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 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 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.
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 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.
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.
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 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.
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:
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!