Grow light numbers, including lumens, LUX, watts, photosynthetic active radiation (PAR), photosynthetic photon flux (PPF), photosynthetic photon flux density (PPFD), and photon efficiency, are all useful terms for lighting, but they are not equally weighted when it comes to their importance to growers and their grow lights.

Photosynthetic photon flux density is the grow light term that gives us the most information and the PPFD chart tells the story of how these numbers are significantly affected by distance to the canopy, and by movement. With the PPFD chart, look at the readouts (both for the stationary grow light and the light mover grow light), and that’s at 15 inches (38 cm) and at 20 inches (50 cm). This was charted for a double-ended 1,000W grow light. The PPFD chart demonstrates that when a grow light moves along the canopy and on a light mover, an even and predictable PPFD output can be realized. And the grow lights can be positioned closer to the canopy for more maximized grow light interaction. Grow lights can be placed even closer still, so as the canopy gets taller, there is no stress or burn because those grow lights are moving. Conversely, stationary grow lights are limited and there is no sweet spot.

To understand this on another level, stationary grow lights are always at a reduced strength because of the limitations of stationary grow light rules. It’s a paradox of having a beautifully strong, quality grow light that is forced to be positioned up high so that it is safely diluted, so as not to cause harm. Then, as a strategy to get some degree of grow light strength back, a grower is forced to place their grow lights in a tighter formation, resulting in more grow lights in use. Beyond the logical conclusion that this is wrong for cost and for practicality, it is also wrong for efficiency of electrical usage. With light movers, the grow lights can be positioned much closer and they can cover about 30 per cent more area — and that’s per grow light. That’s with each light mover consuming only about five to nine watts. In a time when electrical savings and energy efficiency are as important as yield numbers, moving grow lights is a way to have both variables significantly improved.

The PPFD chart tells the story of an even canopy, along with faster growth and yield increases due to the moving grow light. A side story is the effect of the optimized moving grow light that moves off-side slightly and then back intensely overhead, again and again. This ongoing action reduces shadow patterns and is how more of the leaves interact with the grow light. It mimics how plants have evolved over millions of years to best receive light. Plant receptors will open more when the light is intense but intermittent. So, the light mover’s effect on PPFD readouts is perhaps the most important grow light variable to know. It’s a story of better coverage and the effect goes even beyond the chart numbers. But, grow lights have their own input and there are many grow light numbers beyond PPFD to decipher.

Breaking Down Grow Light Numbers

The following breaks down some of the most common grow light numbers used, along with the ones to pay particular attention to.

Lumens and LUX: Lumens are actually an eye measurement, so these are numbers strictly from a human perspective. It revolves around a photopic bell curve where humans are sensitive more in the middle for green light and less sensitive on the ends for blue and red light. LUX is simply a measurement of lumens in feet as lumens/ft squared. Because lumens and LUX deal in human terms for light, they are somewhat weak in the far ends of the spectrum. Humans don’t need the far end colors to see well but plants absolutely use the reds/blues as the fuel for photosynthesis. Because of this, we need those far spectrum numbers represented. Lumens and LUX don’t give us grow light numbers at the plant’s perspective and because of this, they are not really useful terms for grow light output.

Watts: Watts is a term used for an overview only and is not used for accuracy. In other words, our general acquired knowledge might be to know which wattage is ideal for what we grow and at what stage.

PAR, PPF, PPFD: PAR is the wavelength of light in the range of 400-700 nanometers and this is the range for photosynthesis. For the acronym components, it is photosynthetic active radiation which is important in a conceptual way, but it is without true measurement because it lacks time and space. We can, however, dilute it with distance from the light source, or we can concentrate it by positioning the light source closer. That’s an example of Inverse Square Law which is also highlighted in the PPFD chart, with the PPFD numbers hugely diminished, and that’s geometrically diminished, with distance. Photosynthetic photon flux gives one more piece of information for PAR, which is time. We can see photon output per second. A quality grow light emits a continuous strong output, and PPF tells that story. The measurement is micromoles per second (μmol/s). The best grow light number to know, however, is PPFD because it incorporates space and time into the calculation. Basically, it is PPF plus micromoles per meter. It is the measurement of PPFD. Photosynthetic photon flux density readings must be done properly and include distance to grow light, horizontal coverage, and the number of readings taken over a period of time. For the PPFD chart, the distance and the horizontal coverage are listed. For other needed information, the meter took a reading every 30 seconds, so 120 readings per hour and every 30 minutes, calculated for the average, and that was per area.

Photon Efficiency: This is all about how efficient a grow light is in converting electricity into PAR. It’s important to know because we are not just thinking of electrical input but are now having the added information of realized output. For the math and knowing PPF (µmol/s) plus knowing wattage (Joules or J/s), the formula is µmol/J because the “s” for seconds cancels out on both sides of the equation. The higher this number, the more energy efficient a grow light is at converting electricity, which is input, into photons of PAR, which is output.

The two most important grow light numbers to know are PPFD and photon efficiency. A quality grow light will be outstanding for PPFD and should be noticeably better than most for photon efficiency. Then, placing that grow light on a light mover and getting it closer than would be recommended in a stationary setting is ideal for optimized results. This is how to best maximize PPFD with stronger/better coverage while creating better efficiency than with a stationary grow light. And, that’s by covering 30 percent more area as a moving grow light.

Grow lights can be powerfully effective while being energy efficient, and light movers are the single best way to maximize this. So, know a grow light’s PPFD, know the photon efficiency and, just as importantly, know the possibility of those grow light numbers when maximized on light movers.