in a plant absorbs light energy in different parts of the PAR spectrum.
PAR is just a way of talking about the spectrum between 400-700nm, and when people are talking about the "PAR of a grow light," they are talking about how much light is put out in that PAR range, or what is called PPFD (Photosynthetic Photon Flux Density).
So when most people talk about PAR, they are talking about "PPFD," or how much light is sent out for photosynthesis. When scientists and plant biologists measure the amount of light produced by grow lights through spectral experiments, they are almost always measuring it in PPFD.
However, even though plants don't absorb energy from all types of light equally, cannabis plants can use light from all parts of the spectrum through photosynthesis, including the green part (as shown in the image above).
The mechanism by which green light helps plants grow is that it penetrates deeper down into the canopy than red and blue light, which are mostly absorbed by the upper leaves, which increases the energy for photosynthesis, allowing photosynthesis to travel further down the plant.
Furthermore, since the light spectrum also affects how plants grow completely separate from photosynthesis, you may not get the best results by focusing only on photosynthesis! As NASA and others have demonstrated, many plants can get at least a little light from the green part, even if it is not the most efficient photosynthesis for green plants, and thus grow healthier and faster.
What affects the quality of light in grow lights?
1) Power is more important than the spectrum
The total amount of light from the grow light, that is, the light intensity (photosynthetically active radiation, PAR) provided is more critical to plant growth.
In other words, even if the spectral distribution is not optimal, as long as the light intensity is high enough, the plant can still photosynthesize well.
2) Blue light promotes stem and leaf growth
Blue light (wavelength of about 400-500 nm) is very important for the development of stems and leaves of plants.
Providing blue light will promote plants to produce stronger and wider leaves while inhibiting excessive elongation of stems and maintaining a compact plant shape.
3) Red/yellow light promotes flowering and fruiting
Red light (wavelength of about 600-700 nm) and yellow light (wavelength of about 570-590 nm) can stimulate the reproductive growth of plants, including germination, flowering and fruiting.
These light bands are particularly important in the later stages of the plant life cycle because they have a significant impact on flowering and fruit ripening.
4) Recommendations for reducing green light
Plants have low absorption efficiency for green light (wavelength of about 500-570 nm), and most of the green light will be reflected by the leaves instead of being absorbed for photosynthesis.
Therefore, it is recommended to minimize the output of green light to improve energy efficiency.
However, most modern LED lights have reduced the green light component to a reasonable level, so there is usually no need to pay special attention to this.
