Grow lights for indoor hydroponics: understanding the science behind the light

Lighting is an indispensable element for growing plants using an indoor hydroponics system, because most indoor locations do not receive sufficient and consistent amounts of light for healthy plant growth. The mechanism behind how plants absorb and utilize light, however, is very complex and often misunderstood. This 3-part guide will help you understand, compare and analyze LED grow lights by diving into the basic mechanism behind how plants use light as well as the fundamental physics and science involved in light spectra and grow light design.

PART I: Understanding the science behind the light

What is light? What is color?

Visible light is type of electromagnetic radiation (energy) that has a wavelength between 380 nm (nanometers) and 780 nm. Within this range of wavelengths, each wavelength corresponds to a particular color that the human vision system perceives. For example, as you can see in the chart below, energy in the wavelength range between 570 nm to 590 nm corresponds to the sensation of "yellow" in our visual system. Color is a socially constructed concept but is roughly synonymous with wavelength for purposes of discussing LED grow light spectra, and can be more convenient than providing a specific numeric wavelength range. Do note that wavelength is much more specific and scientifically accurate when discussing photosynthesis and plant growth, however.

You will likely recall that light is a form of energy that is both a wave and a particle. For grow lights, the "wave" aspect of light becomes relevant when discussing color or wavelength, and the "particle" aspect is relevant when discussing light intensity.

Some light sources like lasers are called "monochromatic" and emit only in a very narrow wavelength range, but most light fixtures are made up of a varying mixture of wavelengths. White light, for example, can be made by an approximately equal mixture of all wavelengths across the entire visible spectrum.

Not all light is equal, and there is more than meets the eye

You may be tempted to conclude that two light sources that appear to be the same color and equally bright are, in fact, the same. But are they?

We mentioned above that light is made up of a combination of different colors, or wavelengths, and it is the various mixtures and distribution of this energy across the spectrum of wavelengths that creates the sensation of different colors in humans. You will likely recall from your schooling days the acronym ROYGBIV to describe the various colors of the rainbow. Indeed, these are the colors that are present in natural sunlight, which, when combined, create the perception of white. But, due to the way in which human vision works (metamerism), we can actually recreate white light without including all of the colors of the rainbow, or scientifically speaking, all of the wavelengths across the electromagnetic spectrum.

Virtually all forms of artificial white lighting take advantage of this fact and do not include all of the wavelengths that are present in natural sunlight. Residential & commercial LED lights, for example, recreate the sensation of white using a combination of just [blue + yellow], or [blue + green + red] channels. Lighting manufacturers must relentlessly pursue efficiency increases and cost reductions so that they can meet climate change goals and energy saving initiatives, and the easiest way to do this is to recreate "white" using as few colors as possible, and cranking up the intensity at only these wavelengths to make it appear brighter. In other words, it doesn't really matter to them how they get there, as long as they can answer the two questions: is it white, and is it bright?

Cree LED bulb packaging showing 800 lumens (brightness) and color (warm white). No mention of how they got there!

But, here's the key: humans use light to see, and plants use light to undergo photosynthesis. Whiteness and brightness as humans perceive it, is an entirely different mechanism from plant photosynthesis. So although a light source might appear white and bright, this does not tell us with any certainty as to whether it is an effective grow light. In order to be able to differentiate this, we must look at a light source's SPD, or Spectral Power Distribution.

How to read and understand an LED grow light's spectral power distribution

When evaluating a light source such as an LED grow light, light meters and measurement devices can measure the amount of energy present at each wavelength, and create a spectral power distribution plot. In almost all cases, the x-axis has wavelength plotted in increasing wavelength (usually 380 nm to 780 nm), while the y-axis indicates energy intensity at that wavelength, and is typically measured in watts/nm or in arbitrary units.

An SPD chart shows us how much light energy is present at each wavelength (color) and this is critical in understanding the true nature of a light source, regardless of how it appears to the human eye. The two following SPD charts, for example, are of natural daylight and an LED, respectively. They are both cool white (6500K) and appear to be the same color to the naked eye. But if you look closely, you will see the wavelength makeup is drastically different.

