Lumens, PAR, PPF, and PPFD: Measuring COB Grow Light Output

Want to figure out how much light your COBs are giving your plants? Getting an accurate measurement is a little trickier than you might think.

A quick glance at your standard COB data sheet will probably show you one related specification: its luminous flux rating. While luminous flux is a good indicator of how bright a COB is at a certain wattage compared to others, it’s not the ideal way to quantify how much light your plants are getting. There are many different units and methods used for measuring light, like luminous flux, lux, foot-candles, PPF, and PPFD. Let’s have a lil’ gander at each.

Lumens

Lumens are probably what most people are familiar with when it comes to defining the intensity of different lights. The lumen is a unit of measure of the quantity of visible light emitted by a source. If you’ve done any research into grow lights, you’ve probably heard the phrase “lumens are for humans”. This is derived from the fact that lumens are weighted according to a model of the human eye’s sensitivity to various wavelengths. This weighting means that light in the green-yellow spectrum will register significantly higher in lumens than red or blue light – two colors that are very important for photosynthesis in plants.

If you’re comparing 2 grow lights and measure 500 lumens per square meter from a light with a high (blue) color temperature vs. 1,000 lumens per square meter from a light in the green-yellow range, you might think the 1,000 lumen light is the clear winner, but this is not the case. While lumens may reflect how much light humans perceive, they do not adequately account for how much light your plants are actually receiving. There’s a name for the specific type of light that we want to measure called PAR, and we’ll get to this shortly.

Lumens are involved in a few different measurements, like luminous flux, lux, and foot-candles.

Luminous Flux

Luminous flux refers to how much light energy is emitted per unit of time in all directions, and is measured in lumens. To properly measure luminous flux, you would need to place your light in a device called an integrating sphere, which is able to measure all of the light that the source produces. Luckily, this value will be provided on the data sheet for your COB, so you can save the $10,000 you were going to buy the sphere with for something else.

You can use luminous flux ratings to compare COBs against one another, so long as you have the voltage and current at which the reading was taken. If you compare 2 COBs and both are rated for 10,000 lumens, but one does it at 36 volts and 1 amp (36 watts), and the other does it at 36 volts and 1.5 amps (54 watts), the first one is more efficient and is a better choice.

A commercial integrating sphere.

Lux

Lux is a measurement of how many lumens fall on a 1 square meter surface, when lit by a source 1 meter away. 1 Lux is 1 lumen per square meter. Lux meters can be purchased pretty cheap online, but again – these are measuring lumens, and aren’t very useful for grow lighting. If you’re simply looking to see how even your light coverage is, you could use a lux meter and take readings across the canopy of your plants, but don’t read into the numbers any further.

Foot-Candles

Foot candles are a measurement similar to lux, but are Imperial rather than Metric. A foot candle is a measurement of how many lumens fall on a 1 square foot area, 1 foot away from the light source.

PAR

So, if lumens and their associated measurements aren’t ideal for evaluating light for growing plants, what is? Well, the kind of light we’re looking for is called PAR, short for Photosynthetically Active Radiation. PAR is not a measurement of light, but a range of a light, which, as you probably gathered from the name, factors in all wavelengths that are involved in driving photosynthesis, from 400nm (violet-blue) to 700nm (red). The PAR range corresponds with the range of light that’s visible to humans, but PAR does not intentionally weight various wavelengths of light differently like lumens do, so the reds and blues are properly represented. There are a couple different measurements of PAR that are important to indoor growers: PPF and PPFD.

A graph showing photosynthesis rate in the PAR range of 400nm to 700nm.

PPF

Photosynthetic Photon Flux, or PPF, is a measurement of the number of photons a light source emits per second that are within the PAR range. It is the plant-friendly equivalent of the luminous flux measurement discussed above. PPF is measured in micromoles per second (µMol/S). What’s a micromole? Well, a micromole is a certain number of photons. Photons are particles of light, and are tiny little buggers. To keep people sane, micromoles are used to quantify photons when measuring PPF. 1 Micromole is equal to 602 quadrillion photons (602,000,000,000,000,000!).

PPFD

PPFD stands for Photosynthetic Photon Flux Density, and is the same as PPF, but takes surface area into account as well. PPFD is measured in micromoles per meter squared, per second (µMol/m2/S). In the world of lumens, PPFD would be most similar to lux or foot-candles: PPFD measures the number of PAR photons hitting a certain area, while lux and foot-candles measure the illuminance of a surface in lumens per square meter or foot. For most indoor growers, PPFD is commonly measured with relatively inexpensive quantum meters like the Apogee MQ-500 or Li-Cor 190R.

Apogee MQ-500

Li-Cor 190R

Until recently, quantum meters did not work very well with narrow band LED lighting (e.g. – grow lights that used only red and blue LEDs of a specific wavelength), and conversion factors had to be applied for a somewhat accurate measurement. Now, with the latest sensors, the spectral response curve matches the PAR range much better, and represents blue and red LEDs more accurately:

A graph illustrating the relative response of the original Apogee quantum sensor (black) vs. the new sensor (gold) to the PAR range of light. As you can see, the original sensor was not very sensitive to light in the 400nm blue range and light beyond the 650nm red range.

 

Li-Cor’s 190R relative response.

Many commercial grow lights provide PPFD values, but omit critical information like the distance at which the PPFD reading was taken. If the manufacturer says their light puts out 1000 µMol/m2/S, that’s all fine and dandy, but how far was the light from the sensor for this reading? Was it 12″? 24″? Taking a single measurement of PPFD is also not worth much either – it’s better to have multiple measurements of PPFD in several different places below the light (what is the PPFD reading in the center? How about a foot out from center in each direction? etc.).

Different plants require different levels of PPFD and a term called DLI, or Daily Light Integral, is used when examining how many moles of light is optimal for a plant per day. This is a topic for another post though!

7 Comments

  1. Do you know of any manufacturers which provide the full list of details (PPF/PPFD/distances of measurement,etc)? I have not found anything yet.

  2. That last link, clu48 “top bin” citizen doesn’t bin as for as I knew. The amount of money they are charging, most of these places other than plc at least are counting on people staying uninformed about cobs, like its some kind of high tech science that can’t be understood by the common man. If a person can put batteries in a flashlight the right way then they can build a light, some people are just too lazy to even read or watch a video, my bet would be the weed growers mostly. and would rather fork out 600 bucks for a 200 dollar light.

    • LEDGardener

      February 20, 2017 at 1:19 pm

      Yeah, I don’t think Citi bins their chips. I’m guessing it was probably a copy/paste error for that page.

      I guess if you’ve got the cash and feel your time is worth more than what it’d take to do the research, shelling out big bucks for a light would make sense. Like you say though, a day of research could save you 50% or more on most of these rigs.

      • I think plc would be the only light I would buy, that is if I had that kind of money. That guy builds a nice light that is much better than those spiderman lights that cost 1000+ and he is totally open about what goes into them and is very active on the diy stuff on youtube. Greengenes and gromau5 videos are where I got a lot of my info on the cob stuff.

  3. Nice post! I would also point out that PAR itself is an imperfect metric because it weighs all wavelengths between 400 and 700 nm equally. We know from the chlorophyll curves though that blue and red wavelengths are “worth” more in terms of photosynthetic absorption. So a grow light with a green bias will still score well with PAR, even though all that green light might not be as useful for the plant.

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