BY GREG RICHTER
R A P
PAR is one of the most misunderstood terms in growing, right up there with EC vs PPM. And yes, light makers sometimes use this confusion to sell or upsell growers on lights, ballasts and reflectors. Here’s my take on PAR, PPF, and PPFD. If you had a set of sunglasses that passed only the light that a plant can use, you’d see a lot less green and a lot more red. You’d have PAR glasses on, and they’d clear up a lot of confusion about how a plant uses light.
PAR MEASURES LIGHT THAT MAKES A PLANT GROW
PAR (Photosynthetically Active Radiation) measures light that makes a plant grow as opposed to Lumens, which measures light a human sees. This is now old news, but until recently Lumens were the only way we discussed grow light power and efficiency. It’s like this: Light is made up of quanta called photons, and there’s a small bazillion of them streaming out of a 1000 Watt grow light you can actually count them. Light output from a grow light is measured as PPF, Photosynthetic Photon Flux, which tells you how many photons are coming out of your light per second. You’ve seen specs for uMoles/sec, or microMoles per second, which is how many photons in plant-growing colors stream out of your light each second it’s on. A Mole is a quantity, like a dozen, but it’s a fantastically big number, about 6.02 x 10^23, which is just plain huge, but what matters is that PPF can be compared from lamp to lamp... Apples to Apples.
PPF is a more accurate way to determine how “good” a light source is, but is still doesn’t tell the whole story. Plants
use light energy like they eat food. Nutritional needs change in a plant as it matures, and so do its light energy needs. Most fast flowering annuals require regimented changes to light cycles. They prefer more of the red spectrum during the flowering/ fruiting phases, and more of the blue during vegetative cycles. Plants require trace amounts of several elements. Without them the plants will suffer, and could even die. UVA, UVB, and IR are also important to a plant’s development, but in terms of their contribution to the PPF, the number is miniscule. So, when measuring PAR you count all those photons, regardless of where they are within the PAR spectrum, and add them all together to get your number. It is better than lumens, but not perfect. The takeaway is this... when comparing output from equal wattage bulbs – more PAR uMoles per second equals more plant growth per dollar’s worth of electricity. That’s all there is to it. A 2100 uMole lamp grows more plant mass than a 1900 uMole lamp with both lamps running at 1000 Watts. More growth for the same power – just what growers want. Ballast makers like to pretend that the same 2100 uMole/s lamp, fed the same 1000 Watts of power from their Boom Daddy Ballast puts out more (never less?) than Brand X ballast. It’s just not true. Power to the bulb = PAR to the leaf. It’s the bulb, reflector, and room layout that counts guys, not so much the power supply.
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