An SPD chart for sunlight (6500K) showing the distribution of energy across various wavelengths.

The SPD chart for a typical white LED (also 6500K).

A light source's SPD is like a person's DNA - it is the essential data upon which almost all photometric calculations are based on. With an SPD chart, we can immediately tell if energy of certain wavelengths are missing or over-represented, and we can make both quantitative and qualitative judgments from the SPD data, especially when they would otherwise appear to be the same or similar to the naked eye.

Relative vs absolute irradiance

With SPD charts, it is important that you pay attention to the y-axis when making comparisons. Most SPD charts will be normalized such that the maximum peak will be equal to 1.0. This can be misleading, particularly with "spiky" SPDs.

For example, below is an example spectrum of a fluorescent bulb at 3000K, normalized to 1.0. You will see that the yellow peak is scaled to match the maximum of the chart. From the looks of it, it is deficient in many parts of the spectrum - blue, green, red - and has a b yellow and green component. There is a lot of white in the chart - the chart background color - showing that there is very little light output.

Below is the exact same spectrum, but scaled differently. This time, we've scaled it so that the yellow peak is equal to 12.0. This time, there's quite a bit less white, and a bit better coverage. Or is there? It's the same spectrum but we just changed around the axes.

Be aware of what the axes mean and how this could lead to misleading results. Some manufacturers may attempt to make their results look better than they are by manipulating their results like this.

PART II: The science behind how plants use light

Why do plants need light?

Plants generate their energy through a process called photosynthesis. Pigments in leaves called chlorophyll absorb photons (light particles) and converts carbon dioxide and water into sugar (C6H12O6) and oxygen. Without enough of the right kind of light shining on plant leaves, it will not be able to grow, let alone survive.

As you will see below, this is a crucial difference from how humans are accustomed to interacting with light - we primarily use and evaluate light in the terms of our vision system and how it helps us see, rather than as an energy source.

In short, plants need light to perform photosynthesis.

What wavelengths of light do plants need?

The most studied and understood pigments that aid in photosynthesis are chlorophyll A and chlorophyll B. The charts below show how well they absorb light of various wavelengths which in turn contribute to photosynthesis. You can consider this to be a chart explaining how efficiently a plant is capable of converting different wavelengths into energy.

You will immediately notice that chlorophyll absorbs light best in the 400 - 500 nm range (violet - blue) and 600 - 700 nm (red) range. Many of the first generation grow lights use this fact to their advantage and integrate a blue LED and a red LED, which peak at 450 nm and 650 nm, respectively, to attempt to match the chlorophyll absorption curves. These are your classic LED grow lights with the purple appearance.

This is very much consistent with our classical understanding of plants and photosynthesis and our intuitive view of leaf color. When we see a green leaf, it is the green light that is being reflected - i.e. bounced off and not absorbed for photosynthesis, while the red and blue wavelengths are absorbed.

Continued research into plant physiology and photosynthetic mechanism has revealed, however, that chlorophyll is not the only pigment that plays a role in capturing photons in a plant and assisting with photosynthesis. Scientists have only just recently begun researching these secondary pigments and the extent to which they influence photosynthetic efficiency, and there has yet to be any decisive evidence or conclusions drawn. Furthermore, different species have different types of pigments present in their leaves, which means that even conclusive research will be limited in scope to being applicable to only several species. In other words, there remains a lot of uncertainty regarding what wavelengths get absorbed by what pigment, and how this affects plant growth.

Some of the general findings to date include the discovery that a wide variety of pigments such as carotenoids, phycoerythrin and phycocyanin are also important for photosynthesis, and they absorb light across a wide range of wavelengths. In the chart above: beta carotene has absorption sensitivities towards the 500 nm mark, while phycoerythrin has absorption peaking in the middle of the green spectrum at 660 nm, and phycocyanin peaks in the orange portion of the spectrum at 600 nm.

