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The Complications of Measuring Horticultural Lighting

Written by Eric Hopper
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Getting the best out of your indoor lighting is a tricky business.

The driving energy behind the success of any indoor garden is the light source. It comes as no surprise that, because of its importance, growers want an efficient way to measure the light. Unfortunately, this is easier said than done, as the entire lighting industry and the way light is measured is based on the way humans perceive light. This is why horticultural lighting is still rated on initial lumen output. One of the most important sayings growers of high-value plants should remember is “lumens are for humans”.

PAR and Measuring Light for Plants

Photosynthetically active radiation, or PAR, is defined as light energy which is usable by plants. In other words, some of the light spectrums that humans see aren’t necessarily usable by plants. This is why there has been such an issue when trying to measure light for horticultural purposes. For example, plants reflect the majority of green light (hence their color) but the human eye is most sensitive to yellow-green light. This means a bulb which emits mostly green light can still have a high initial lumen rating. The green and yellow light wavelengths create an obstacle for producing accurate horticultural light measurements. Plants have two types of chlorophyll that absorb light; they are classified as chlorophyll a and chlorophyll b. Combined, these two types of chlorophyll have peak absorption at 410-460 nm (blue spectrum) and 630-670 nm (red spectrum). These peak absorption zones create another large obstacle for producing accurate horticultural light measurements.

Photosynthetic Photon Flux Density (PPFD)

Photosynthetic Photon Flux Density (expressed in umol/m²/s) is currently the best measurement we have to determine the usable light from a source. Many hydroponic suppliers now offer some sort of PPFD meter, sometimes sold as a PAR meter. This meter is the best tool available for horticultural light measurement. However, it is far from perfect. A PPFD reading expresses the total photon flux density between 400-700 nm. The meter does not take into account the spectrums most sensitive to absorption.

The Future of Light Measurement for Horticulture

In the not too distant future there will be a new type of light meter available to indoor horticulturists. As we learn more about plant physiology and the way chlorophyll a and b react to light, we will create meters that consider all the variables that affect the way the plants process light. I predict that the light meters of the future will give two readings: one for the blue light spectrum and one for the red light spectrum. I also believe we will have to find a new way to define each wavelength based on the sensitivity to absorption. In other words, there will have to be a multiplier in the equation based on the wavelength’s sensitivity to absorption. This is, of course, easier said than done and is years away from fruition. One thing is certain, as our ability to accurately measure light increases, growers of high-value plants will have the proper tools to determine the best light source for their gardens.