Metrics Matter… Wait, What Are Metrics?
Trying to find the best grow lights for your application can be an overwhelming process. In fact, researching the topic might leave you with more questions than you started with. After all, not all of us are qualified engineers or plant physiologists, and lighting system specs can get very technical very quickly. One of the most challenging issues that shoppers face when researching horticultural lighting is learning to interpret and understand the many metrics that are used to measure and describe the light that systems emit (watts, lumens, PAR, PPFD, YPF, and the list goes on). Unfortunately, without a solid grasp on these, the shopping process can feel a bit like herding cats with your eyes closed. Therefore, the purpose of this article is to define and discuss the most common metrics used to describe grow light systems. We hope that this will improve your understanding of lighting specs and help you compare any light systems that you might be considering.
Types of Light: Visible Light vs. PAR
To understand the differences between the various metrics, it is first important to understand a little bit about light in general. This includes understanding the differences between the types of light that are more important for human sight and those that are more important for driving photosynthesis. It is important to remember that not all light is visible to the human eye and, further still, that light only makes up a small portion of a class of energy known as electromagnetic radiation, which also includes x-rays, microwaves, and even radio waves.
Different types of electromagnetic radiation are defined by their wavelengths and frequencies, which are expressed in Hertz and meters, respectively. Most x-rays, for example, have wavelengths of between 0.01 and 10 nm. In contrast, the wavelengths that are visible to the human eye (i.e., visible light, or the visible spectrum) range from 400 to 700 nm, and electromagnetic radiation that falls outside of this range is imperceptible, at least to humans. However, wavelengths outside of the human range are still visible to many other animals. For example, UV (10-400 nm) is visible to many fish and insects, and infrared (700-1000 nm) is visible to many snakes. More important, though, is that these types of light are also detected and used by plants. In fact, plants are able to sense wavelengths as low as short as 260 nm (UV-C) and as long as 730 nm (far-red).
That being said, most of the light used by plants for photosynthesis, which is known as photosynthetically active radiation (or PAR), falls within the visible spectrum (400-700 nm). However, plants are much more sensitive to red (640-680 nm) light than to other wavelengths, whereas the human eye is more sensitive to green and yellow. This is important because grow lights that appear bright to the human eye might be much less useful to plants than you might expect, especially if yellow light is over-represented, and grow lights can be more effective than they appear if the systems produce disproportionately more red light.
Lumens are for Humans
Now that we understand a little about the types of light, we can talk about how to measure it. For human applications, like lighting homes or workplaces, the intensity of visible light can be gauged by measuring radiant flux (or power), which is calculated as the sum of visible light (in Watts, or J/s). This metric, however, can be misleading because systems that fail to produce the full range of visible wavelengths can still produce high radiant flux values. For example, a lighting system that produces high levels of violet and red light might have a high overall radiant flux output, but since the human eye is more sensitive to yellow and green light, the light wouldn’t seem as bright as the radiant flux measurement suggested.
Instead, measurements of luminous flux are much more meaningful. This metric, which is expressed in lumens (lm), is similar to radiant flux but is weighted according to the sensitivity of the human eye to different wavelengths of light. Accordingly, light sources with higher lumen ratings are perceived as brighter. However, because the human eye is most sensitive to green/yellow light (550 nm), the metric is heavily biased toward these wavelengths. This means that measures of luminous flux underrepresent red and blue wavelengths and, consequently, that luminous flux is a poor indicator of the usefulness of light sources to plants, which mainly rely on red light for photosynthesis. For this reason, measures of luminous flux and lumen ratings are inadequate for assessing and comparing grow lights. Beware that lux, foot candles, and candelas are all additional measures of luminous flux. These units of measurement are based off the lumen (lm) and are equally inadequate.
Photons are for Plants
So how should measure the usefulness of light to plants?
Interestingly, the idea that plants and humans use different wavelengths of light isn’t a new one. In fact, quite a few metrics have been developed for specifically measuring PAR (photosynthetically active radiation). However, confusion regarding the assessment and comparison of grow lights has a similarly extensive history. To make sure you aren’t fooled by meaningless specs, the first thing that you want to make sure you understand is that PAR isn’t a metric in itself. Rather, it is the name given to the range of light that drives photosynthesis (400-700 nm). In other words, PAR is the plant version of radiant flux (the total sum of visible light), and the only real difference between the two is in the way that they are measured (energy vs. photons). Two of the main metrics that are based on photon flux are PPF and PPFD.