Remember the original chlorophyll spectrum that said we only need red and blue? Well, if we start considering these other photosynthesis pigments and addressing their absorption wavelength needs, we need to start adding some green and orange wavelengths back in. At a certain point, we have to ask: aren't we just headed back towards re-creating a spectrum similar to natural daylight?

So should a grow light have a natural daylight spectrum? This is naturally an intuitive and convincing argument to make - after all, plants have evolved for many generations under natural daylight and have done well; we know for a fact that natural daylight is not bad for a plant. If so, a grow light that also comes close to mimicking natural daylight must also be more effective for plant growth. However, how much more effective, and does it justify the additional system and operational costs?

The best answer we can give at this point, is that plants need blue and red light, and maybe more.

Is it just about photosynthesis? Look at quantity vs quality vs function

Another question to ask at this point, is what our ultimate goals in indoor growing are. A common and obvious one might be maximum yield or production amount (quantity) but for some applications, it may be other factors such as aesthetics (subjective & difficult to measure) or nutritiousness (even more difficult to measure in a non-lab setting).

If we are looking at quantity exclusively, we would want to know what wavelengths induce the most photosynthetic action for the least amount of energy input and cost.

If we begin to look beyond just quantity, however, other mechanisms and processes that we do not yet fully understand may involve other wavelengths and influence other factors such as aesthetics. If this is the case, it may be advantageous to hedge your bets and include light of all wavelengths by pursuing a grow light that comes closer to natural daylight given that it has a proven track record of thousands of generations of successful growth.

Also consider the fact that you may not be after vegetative leaf growth at all, but rather flowering and fruit production. Many of these mechanisms are governed by changes in light wavelength that naturally occur as the seasons change. 730 nm (far red), for example, is known to be a trigger wavelength for a photoperiod receptor that controls flowering and fruiting.

Here's an analogy: if you are trying to train a bodybuilder to gain maximum muscle mass (legally), what types of foods would you prescribe? If you narrowly follow conventional science and decide on just protein, you might succeed at achieving high muscle mass. But what about their overall health and well-being? Without a balanced diet, your bodybuilder will likely have significant health and nutrition issues despite their large size.

In other words, the closer you can get to a full daylight spectrum, the more of a balanced light "diet" you are providing for your plants.

Practical arguments for using a natural daylight spectrum

In addition to the potential biological and photosynthetic benefits of using a natural daylight spectrum, several practical reasons are also worth keeping in mind.

Before we move on, one important caveat: the white spectrum must include sufficient coverage in the blue and red portion of the spectrum. Many white lamps and fixtures are deficient in the red and blue, which are necessary for chlorophyll absorption and photosynthesis.

First, a natural daylight spectrum grow light appears to humans the same as natural daylight itself, and this is a critical advantage. Take a look at the photo below - this is a facility that uses the traditional blue + red LED approach, which creates a b purple cast on everything.

Can you imagine working in this environment for a long time? Not only will this strange color confuse your brain, it will also prevent you from properly conducting observations and discovering issues such as diseases whose symptoms would otherwise be noticeable under white light. So much so that there are specialized glasses that attempt to mitigate the vision issue.

Secondly, a daylight spectrum white light is a more commonly encountered type of light, which makes it easier to evaluate and compare without specialized horticultural measurement devices and formulas. It also makes visual assessments of relative brightness much easier and reliable.

For example, want to know if your grow light is providing enough light for your plants? Simply take a light meter and use commonly known units such as lux to measure the amount of light falling on plant leaves, and compare this to what is commonly measured for natural daylight. You can even download a free app for your smartphone that will measure lux values through the camera. No need for complicated PPF and DLI calculations (more on this later).

Our perspective on the best light recipe for grow lights

To sum up, we would summarize our recommendations as follows:

1) Be certain that at the very least, there is sufficient light output in the 400 - 500 nm range, as well as the 600 - 700 nm range. This is to ensure that your plants will at minimum receive the light that is necessary for chlorophyll A and B, the primary drivers of photosynthesis.

