Lumens Are For Humans, Not Cannabis
One of the most common and misunderstood marketing ‘buzzwords’ in cannabis lighting is lumens. Luminous flux is a measure of light intensity as perceived by the human eye, and lumens are the unit used to describe this measurement.
So, why don’t lumens matter when it comes to growing plants? Basically, lumens will show you how well a human eye will be able to see under a given light source… But plants don’t have human eyes.
Plants have a variety of pigments that use the energy in light to convert carbon dioxide and water into sugars in a process known as photosynthesis. Different pigments use different wavelengths (colors) of light to accomplish this task: chlorophyll absorbs mostly deep red, ultraviolet (UV), violet, and dark blue light, while chlorophyll b absorbs mostly reddish-orange and light-blue light. Accessory protein pigments can funnel some yellow and green light to chlorophyll, but this is less efficient for photosynthesis. In fact, plants reflect more of the green light that hits their leaves than any other color- this is why they look green to us.
Besides the photosynthesis that powers plant growth, other compounds control hormones within the plant that affect how it grows. Carotenoids absorb UV, violet, and blue light, and phytochrome utilizes red and far-red light.
While there is overlap between the wavelengths of light the human eye can see and those useful to plants for photosynthesis, different wavelengths are more important for each function.
For example, the human eye is most sensitive to green and yellow light, so the measure of lumens is weighted with respect to that particular range of wavelengths, while red and blue light count less toward lumen measurements, and UV and far-red light count for nothing. A light source with a lot of yellow and green light will have a high lumen rating and appear very bright to our eyes. Plants however photosynthesize best with red and blue light, so a high-lumen light doesn’t mean that plants will grow well under it. In fact, Low-Pressure Sodium (LPS) lights are among the most-efficient artificial light for humans- in terms of lumens per watt– but plants will die underneath them.
Another important consideration is that most plants utilize wavelengths of light invisible to the human eye. For example, besides helping to drive photosynthesis, UV light stimulates the production of defensive chemical compounds and structures in many plants; in cannabis, this includes THC, CBD, terpenes, and trichomes. Far-red light enhances photosynthesis through something called the Emerson Effect. Lumens don’t provide any information about the UV or far-red content of a light source. In terms of lumens-per-watt efficiency, lots of red and blue light- or any UV or far-red counts against the light’s efficiency when measured this way.
So, what intensity metric should you look for in a plant grow light, if not lumens?
Many grow lights are evaluated using Photosynthetically Active Radiation (PAR) and Photosynthetic Photon Flux (PPF). PAR counts all photons in the human-visible 400-700 nanometer (nm) range, corresponding to violet through a red light, and doesn’t weigh them differently depending on their color in this range. PPF counts the flux (number of photons per second) of photons in the PAR range and is typically measured in micromoles of photons per second (µmol/s), PAR / PPF is a far better gauge of how intense a light will be for plants, rather than just the human eye. However, UV light (less than 400 nm) and far-red light (more than 700 nm) are not counted in the PAR range or PPF measurements, even though plants use these wavelengths of light, and an all-green light can still have a high PPF measurement, so PAR / PPF still isn’t a perfect measure of how a grow light will perform.
Photo-Biologically Active Radiation (PBAR) counts all photons that are biologically active in plants, from 280 nm to 800 nm. Photo-Biological photon Flux (PBF) is a count of the number of photons per second that a light source emits in the PBAR range, usually given in µmol/s just as PPF is. As this measurement includes all wavelengths of light that plants use from UV to far-red, not just those that are visible to humans, it is by far the best way to evaluate the intensity of a grow light. However, PBAR and PBF still have shortcomings as measures of grow light performance. Because PBF counts all photons in the PBAR range equally, it still does not tell you if the light has any particular wavelengths of light, such as UV or far-red – it is necessary to look at the actual spectrum of the light to determine what colors are present. In fact, it is possible to have a light with a good or even great PBF score that won’t grow plants at all, such as the previously-mentioned LPS light.
When evaluating efficiency in terms of any of these intensity measurements per watt the light pulls, it is also important to recognize that different wavelengths of photons require more energy to create than others. The lower the wavelength, the more energy the photon has, and the more energy it takes to create. Lights that produce UV light (which has the lowest wavelength) will take more energy (watts) to create those photons than lights that don’t include any UV. Lights that have a lot of higher-wavelength, lower-energy red and far-red photons will create more photons per watt and may look better on paper, but without enough blue light cannabis plants don’t grow as well and flower quality will suffer. In other words, just because a light has a high PPF / PBF score, or high PPF/watt or PBF/watt, doesn’t necessarily mean that it will out-yield a light with a lower score. It’s important to have a balanced amount of blue and red light to drive photosynthesis, with some of all the other colors from UV to far-red to get the best results. You have to look at both the intensity and spectrum to really evaluate a grow light.
However, when looking at intensity measurements, lumens are probably the worst possible way of evaluating grow lights since lumens weight light that is least-useful to plants over the colors plants really want!
Kevin Frender is Chief Science Officer and Chief Technology Officer at Black Dog Grow Technologies. He has been growing indoors with artificial light for more than 35 years, using every commercially available lighting technology along the way. He also starts plants indoors for his summer outdoor garden every year and has decades of experience with hardening-off seedlings for transplantation outdoors. Combined with a vast knowledge of plants and a penchant for science, Frender is an expert in the field of lighting.