Lumens are for Humans and PAR is for Plants

It blew my 10-year-old mind when my all-knowing grandmother told me that the blue jay we were watching was in fact not blue. She explained that light is composed of many colors, and it is the colors that are reflected, not absorbed, that our eyes perceive as the color of an object. This is a necessary reminder that what is perceived might not be what it appears to be. For decades, the indoor gardening community has used lumens as the standard increment for the measurement of light. Lumens were unfortunately a poor choice, and here’s why.

Lumens are essentially a measure of brightness based on human perception. A lumen is equal to the light emitted by one candle falling on one square foot of surface located one foot away. This measurement presumes a human is the perceiver of the light. Plants perceive light differently; from a plant’s perspective, light that is useful for photosynthesis is not necessarily bright. Light– more specifically, visible light– is made up of wavelengths of energy on the electromagnetic spectrum ranging from 380 to 770 nm (nanometers). Plants use wavelengths from 400 to 700 nm for photosynthesis. Brightness does not accurately describe if the light will be more or less useful to a plant.

Light can be characterized in other ways when discussing its benefit to plants. Color temperature is often referred to in the horticultural industry on lamp boxes to describe the color of the light emitted by the lamp. Does 4,000 K grow a plant better than 7,500 K? Color temperature is listed in Kelvin (K), which is a measurement of temperature.

The temperature of what you might ask? It is a description of the relative whiteness of a piece of tungsten steel when heated to that particular temperature in degrees Kelvin. This accurately characterizes the color of the light as we perceive it, but color temperature again fails to address how effective a particular light source will be at providing the energy necessary to drive photosynthesis.

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But not to worry, there is in fact a measurement that precisely describes how effective a particular light will be for growing plants– PAR, or photosynthetic active radiation. PAR spectrum accounts only for light, or more precisely, photons emitted between 400 to 700 nm. Scientists have concluded that it requires about nine photons to bind one CO₂ molecule in photosynthesis.

Even though blue photons have more energy, scientists have also found that there is little difference between the effectiveness of red versus blue photons at driving photosynthesis as long as the photons are within the 400 to 700 nm range. This leads to a direct correlation between the number of photons produced in the PAR spectrum by a given light, and the photosynthetic potential of that light.

Photons are emitted by light sources in large numbers so we do not talk about billions or quadrillions of photons; instead we refer to them using the multiplier moles, which stand for 6.0221415 x 1,023. To make the numbers even more accessible, the number of moles is often divided by one million resulting in micro-moles. Light sources emit photons continuously over time so the number of micro-moles is more accurately described as μmol per unit of time (most commonly, seconds).

When trying to quantify how effective a light source is beyond the total output of μmol/second, you must consider one last piece of information– the size or area of your garden. Inevitably some of the photons produced will not reach your garden, so the most accurate representation of a light source’s ability to drive photosynthesis will take into account the area being lit and how many photons reach that given area per second. The formula that represents the effectiveness of a light source for photosynthesis is written as μmol/sq. ft./s. This descriptor is actually referred to as photosynthetic photon flux density, or PPFD for short.

In light of all of the information above, let’s remember that lumens are not a useful descriptor of a light’s ability to drive photosynthesis. I think I will sit back with a drink, and digest all of the information about PAR and PPFD while I watch the not-blue blue jay outside my window.