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Comparing Grow Lights

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How can I compare different grow lights?

Comparing different grow lights is actually incredibly difficult since you have to look at power, spectrum, cooling, and a host of other factors. You may not have all the gear handy to do your own tests on lights but at least you can learn how to compare them and what to look for in your next indoor gardening grow light by reading our blog post where we cover all of this in detail.

What are lumens, and are they useful for evaluating grow lights?

Lumens are a measure of luminous flux, or the total amount of visible light radiating from a source, weighted by the human eye's sensitivity to the particular wavelength of the light. Lumens are the best measurement to use when evaluating how well a light will illuminate an area for human eyes. The human eye is most sensitive to light in the yellow range of the spectrum, so 100 photons of yellow light have a higher lumen rating than 100 photons of blue light or 100 photons of red light.

Plants preferentially absorb red and blue light. Lumens preferentially weight yellow light and de-weight red and blue light, making lumens just about the worst light intensity measurement possible for evaluating how well a light will grow plants.

Lumen Weighting (yellow) versus Photosynthetic Efficiency (green):
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Select Spectral Data
Relative Photosynthetic Efficiency by Wavelength
Lumen Weighting Function (Relative Spectral Sensitivity of the Human Eye)

Lumens' measurement of human-visible luminous flux differs from PAR, which measures radiant flux -- the total number of photons in the visible spectrum without weighting for human visibility. Yield Photon Flux (YPF) is like lumens in that photons are weighted based on their wavelength, but YPF weights them based on their usefulness to a plant rather than to the human eye, and YPF considers photons outside of the human visual range. For this reason, YPF is the best measurement of light intensity for growing plants, although it still has significant drawbacks as we explain here.

What is PAR, and is it useful for comparing grow lights?

Photosynthetic Active Radiation (PAR) designates the spectrum or range of colors of light from 400 to 700 nanometers that plants are able to use for photosynthesis. PAR measurements are usually expressed as photosynthetic photon flux density (PPFD) in units of μmol m-2s-1 -- how many micromoles of photons (602,214,150,000,000,000 photons) within the PAR wavelengths of 400nm-700nm that go through 1-square-meter each second, though most devices that measure PAR do it only at a single point, rather than over a whole square meter.

There are important caveats, but in general, the higher the PAR measurement a light has, the better it will grow plants. Too much PAR (too much light) is wasteful and can even damage plants, although this is almost never a problem with artificial grow lighting. More fundamentally, PAR measurements do not account for the relative usefulness of particular wavelengths to the plant-- leaves' preferential absorption of different spectra (colors) mean some photons are more useful to the plant than others, even within the PAR range. For example, plant leaves reflect much of the green light that hits them-- most of it is not used by the plant, but this is right in the middle of the PAR spectrum at 495nm-570nm. In addition, plants require many spectra to perform well, although a light with a single color can have the same PAR measurement as a multi-spectrum light. Just having a high PAR value doesn't indicate that a light will grow plants well; the spectrum must also be considered.

PAR also assumes that all photons outside of the 400nm-700nm range have no use in photosynthesis or for overall plant health. However, plants do use light outside of PAR, such as far-red light above 700nm which increases photosynthetic efficiency (due to the Emerson effect) and for hormone regulation. UV light (below 400nm) plays an important role in triggering plants to create pigmentation and other substances, such as vitamins, THC and CBD.

PAR measurements can vary significantly within the lighting footprint of a light, so any single PAR measurement is uninformative as to how the light will grow plants. For example, a laser can focus all of its light onto a PAR-measuring instrument and have an incredible PAR value, but this doesn't mean it will grow plants well. Many LED grow lights are sold with secondary lenses which focus the light into a narrow cone. These lenses give the light to achieve a very high PAR measurement at the center of its footprint, but just off to the sides the PAR falls to a point it can no longer sustain a plant.

PAR measurements are also higly dependent on the distance they are taken from the light source. The inverse-square law of light means that the photon flux density (what PAR is measuring) will decrease by the square of the distance from the source-- so if a PAR measurement was 100 at 1 inch from the fixture, it will be 25 at 2 inches, and 11.1 at 3 inches. It's easy to claim a high PAR reading for a light fixture if the measurement is taken close to and directly below it. It is therefore critical to know how far from the light PAR measurements were taken, in addition to where in the footprint the measurement was taken.

