What Indoor Growers Need To Know About Radiant Intensity


January 25, 2021

This article originally appeared in Garden Culture Magazine UK32 and US30.

As recently as a decade or so ago, indoor growers changed the language they used to describe light intensity. Lux (lumens per square meter) was out; PPFD (micromoles per square meter per second) was in. Accordingly, you may have heard folks say stuff like:

“Lumens are for humans!”


“Lux sucks!”

And if you’ve ever looked into why, you’ll know that lumens are based on the standard luminosity function of the human eye’s sensitivity to light. This sensitivity peaks in the green zone of the visual range at around 555 nanometers. However, as beneficial as our human evolutionary capacity to see plant foliage is, it has nothing to do with photosynthesis or light intensity from a plant’s perspective.


Perhaps sensing the shortcomings of lumens and lux, growers began to attach themselves to the concept of PAR (photosynthetically active radiation). They started measuring PPFD (photosynthetic photon flux density) in micromoles per meter squared per second (µmol m-2 s-1) using handy devices known as PAR meters (aka Quantum meters). These devices use a tiny silicon photodiode (think “reversed LED” as it takes light as its input energy and transforms this into electrical current) encased within a translucent, acrylic cap to estimate the aggregate light intensity over a square meter. The more light that hits the sensor, the greater the electrical current, and the higher the PPFD shown on the screen. The acrylic cap diffuses incoming light, although light arriving from directly above most stimulates the sensor.

In one sense, growers have moved from an energetic paradigm (lumens are based on the Candela, the base unit of luminous intensity, which is defined using watts) to a quantitative approach where photons are counted as quantifiable elements. PPFD, then, refers to how many photons are arriving on a hypothetical flat square meter surface during one second. Each photon (in the PAR range—between 400 and 700 nanometres) is counted as equal rather than weighted according to the standard human eye, luminosity function, or anything else.

What’s Next?

So, where does that leave us? Well, it’s easy to believe that we’ve automatically upgraded our level of scientific rigor by moving from plain vanilla “Lux” to the far more exotic and technical sounding “µmol m-2 s-1”. (Ooooh! Aaaaah!) You might even believe that PAR, PPF (total output), and PPFD represent the apex of precision when it comes to describing light intensity for plants. Maybe it’s the slightly esoteric looking “µ” symbol in “µmol” or the tiny exponential values in superscript that imbues us with that fuzzy, freshly-laundered lab coat feeling. Still, before we start swaggering around industry tradeshows waving our PAR meters around, we might want to allow ourselves a moment of sober reflection, so we don’t become too confident about these new metrics.

Okay, okay, I’ll cut to the chase. It seems many of us are mistaking our PAR meters for “photosynthesis prediction devices”. These days, indoor growers in the market for a new grow light are bombarded with various, colorful “PPFD maps” showing light intensity distributed across a two-dimension flat plane. Great if you’re growing a plate of algae—but not so much for a 3’ tall three-dimensional, light-loving plant! These PPFD maps are usually presented as illustrative evidence of the output and uniformity of a given lighting fixture. Some lighting manufacturers go further and compare the efficiency of disparate light sources using “µmol per Joule”. In other words, how many photons does my light source produce for each Joule of input electrical energy? The result is a bunch of very convincing pseudo-science leading to the inevitable comparative and competitive glances over the urinal:

Grower 1: “My new generation 315W CMH produces 1.93 µmol per Joule!”

Grower 2: “Not bad … but my 1000W DE-HPS achieves just over 2 µmol per Joule!”

Grower 1: “Yeah—but my CMH has more blue…”

Grower 3: “Both of you shut up! My DIY Quantum Board with Samsung [insert long, obtuse SKU] diodes is pushing 3 µmol per Joule! Therefore, I am the victor! Kneel before me, Edison globe luddites!”

There are countless variants of this conversation on every growers’ forum out there showing, among other things, how easy it is to get drawn into the polemic, with growers arguing from the camp of their favored lighting technology in an endless battle of contradictory anecdotes. However, these exchanges are a distraction from a far more fundamental logical fallacy that undermines and invalidates the whole conversation. This, ladies and gentlemen, is what I call the “PPFD trap”.

First, the fallacy:

Premise 1 (TRUE): IF my PAR meter measures photons traveling between 400 and 700 nanometres and…

Premise 2 (TRUE): IF my PLANTS use photons traveling between 400 and 700 nanometres for photosynthesis then…

Conclusion (FALSE): My PAR meter predicts the rate of photosynthesis for my PLANTS.

Tempting, isn’t it? While both premises may be factually correct, the devilish detail lies in how we sleepwalk to the false conclusion. (The fallacy at hand, by the way, is known as “affirming the consequent”.) So, where does this deceptively simple-sounding logic fall, exactly? For one thing, you probably already know that photosynthesis is dependent on several other key variables, including leaf temperature, air temperature, cellular moisture content, atmospheric carbon dioxide, spectral quality, and relative humidity. But even when we consciously disregard these additional factors, there remains a huge elephant loitering in our grow rooms.