2) The wider the coverage in this range, the better. Although an LED emitting in the range between 440 - 460 nm would qualify, multiple LEDs covering the full 400 - 500 nm would be even better.

3) Additional coverage outside of the blue and red wavelength range may be helpful, and is almost never harmful. Additional green light would also be helpful for allowing for visual inspection, as the resulting spectrum would produce white light that is easier to work and see under.

Putting all of this together

So far, we've discussed how the spectral power distribution plays a very crucial role in describing the amount of energy that is present at each wavelength. We've also taken a look at how plants react to light of different wavelengths, but that it is still uncertain the extent to which non-chlorophyll wavelengths contribute to plant growth. Now that we have taken a look at the quality aspect of light, next, we'll look at the quantity of light needed for sufficient growth.

PART III: Understanding the science behind the lighting metrics

Before we get into the alphabet soup of acronyms that are used as grow light units, there are two sets of distinctions that are often confused and are very important to clarify. The first distinction is between light illuminance and light output, and the second is between brightness metrics vs photosynthesis metrics.

Illuminance vs output

Light illuminance is about the subject, or target of the lighting system and describes how much light falls onto a particular surface. You can think of it as how "well lit" a surface is. Note that a highly focused, small flashlight can create a higher level of light illuminance over a small surface area than a large, powerful light bulb emitting light in all directions.

Light output, on the other hand, is about the light source itself. This measure describes how much light is being emitted, regardless of its direction and where it ends up. A higher powered device will almost always have a higher light output than a lower powered device, assuming that the efficiency levels are the same.

Brightness metrics vs photosynthesis metrics

The second distinction is about the units used to describe the quantity of light for brightness versus photosynthesis. Lux and lumens are brightness metrics used for white light that more accurately describes the amount of brightness, while PPFD and PPF are photosynthesis metrics that more accurately describes the amount of light capable of contributing to photosynthesis.

For residential and commercial LED lighting applications, the metrics lux (sometimes footcandles) and lumens are most often used to describe the brightness of white light, because that is what these products are designed to do ("cause brightness"). Lux is a measure of light illuminance, and lumens is a measure of light output. For example, lighting designers will typically recommend that a lighting illuminance level of 300 lux is needed for basic tasks like reading. A 10-Watt LED bulb, on the other hand, may emit a total of 800 lumens of light output. Depending on the size of the room, positioning and other factors, a room may need several 10-Watt LED bulbs to achieve 300 lux over a work area.

It is absolutely critical that you understand lumens and lux are brightness metrics designed for humans, and not plant photosynthesis. They are calculated by taking the SPD of a light source, then applying a non-equal weighting called the luminosity function to describe the amount of brightness that the human eye perceives. It answers the question: how bright is it?

If you take a closer look at this curve, what should become immediately evident is that this explains that humans perceive light energy at 555 nm (green) to be 10x as bright as light energy at 465 nm (blue) or 650 nm (red). In other words, if you want to create the same amount of perceived brightness, a 1-watt LED at 555 nm would work just as well s a 10-watt LED at either/both 465 nm and 650 nm. (Ever wonder why high-visibility clothes for cyclists / construction workers use bright yellow/neon? It is precisely for this reason).

But it is not the case that a brighter light is necessarily a better light for growing plants, and this is evidenced by the discrepancy between the chlorophyll curves and the luminosity function. There is a glaring paradox between the chlorophyll & photosynthesis curves we discussed above and the human-based sensitivity curves.

While plants absorb green light very inefficiently to conduct photosynthesis, humans perceive green light to be extremely bright compared to light of other wavelengths. This is why lux and lumens is not always a fool-proof way to measure the quantity of light when considering discussing grow lights.

Well, then, what is a better way to measure light quantity in terms of its effect on photosynthesis? The most commonly used concept is PAR, or photosynthetically active radiation, and the corresponding units of measure are PPF (photosynthetic photon flux) and PPFD (photosynthetic photon flux density) to measure output and illuminance, respectively. Rather than asking "how bright," these metrics ask "how many photons that are capable of inducing photosynthesis?"