For these reasons, single PAR measurements and PAR alone should not be used as a measure of how good a grow light is, and can be very misleading when comparing lights. Even multiple PAR measurements over the entire footprint at the recommended height will not indicate how well a light can grow, as the right spectral distribution of light is critical and is not considered by PAR. Only by measuring PAR over the entire footprint of the light, at the recommended hanging distance above the plants, and considering the entire spectrum (including outside of PAR!) can useful comparisons be made.

What is YPF, and is it good for comparing grow lights?

Yield Photon Flux (YPF) is a measure of light intensity, weighted based on the light's usefulness to plants. Unlike PAR, which considers only photons in the 400-700nm visible light spectrum and weights each photon equally, YPF considers photons from 360-760nm (ultraviolet through near-infrared) and weights each photon based on the plant's photosynthetic response to the particular wavelength of light.

The weighting employed by YPF measurements eliminates some of the shortcomings associated with PAR measurements. For example, 100 photons of pure-green light have the same PAR value as 100 photons of red light, even though most of the green photons will be reflected by the plant while most of the red photons will be absorbed. YPF accounts for this and 100 photons of green light have a lower YPF than 100 photons of red light.

Unfortunately, YPF still has shortcomings when used as a measure of how well a particular light will grow plants. YPF does not account for the fact that different wavelengths of light are used by plants to initiate different biochemical reactions and that a wide spectrum is necessary to grow plants to their maximum potential. All-red light will have a higher YPF score than a broader spectrum containing all the wavelengths of light plants require for photomorphogenesis (creation of secondary metabolites such as pigmentation, flavonoids, THC and CBD).

Like PAR, YPF is also measured at a single point, which does not indicate how well plants will grow over the entire footprint of an artificial light source. If light is being focused into a narrow beam by secondary lenses, the YPF score in the center will increase, even though plants can only be grown directly under the light. The inverse square law of light means that YPF measurements will decrease by the square of the distance from the source-- so if a YPF measurement is 100 at 1 inch from the fixture, it will be 25 at 2 inches, and 11.1 at 3 inches. As with PAR, it's easy to claim a high YPF reading for a light fixture if the measurement is taken close to and directly below it.

YPF is a better measurement of how useful light is to plants than PAR, but still shares most of the shortcomings of PAR. A red laser pointer has an amazingly high YPF score but can't be used to grow plants.

Only by considering multiple YPF measurements taken over the entire footprint of the light, at the recommended hanging distance above the plants, and considering the entire spectrum, can useful comparisons be made between grow lights.

What is Correlated Color Temperature (CCT), and is it good for comparing grow lights?

When you get anything warm enough it will start to give off light, and the hotter it gets, the more energetic the light gets, shifting from the red end of the spectrum at about 1350 °F to blue when temperatures get closer to 17500 °F.

Correlated color temperature is a measurement of the average hue of light as it appears to the human eye, expressed as the temperature (in Kelvins) something would need to be heated to glow at approximately the same color. We have a more in-depth discussion of what color temperature is (and isn't) here.


Color temperature is useful whenever you're comparing different lights that look white to the human eye, and for things such as adjusting "white balance" in photographs, to make them look right to humans.

When comparing "white" grow lights (multi-spectral lights that appear white to the human eye), the color temperature can be useful as an indication of the relative balance of red light to blue light in the spectrum- the higher the color temperature, the more blue there is in the spectrum (compared to red). But if the light doesn't look roughly blue, white, yellow, orange or red to human eyes, color temperature doesn't apply-- no matter how hot you get something, it will never glow purple or green, so color temperatures for these colors don't exist.

So while color temperature can be a useful comparison among grow lights that look white to humans, it isn't useful overall in comparing grow lights. The correlated color temperature definition is based on how humans perceive the light, not on how plants perceive it.

Which is better- UVA or UVB?

Studies have shown that plants respond to both UVA and UVB light, but UVB light damages cells and can cause cancer. We have an in-depth discussion of UVA and UVB light here.

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