Plants, Light, and Photosynthesis

To start patting down this elephant’s trunk (you keep telling yourself it’s a trunk), we need to step away from our PAR meters, just for a moment, and reconsider the fundamental, core reality of plants, light, and photosynthesis. The two most important things to consider are, 1) the photosynthetic characteristics of our chosen plant species, and, 2) the energy source they have evolved over millions of years to exploit: the sun. Not all plants use the sun in the same way. Some species have evolved to brave the intensely lit and atmospherically harsh conditions of elevated, sub-tropical, mountainous terrains. Others are happier on the barely illuminated (but more forgiving) floors of temperate rain forests. Different plant species evolve specific adaptions enabling them to exploit all that’s on offer within their natural environment—that’s why it’s so important to consider the native habitat of your chosen species.

Getting back to the light, we perceive the sun as a single, intensely bright yellowish-white blob in the sky, 93 million miles away from the Earth.  As such, the vast majority of photons that reach our planet travel in a similar direction, along a similar path. Of course, some photons bounce off clouds, refract through water vapour, or bounce from leaf to leaf within a dense forest canopy to create a measure of diffusion. As an analogy for direct light, think of a garden hose emitting a powerful “soaker” style jet of water and think of diffuse light as the same hose with the nozzle set to the “mister” setting. For argument’s sake, the volume of water in both cases is the same. And yes, both will get you wet! But the fact remains, light-loving plants have evolved to exploit the “soaker” (direct light) not the “mister” (diffuse light). Unfortunately, your PAR meter, by design, does not “see” the difference.

Science has only recently started to explore the difference between direct and diffuse light on plants. Craig Brodersen, assistant professor at Yale’s school of forestry and environmental studies, performed experiments on leaves using both direct and diffuse light and his results showed that direct light penetrates deeper into the leaf tissue—especially green light.

The images on the left show the penetration of the light into the leaf surface from direct light sources — the pictures on the right are from diffuse light sources. It’s important to note that both the direct and diffuse light sources gave the same reading on a PAR meter—but here we can see clear evidence that leaves do not process direct and diffuse light in the same way. (We also see how green light penetrates deepest, but that’s a subject for another time!)

If you choose to grow a plant that has evolved to cope with very high levels of solar intensity, then you must first acknowledge the fact that it must have developed specific mechanisms to deal with (and thrive in) this environment. A plant species native to elevated, subtropical habitats, for example, perched several thousand feet above sea level in places like Nepal, India, or Afghanistan, needs to withstand both very high levels of PAR and increased ultraviolet radiation. From a plant’s perspective, the sun rises every day in the east, moves up towards its apex in the sky, and then heads west for sunset. Throughout the day, the sun sends its intense, angular rays towards the plant. As such, the whole plant benefits from being “soaker hosed” with light, drenching it from the side, the top, and then the other side. The incident sunlight has very high radiant intensity capable of penetrating deep into the plant, striking some leaves directly and passing between others, depending on the angle or time of day.

The fascinating characteristic of light-loving plants, however, is found at the microscopic level. Here, deep inside the chloroplast, you will find tall “stacks” of light-harvesting compartments called “thylakoids”. These thylakoids are ubiquitous throughout the world of higher plants (as well as algae and bacteria) as the site of photosynthesis itself. But it’s worth noting that shade-loving species (e.g. ferns) develop short, stubby little stacks of thylakoids because the light they receive is low intensity and diffuse.  The light that ferns capture doesn’t have much “punch” and neither does it need to as exploiting low light is encoded into the fern plant’s DNA, and therefore, reflected in its physiology.

The tall stacks of thylakoids characteristic of light-loving plant species (as well as a double-palisade layer) on the other hand are arguably a means of squeezing every last joule of incident energy from the intense solar beams they’ve evolved to exploit. Often, the stacks of thylakoids (known as “grana”) are positioned so that the top of the stack is closest to the adaxial plane of the leaf. To understand why you need to appreciate that there’s a finite limit to how much photosynthetic yield a single thylakoid can generate. So, if a thylakoid at the top of the stack, nearest to the leaf’s adaxial surface, is “maxed out” with incoming photons, these otherwise “surplus” photons can transmit through and be processed further down the stack. Tall stacks of thylakoids are light-loving plants’ evolutionary mechanism for dealing with lots of direct, intense sunlight. That’s what they do better than other lesser adapted plant species, and that’s why they exist in the first place. The phenomena of “positive phototropism” (where a light-loving plant dynamically orientates its leaves, so they receive as much direct light as possible) shows us the lengths plants will go to grab their essential energy payload.