Photosynthesis lighting metrics do not use the human-response curve, and focus instead on the wavelengths of light capable of causing photosynthesis. Specifically, PAR is calculated by measuring the total amount of light energy emitted between the wavelength range of 400 nm and 700 nm, and then converting this aggregated amount into the total number of photons emitted.

Unlike brightness metrics which treat green light as having more benefit to contributing to "brightness" than blue or red light, PAR treats all wavelengths equally, as long as it is between 400 nm and 700 nm (with an additional conversion factor to account for the fact that shorter wavelength light contains more energy per photon).

In many ways, PAR is a much more applicable metric for grow lights because it is about photosynthesis and not brightness. Unlike brightness metrics, PAR allows all colors of the spectrum to be on equal footing - i.e. green energy is "worth" the same as red and blue energy.

But there are two caveats to be aware of:

First, PAR considers energy between 400 and 700 nm only, with a sharp cutoff at both ends. This means that any supplemental energy in the far red (>700 nm) or near-UV (<400 nm) will have absolutely no contribution to your PAR numbers.

Secondly, PAR will not tell you how the energy is distributed across the range between 400 - 700 nm. As we saw above, chlorophyll absorbs very bly in the 400-500 nm and 600 - 700 nm range, but not so much in between. In other words, you could have two grow light emitting the same number of photons, giving you the same PAR numbers, but one could be emitting light only in the green, and not hitting any of the chlorophyll absorption ranges in the red and blue. In this case, these two grow lights would perform very differently, but PAR numbers alone would tell us otherwise.

So which one do I use? Brightness metrics or photosynthesis metrics?

Well, given all these challenges with measuring grow lights, what are you supposed to do? Unfortunately, there is no perfect answer, but you can consider the following when evaluating grow lights:

1) Is the emitted light white, AND does it contain the necessary wavelengths between 400-500 nm and 600-700 nm necessary for photosynthesis? If so, you may use either lux/lumens or PPF/PPFD. The reason for this is that you have a balanced, full spectrum grow light and both methods will give you a reliable indication of photosynthesis effectiveness.

2) Is the emitted light not white? Then absolutely DO NOT use lumens or lux. Evaluate the spectrum and ensure that there is sufficient coverage in the wavelengths between 400-500 nm and 600-700 nm necessary for photosynthesis. You can use PAR to evaluate the quantity of light useful for photosynthesis, but be aware that this is dependent on the spectrum and may result in misleading results.

How to measure light output - measured in lumens or PPF (photosynthetic photon flux)

Light output describes how much light energy is being emitted from a light source, and is measured in luminous flux for white grow lights, and PPF (photosynthetic photon flux) for non-white grow lights. The unit of measure for luminous flux is lumens, while the unit of measure for PPF is micromoles per second.

As a rough estimate, you can keep in mind that a typical 4-ft 30W T8 fluorescent or LED tube would emit about 1500 - 2500 lumens, and about 40 - 60 micromoles per second.

You cannot measure this without a specialized lab device called an integrating sphere, because it requires you to capture light emitted in all angles.

How to measure light illuminance - measured in lux or PPFD (photosynthetic photon flux density)

Light illuminance is a measure of how much light falls on a particular surface, and is dependent on the distance from and distribution of the light as it is emitted from a light source. For white grow lights, illuminance is used, and for non-white grow lights, PPFD (photosynthetic photon flux density) is used to measure how much light falls on a particular surface. The unit of measure for illuminance is lux (lumens per sq meter), while the unit of measure for PPFD is micromoles per second per sq meter.

A good comparison to sunlight would be as follows:

  • Dark, cloudy day: 1,000 lux, or 20 micromoles per second per sq meter
  • Indirect daylight: 10,000 lux, or 200 micromoles per second per sq meter
  • Direct daylight: 100,000 lux, or 2,000 micromoles per second per sq meter

Keep in mind that these comparisons between lux/lumens to PPF/PPFD are appropriate only because we are comparing approximately white light sources. Again, you cannot usefully measure and use lux/lumens for a non-white grow light!

