Did you know that lux, lumens, and PAR all measure the same visible spectrum of light? It’s true that they go about it in different ways, but nonetheless they measure the same visible range of light.
It can get confusing, but the thing is, plants don’t care about only the 400-700nm (visible light spectrum) range. It’s true that it is the bulk of what they use, and older science swears by that range, but as The Dude says, “new shit has come to light, man”. The old “lumens are for humans” argument can be amusing when you consider that many who have said that also praised PAR, which measures the same range. Guess what? Lux measures that same range too, as do candelas! They don’t all measure in the same manner, but they all measure the visible spectrum. PAR (Photosynthetically Active Radiation) weighs certain reds “heavier” than other colors, for example, as being a notable difference to lux or lumens, which are more focused on quantifying intensity. PAR is a good indicator, as are lux and lumens, and they all measure the biggest part of the plant spectral range requirements…just not all that plants need and use.
As the tools to more adequately measure what plants actually use come to be, and as new horticultural lighting standards come into play (such as UL8800 and ASABE S640), we see a need to better understand all that is involved in having a grow light pass certification. Then there is the issue of getting lights qualified on the DLC SSL QPL, which shows that they are highly efficient and as a result, may be able to qualify for rebates from power companies. Additionally, new industry buzz phrases and terms have popped up in recent years, and it’ll take some time to get familiar with and accustomed to them.
To keep it simple, in today’s new lighting landscape, there are terms and phrases you need to know.
The Helpful Terms
You’ll see PAR, which can also be referred to as PPF (photosynthetic photon flux) as well as PPFD (Photosynthetic Photon Flux Density). That is still the visible spectrum. YPF (Yield Photon Flux) and YPFD (Yield Photon Flux Density) measures more of the total range that plants actually use (360nm-760nm), which includes the PAR range (400-700nm) of visible light along with a range below 400nm (UV aka ultraviolet) and another range above 700nm (IR aka infrared).
If a light has been truly made for horticultural use, then the YPF number will be higher than the PAR number. This is because YPF includes a wider range of light that includes UV and far reds in addition to the visible spectrum. YPF measures closer to the entire range of light that plants actually use, whether visible to the human eye or not. UL8800 measures 350-850nm, which in the scheme of things, is a more complete range of light that plants actually use. We also have ASABE S640 which measures 280-800nm. This is what other plant specialists have said is the range that we should be measuring. It’s getting closer to settling between these minor variations in range, but it is clear that plants are using more than just 400-700nm bandwidths of light.
Part of the industry didn’t want to have people measure more than 400-700nm for personal reasons. As spectroradiometers get more accurate and more cost-effective with more people using them, we see that HPS bulbs, for instance, create an absurd amount of infrared light, which is pure heat and that plants have no use for at such high doses. Most of it is actually outside the range of what plants prefer.
The more you know, the better informed you can be in your decision-making process when buying a light with which to grow your plants. Nothing beats the satisfied look on a gardener’s face than seeing happy plants.
Let’s drop the last key detail. Horticultural lighting is measured in umol/joule, which tells you how well a light converts electricity into photons. This is called PPE (Photosynthetic Photon Efficacy). It’s an efficiency rating and is important to know for grow lights. The higher the number, the better it is for plants. These days, anything above 2.5 umol/J is considered good.
Remember: plants eat light, so the healthier dose of what they want gets you to those better results that you’ve been wanting!