Photosynthetic Photon Flux
PPF (photosynthetic photon flux), or light output, for example, is a measure of the total PAR produced by a light source per second. In contrast to radiant flux, though, which is expressed in terms of energy (Watts or J/s), PPF is expressed in terms of photons, which are the basic unit of electromagnetic energy. Furthermore, since the number of photons that are counted is usually on the order of quadrillions and quintillions, the number of photons is typically expressed in micromoles (μmol), with each μmol representing roughly 6.02 × 1017 (602 quadrillion) photons. Therefore, since PPF is a measure of PAR produced per second, the metric is usually reported in μmol/s. However, PPF is measured at the light source, and although this means that the measure can be standardized (0 distance from
However, because PPF is measured at the light source, the metric doesn’t accurately represent the amount of light that actually reaches the leaves of plants or the distribution of the light after its emission.
Photosynthetic Photon Flux Density
In contrast, PPFD (photosynthetic photon flux density), or light intensity, is a measure of PPF that reaches a specific area (m2) of a given surface. It is expressed in μmol/m2/s. Because PPFD only considers the light that reaches plants, it is generally considered a better metric than PPF, and the metric is currently one of the best ways to measure and compare light intensities.
Unfortunately, PPFD is still far from perfect. For example, the metric gives equal weight to any photons that fall within the 400–700 nm range, even though red light is more important for driving photosynthesis. The metric also ignores both UV and infrared light, even though a large number of studies have shown that UV light stimulates the production of secondary metabolites – like pigments, flavonoids, and THC – and infrared light plays important roles in driving circadian rhythms.
Furthermore, PPFD is also easy to manipulate and exaggerate. One way that the metric can be exaggerated is by reducing the distance between the light source and point of measurement. Because light intensity is inversely proportional to the square of the distance it travels (inverse-square law), the PPFD measured at a surface closer to the light source will be greater than that measured at a surface that is further away. Taking advantage of this, some manufacturers report close-range PPFD values, even though the distances are unrealistic, either because of heat generated by the lights or other factors. In addition, since the area under a lighting system isn’t necessarily illuminated evenly (in other words, some spots receive more light than others), PPFD can also be exaggerated by taking measurements from points that receive more light and failing to account for areas that receive less.
For these reasons, even though PPFD is currently the best metric out there, it should be interpreted with caution. Manufacturers should report the distance at which PPFD values are measured and should also describe the distribution of light output, either by reporting the average PPFD from multiple sample points or by reporting the ratio of the minimum to maximum PPFD measured within a specified area, or footprint. If manufacturers fail to provide this information, we recommend that you contact them to see if it is available. Without this information, you will be unable to accurately assess the manufacturer’s product(s).
What about YPF?
Admittedly, PPFD is analogous to measurements of power/radiant flux in that it only considers the total sum of PAR and ignores the actual wavelengths that are represented. Ideally, PPFD would be weighted by the effectiveness of individual wavelengths in driving photosynthesis. In fact, this is the exact principle embodied by YPF (yield photon flux), or quantum yield. This metric takes PPFD one step further by weighting the photosynthetic response of plants to photons with different wavelengths (action spectrum) according to the effectiveness of plants in absorbing those photons. Therefore, the calculation of YPF is analogous to the calculation of luminous flux, which weights the visible spectrum according to the sensitivity of the human eye. This idea was first explored in the 1970’s by Dr. Keith McCree, who concluded that plants absorb relatively less green light than other wavelengths and that, as a result, photosynthesis is mainly driven by blue and red light.
However, this conclusion is not exactly correct. Because McCree analyzed the absorbance of light by individual leaves, YPF fails to represent the photosynthetic response of whole plants. More recent studies have shown that the green light that passes through plants’ upper leaves is often absorbed by lower “shaded” leaves. Therefore, YPF underestimates the absorbance of green light and underestimates its importance for driving photosynthesis. Accordingly, the uncorrected action spectrum (PPFD) is probably a better indicator of the whole plant photosynthetic response.
There are some other issues too. Another major problem with YPF is that different plant species utilize light differently. Therefore, it is unlikely that the photosynthetic response of the plant species analyzed by McCree accurately represents all 352,000 other plant species. This is especially true since the plants that McCree investigated were all crop or agricultural weed species. McCree’s study also involved short-term measurements, which may have skewed his results. Again, more recent studies have painted a different picture. These long-term studies have shown that the quantity of light is more important for plant growth than quality. So, even though McCree’s study was a great contribution to our understanding of photosynthesis and plant physiology, especially when first published in 1972, both the study’s results and subsequent interpretation of those results are fairly flawed and outdated.
YPF also fails to overcome the other weaknesses of PPF and PPFD, in that the measure can be manipulated or exaggerated. Accordingly, the distance over which the metric is measured and the average PPFD or min/max ratio are needed to accurately assess and compare different systems. Therefore, even though the concept of YPF is an ideal solution for measuring light quality, our current understanding of photosynthesis and the difference of photosynthesis between species is insufficient.