Quality of Light

In an indoor growing environment where our plants are reliant on grow lights for every photon, it’s essential to give our plants not just the measured quantity but also the character of light they want. The first thing most growers think of when considering light quality is spectrum / spectral distribution, but that’s not the point I’m trying to make here. 700+ µmol measured 18 – 24 inches beneath a 630W DE-CMH lamp in an open, double-parabolic reflector is qualitatively very different from 700+ µmol measured 15 – 20 inches beneath a 600W multiarray LED, or four or six feet below a bunch of 1000W DE-HPS lamps in greenhouse style, deep-dish reflectors in a compound lighting plan. The key difference is found in the paths those photons travel along relative to each other to reach the leaf. Multi-array un-lensed LED grow lights spread their photons out across hundreds of diodes—each diode representing just a fraction of the overall intensity of the lighting fixture, hence the total output is predominantly diffuse. A bunch of 1000W DE-HPS lamps bolted to a warehouse ceiling may well overlap footprints in a compound lighting plan but, once again, the incident photons measured at any given point have arrived from many different sources, and the resulting light is more diffuse).

The PPFD Trap

So, while the PPFD readings on our PAR meters can give us useful data in terms of grow light positioning, they tell us nothing about the angular quality of the light we’re measuring. A flat photodiode sensor in a consumer-grade PAR meter behaves in a very different way to a three-dimension leaf or a three-dimensional plant. As such, it doesn’t “care” whether the 700+ µmol were extrapolated from a highly collimated, beam-like light source or a diffuse light arriving from multiple fixtures at various angles. Going back to our garden hose analogy, PPFD tells us nothing about whether the incident “water” has been delivered as a fine mist distributed from many different tiny nozzles or a girthy firemen’s hose emitting a powerful soaking stream!

Remember, light-loving plants have evolved to exploit the ultimate soaker hose—the sun! If you’re not giving them the quality of light they crave, they won’t express their genetic potential to the full, as this can only be “unlocked” with the quality of light they want. Regrettably, all this fine detail is lost in those reassuringly simple and colorful PPFD charts.

The “radiant intensity” of a light source (the correct radiometric term we use to describe these angular qualities) has a significant bearing on a fixture’s ability to penetrate plants at both a micro and macro level. As already described, at the micro-level, high radiant intensity “soaker hose light” will cause thylakoids deeper inside the plant to photosynthesize. More diffuse light lacks this penetrative power. At the macro level, high radiant intensity delivers many photons along narrow-angle ranges to penetrate past the canopy. This is the power needed to stimulate second and third-tier flower sites beneath the canopy, leading to more homogenous crop quality.



Radiant intensity is concerned with photons emanating from a light source within a specific angular range. As radiant intensity is a three-dimension phenomenon, it needs to be defined in terms of solid angles—the Steradian. From the light source’s point of view, radiant intensity is about limiting our count to photons traveling at certain angles from the light source. From a plant’s perspective, the ability of light to penetrate (at both the micro and macro levels described) is dependent on the range of incident angles in its photonic diet. In other words, are the photons arriving from a wide range of angles at any given moment, or do they conform to a tighter angular range, like the sun?

Many growers and lighting manufacturers appear to be over-simplifying the complex relationship between plants and their light source into PPFD—as measured by a PAR meter—and spectrum—as measured by a spectroradiometer. Even given the exact same spectra, 700+ µmol of photons arriving from many different spatial origins (i.e. LED diodes or several greenhouse-style DE-HPS fixtures bolted to a warehouse ceiling) is very different to 700+ µmol measured underneath a single HID arc-tube positioned at close proximity. While your PAR meter may read the same value in either situation, your plants will experience and react to the two different light sources very differently.

In conclusion, light with high radiant intensity has penetration power. It can penetrate the canopy itself and also into the leaves and flowers themselves. Light with high radiant intensity can be achieved with HID lighting (HPS, DE-HPS, MH, CMH, DE-MH, DE-CMH) in a well-designed reflector (i.e a reflector that facilitates close placement to the canopy rather than one that runs too hot to be under four feet away from the canopy.) It can also be realized with high power LED fixtures equipped with secondary lensing or a COB style unit when many diodes are packed into a single hemispherical lens. Note how this discussion isn’t centered around one lighting technology being “better” than the other. It’s about identifying and understanding the characteristics of your light source that matter and growing the right kind of plants in a style that suits it! Conversely, you could choose the right light source for growing your historically favored plants!

For more information and explanation, please check out my videos below over at Just4Growers on YouTube.

Video references:



Journal reference:

A new paradigm in leaf-level photosynthesis: direct and diffuse lights are not equal
Plant, Cell and Environment
Craig R. Brodersen, Thomas C. Vogelmann, William E. Williams & Holly L. Gorton

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