In another information article on this site we discuss lumens and how they affect plant growth, but you should be aware that a lot of scientists would argue it’s not the lumens that affect plant growth at all – it’s the PAR (photosynthetically active radiation). Understanding the difference between the two different methods of measurement may seem like nit-picking, but it’s not really. If you’ve read up on this topic you might be forgiven for thinking that they’re one and the same thing at first glance but, like any other information, it takes a while to process.
There really are a lot of terms that you need to get familiar with if you’re going to start a successful grow operation of any size, but taking the extra time now to figure all this out will pay dividends for you in the future. We’re going to take a closer look at exactly what the difference is between PAR and lumens and how this might affect your grow operation.
So What Is a PAR?
Essentially a PAR is the amount of light that is actually available to be used during the photosynthesis process. There may be more light emitted from a particular source, but your plants don’t make use of all of it – some of it has no real effect on the photosynthesis process. It might be best to explain this with an example. For the human eye a bright white or yellow light bulb seems to illuminate the room most effectively, whereas blue or red light appears to our eyes to be very dim and so for us we prefer the brighter bulb to light up a room.
The lumen is a measure of brightness or how much light is actually emitted from any particular light source at a given time. Unfortunately, plants don’t really use that bright yellow light for photosynthesis, and while your traditional light bulb may be high in lumens it won’t necessarily be very high on the PAR scale. Plants experience the best growth when exposed to both blue and red lights and this type of light, while still visible to the human eye, is better measured in terms of PAR’s than lumens. As part of the visible color spectrum blue and red light is found between 400nm and 700nm and as you approach 700nm the PAR exposure increases. As a plant grows from a seed to a mature plant its need for light shifts from the blue to the red end of the spectrum, and you can measure how much exposure to light it’s getting in nanometers by using special instrumentation – the closer the light gets to 700nm the higher will be the PAR reading. As long as you keep your plants exposure between that 400nm and 700nm range at the appropriate levels for each stage of the plant’s growth cycle, you’ll end up with healthy and productive plants.