Lumens, Lux, and Micromole Measurements for Plant Growing Lights
If you’ve been researching LED lighting long enough, then chances are you have heard someone say that lumens aren’t a good measurement of the amount of power a light is putting out. When people say this they don’t usually offer a better alternative, they simply mention that lumens are a bad measurement. Well today, we’ll look at why lumens aren’t the best measurement and we’ll mention an alternative that works much better.
Luminous Flux
Anything that has the word flux in it immediately has me thinking about Back to the Future and time travel (the flux capacitor made time travel possible in the movie). Luminous flux doesn’t allow for time travel, but it does measure the perceived or visible power of light. Luminous flux is not a measure of the actual power of light; it is a measure of how powerful the human eye perceives the light to be.
The standard measurement unit for luminous flux is the lumen (lm).
Lux is simply the measurement of the number of lumens in a one square meter area.
Lumens, a Good and Bad Measurement
Luminous flux is an excellent measurement of the power of light for industries that are catering towards humans and not plants. In concert halls, stadiums, and theatres the amount of light the human eye perceives is very important. We’ve all probably been to a movie before that had segments that were too dark to really see what was happening on screen. Obviously the directors of these movies didn’t study their luminous flux!
Luminous flux is a horrible measurement for plants because, unfortunately, most plants don’t have human eyeballs. Plants simply don’t care about how well the light looks to us humans. What they do care about is that the right type of light is available to spark photosynthesis.
Photosynthetic Active Radiation (PAR) Light
The main thing you need to know about PAR light is that it represents the light that plants are able to use in photosynthesis. Many of you have probably already seen PAR charts (like the one below) that show that plants love light in the red and blue spectrum of light.
The funny thing is that humans love lights on very different wavelengths (almost the exact opposite of plants). See the chart below.
As you can see from our charts, plants and humans see light very differently. Lumens are used to measure the intensity of light to the human eyeball. Plants really don’t care how well we humans see light. A light that is really bright to your eyes might actually do little to nothing for a plant.
Micromoles, a Better Alternative
Scientists realized that luminous flux was not a good measurement for light power as it relates to plant growth so they came up with a better measurement. A better measurement needs to have an even cooler name so the scientists came up with Photosynthetic Photon Flux (PPF). That name will scare the pants off of your average gardener! Don’t get overwhelmed by the big name, just know that it’s a method that measures the amount of light good for plant photosynthesis. This method actually measures the amount of photons in the photosynthetically active range for those interested.
The standard measurement unit for photosynthetic photon flux is the micromole.
If you’ve really researched a lot about plant lighting, then you may have come across the word micromole (abbreviated as µmol). Understanding micromoles can be quite complicated, but the main idea behind the micromole light-measurement technique is that it measures the amount of light helping with photosynthesis and not the perceived power according to the human eyeball.
Quick Example
To make this all easier to understand let’s pretend that humans and plants both have eyeballs. When our eyeballs receive light we grow. Humans grow when their eyes absorb light that has a high lumen reading. Plants grow when their eyes absorb light that has a high micromole reading. Lumens don’t matter to plants just as micromoles don’t matter to our human eyes.
Micromole Meters
There are measuring instruments out there that measure micromoles. In general, the more micromoles an LED grow light puts out, the better the light will be for growing.
If you only take away one thing from this discussion it should be that LED grow lights with more micromole power are generally considered better lights.
If you have any more questions on the subject, please leave me a comment below.
Oh, and here’s a great YouTube video that shows a side-by-side micromole comparison of two LED grow lights. I don’t necessarily endorse Apache Tech grow lights (you can read my review of Apache Tech here), but I do like that they are using micromoles to compare the two lights.
PAR is a much better measure of light than lumens because it flattens out the spectrum while Lumens focuses on green. But it’s really the same measurement as LUX, just a diffrent scale. But none of these measurements truely measure the light’s impact on plant growth. Why? Because PAR still does not take into accout the impact the spectrum has on photosynthesis. To do this measurement you have to apply a mask or filter that represents the impact light has on plants. The filter would block about 50% of green and about 30% of yellow.
No-one is doing this yet, at least not in a automated instrument. You can buy filters or gels that pass only blue or red then add the results together but for a good comparison but both light sources must be measured & compared because the filters also block out some of the “good light”.
Need some clarification. I agree that the meter is reading the full (PAR) wavelengths between 400-700nm, however most of the better LED lights are being produced with just the wavelengths that the plants are requiring…example…our blackstar 500 UV produces 380,425,460,630,660,730 and the 12K Cree. Others are 5 band, 11 band, etc… Am I wrong in assuming that when a par meter is measuring an HID or CFL its measure all the wavelengths between 400-700nm. I assumed that an LED grow light with specific lights wavelengths (not having the unnecessary wavelengths), would be even more valuable in terms of real PAR output as its not measuring the visible light wavelengths. any thoughts?