Light Spectrum vs µmols — Why Choosing an LED Grow Light is About More Than High Numbers.

iLogic8 Pushing Past Par

Lies, damn lies, and statistics. If you’re in the market for a new LED grow light in 2021, we encourage you to remain mindful of this time-honored phrase throughout the entire process of choosing a new LED fixture.

With so many new lighting companies and novel horticultural LED lighting fixtures vying for our attention, it’s no wonder that some companies’ marketing departments are trying to distil everything down to a single number!

Moving Beyond Photon Efficiency.

The number we’re talking about is called PPE—or Photosynthetic Photon Efficacy. If you’re new to the world of LED grow lights then acronyms like this may sound impressively scientific upon first hearing.

Certainly, Photosynthetic Photon Efficacy has established itself as the cherished metric of “light efficiency” in the horticultural industry for several years. But all this is set to change.

Delve a little deeper and we discover that Photon Efficiency is measured in µmols per Joule (J).

Now, for some consumers, the inclusion of special Greek symbols and the mention of “Joules” is all it takes to get that “freshly-laundered lab-coat “ feeling like we’re back in science class.

It’s not positive memories for everyone, however—for some of us even a small dose of “science talk” can quickly turn into the voice of Charlie Brown’s teacher…

“Wah-wah-wah micromoles wah-wah-wah, photon efficacy…”

However—if you’re seriously asking yourself “What is the best LED grow light?” then you need to go a lot further than merely comparing Photon Efficiency numbers.

Unpacking Photon Efficiency.

First, let’s make sure we understand what Photon Efficiency is: the total amount of light generated by a fixture (PPF—or Photosynthetic Photon Flux—measured in µmols per second divided by the number of watts required to generate that light.

So, if an LED fixture generates 2,300 µmols per second using 1000W of electrical power, that’s a Photon Efficiency of 2.3 µmols/J. (One Joule is equivalent to one watt of power per second.)

It’s no secret that LED grow lights are generally more efficient than High Pressure Sodium, Metal Halide, and T5 Fluorescents—so we forgive you if it seems perfectly logical and natural to compare the Photon Efficiency numbers of various fixtures (perhaps with different wattages) in an effort to assess the market.

One LED fixture claims 2.3 µmols/J. Next month a competitor announces their new model with 2.5 µmols/J. Then just a few weeks so by and the highest claimed Photon Efficiencies have risen further: 2.7 …. 2.8 …. 2.9….with each increment, the implication is that technology has marched on a few steps.

The higher the number, the more efficient the fixture is at transforming electrical power (Joules) into plant-usable light (µmols).

Not All Photons Are Equal.

Notwithstanding some less scrupulous brands’ tendencies to “vertically massage” their Photon Efficiency numbers, the important thing to be aware of right off the bat is that these “µmols” are just a “blind count” of photons. Full spectrum light is made up of all the colors of the rainbow: from high-energy blue photons, medium-energy green photons, down to low-energy red photons.

All are counted equally when it comes to Photosynthetic Photon Flux, Photon Efficiency, and their associated µmols.

What does this mean in practical terms? Well, let’s take a high-end greenhouse LED for example. This particular breed of LED lighting fixture is designed to supplement low level background solar radiation and top-of-the-line models can boast Photon Efficiencies of 3.5 µmols/J or even higher! They achieve this high Photon Efficiency number by using a majority of super high-efficiency, monochromatic red diodes as opposed to lower efficiency full spectrum diodes common in fixtures used in indoor farms.

This is all great news for a Dutch greenhouse tomato grower. The red-rich spectrum of a greenhouse LED fixture provides an ideal complement to low natural light levels during the winter months.

However, if an indoor grower attempts to cultivate light-loving plants under this light spectrum in a sole light source setting, the results will probably not be that great at all: leggy growth habits, leaf and flower bleaching, and perhaps even necrosis and mold. Not a good look at any time of the year!

As Light Intensity (PPFD) Increases, Light Spectrum Plays a Bigger Role.

Commercial indoor farmers understand that it’s not just about the number of photons they can deliver to their plants, but the quality.

Plants have evolved under continuous full spectrum sunlight—that means they’ve gotten used to enjoying all of the colors, all of the time. What’s more, the ratios of blue to red light, as well as red to far-red light, are also crucial for healthy plant growth and development.

This is why full-spectrum, phosphor coated diodes are a mainstay of LED grow lights designed for grow rooms and indoor farms.

Japanese researcher, Katsumi Inada, discovered that as light intensity increases, plants become more “choosy” when it comes to spectral quality. When light levels are low, plants are like a hungry man at a buffet—they’ll take whatever they can! However, as light intensity increases, in order to achieve a proportional increase in photosynthesis (photosynthetic yield), the light spectrum has to be right.

It’s no good just bombarding plants with the same color of photons or even a reduced range.

The Emerson Effect.

Another key scientist, Robert Emerson, pioneered important research that laid the basis of our modern understanding of photosynthesis.

He discovered that the photosynthetic yield from red and blue light was significantly greater than the sum of red light only and blue light only. In other words, the photosynthesis generated from both colors of light was more than the sum of its parts.

Since Emerson’s work, there have been significant further discoveries. We now know that plants use UV and far-red light for photosynthesis and other critical metabolic processes.

At ILUMINAR Lighting we offer the iLogic™ LED series featuring a cutting edge technology incorporating both UV and Far-Red diodes for next level results. Click here to learn more.

It All Comes Back to PAR.

Photosynthetic Photon Flux (total light output), PPFD (incident light intensity), and Photon efficiency (Photosynthetic Photon Flux divided by Joules) all share one big problem.

They are based on an outdated definition of what constitutes “plant usable light” in the first place. PAR (photosynthetically active radiation) is limited to photons with wavelengths between 400 and 700 nanometers.

However, both UV and far-red fall outside of the traditional PAR range. (PAR was defined before we understood that UV-A and far-red both contribute to photosynthesis when supplied in conjunction with full spectrum PAR light.

Photosynthetic Photon Flux, PPFD and Photon Efficiency are all based on this definition of PAR—so when lighting scientists Zhen and Bugbee proposed extending the definition of PAR to include photons between 380 and 760 nanometers, it fundamentally moved the goal posts for measuring plant-usable light output, intensity and—by extension—fixture efficacy.

And yet many fixture manufacturers are still wedded to these old terms, continuing to boast high Photon Efficiency numbers as if they were the be-all, end-all metric of grow light.

And There’s More…

New metrics (sorry, more acronyms!) are emerging: ePAR, BPF, BPFD and BPE to replace PAR, PPF, PPFD and PPE respectively. (The “B” stands for “biologically active”.) ePAR defines the new range of photons and BPF, BPFD and BPE all respect this. So, if a lighting manufacturer is using these terms, that’s an encouraging sign.

Other important things to consider when choosing an LED grow light include the recommended canopy distance, dimmability, controller compatibility, diode L90 (working life span), diode variety and layout, water resistance, form factor, as well as driver and heat sink quality.

Suffice to say, if an LED grow light reseller or manufacturer is crowing on about Photon Efficiency and little else, you should now know enough to at least ask some pertinent questions, or—better yet—find a company who clearly presents all the relevant stats you need to make an informed choice.

More questions about this topic? Feel free to drop us an email at - we love to chat about these new discoveries.