These metrics can be measured using a light meter, or even simpler by using a smartphone app via your camera sensor.

Daily light integral (DLI)

If you take a look at the units above, you'll notice that PPF and PPFD are measures of how many photons per second are being emitted or land on a surface. The duration of time that a particular amount of light is falling on a plant is certainly of importance for it, but is not captured by PPF and PPFD metrics alone. After all, the amount of energy a plant can produce via photosynthesis is dictated by the total amount of light it can capture over a period of time, usually defined as a 24-hour period.

Therefore, what ultimately must be addressed is a particular plant's total photon need per day, measured in moles per day. Below are some ranges for DLI requirements of common indoor hydorponics plants:

  • Lettuce: 13 - 17 moles per day
  • Mustard greens/cabbage: 15 - 20 moles per day
  • Tomatoes: 14 - 30 moles per day
  • Peppers: 14 - 30 moles per day

To calculate moles per day, take the PPFD value and multiply this by a factor of 0.0864 (this is the number of seconds in a day, divided by 1 million to convert from micromoles to moles).

For example, say we want to target 15 moles per day for hydroponic lettuce. In this case, we would divide 15 by 0.0864 to give us 173.6 micromoles per second per sq meter. What this means is that you will need to provide light illuminance levels of this level (or approximately 8700 lux) all day (24 hours) to acheive this level of DLI. You may want to instead provide 2x the illuminance level (350 micromoles per second per sq meter or 17,000 lux) for half the time (12 hours), both of which would give you the same DLI result, and in theory, the same level of growth (ignoring photoperiod effects).

When evaluating grow lights, their specifications will typically indicate the total number of lux or PPFD at a certain distance. If there is no distance indicated, there is likely something wrong with their specification. (Lumens and PPF should not, on the other hand, have any distance specified). Use this information to decide if a grow light will provide enough illuminance for the type of plant you are looking to grow.

Measuring the efficiency of a light source as well as TCO (total cost of ownership)

Now that you've established how much light you need, and have several grow light prospects, you will also want to evaluate how much electricity it will take to run this thing. Don't forget that electricity costs money, and will be the primary variable cost for your indoor hydroponics operation.

Depending on where you live, electricity costs are between 10 to 15 cents per kWh (kilo-watt hour). The simple rule of thumb is to know that this is you cost to run a 1000 W device for 1 hour. A 500 W grow light, run 10 hours a day, for example, will cost you 50 cents to run per day at 10 cents per kWh. This might seem like a lot, but it can add up quickly - 50 cents per day is 15 dollars per month!

Grow light longevity and lifetime claims

Another question to ask is how long you can expect the light to last. LED devices are designed to last up to 50K hours, which comes out to 13.7 years at 10 hours use per day. On the surface, this sounds great, but two things to be aware of here:

First, understand that LEDs do not "burn out" catastrophically like an incandescent bulb. Instead, their brightness will slowly degrade over time, but what this means is that the definition of "failure" can be somewhat vague. Most commonly, manufacturers will use a definition based on "L70" which means that the lifetime rating is the number of hours it will take for the light to degrade to just 70% of the original brightness. 30% less light output is perhaps acceptable for a residential light bulb, but can be a significant deficit for plant growth especially if you need to hit a particular DLI target. If it isn't noted on a specification sheet, you should ask the grow light supplier.

Second, there is a large element of trustworthiness when evaluating manufacturer claims about longevity, especially when many products and companies have only been in existence for a small fraction of the time they claim their lights will last. Although there are ways to reverse-engineer light fixtures to determine if the product is designed properly, the easiest way to determine if the manufacturer's rated lifetime has merit is to evaluate their warranty offerings. A reputable, trustworthy manufacturer should offer you 5 years of warranty. This means that the manufacturer has a certain level of confidence in their product's longevity and this means that they have invested sufficient R&D resources into ensuring the light output does not degrade prematurely.

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