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Graphs Comparing the Wavelengths Plants Respond to vs. the Wavelengths the Human Eye Responds To

Plant Lighting: Demystifying Lumens, Lux and PAR

Are you using lighting in your grow setup? If you are, you need to change the way you look at measuring light for the use of your plants.

Most growers are familiar with “lumens”. The definition of the lumen is the total light produced within the range of the human visual response. It tells us nothing about the distribution of that light over the spectrum, and even more importantly, it tells us nothing about how much of that light is useful for plants.

The problem with lumens is especially pronounced when measuring light at the far ends of the human visual response curve. For example, take three lamps - a red one, a green one and a blue one - each emitting the same number of watts of optical energy. The red lamp and the blue lamp would have much lower lumen ratings compared to the green lamp, simply because the human visual response is very low at red and blue and highest at green so that is why a high lumen rating does not necessarily make a lamp better suited to growing plants.

Similarly, light meters that measure “lux” tell us very little about a lamp’s plant growing power. The light sensors in lux meters have their own spectral response curve which may over/under measure light at various colours. This is why lux meters usually have different settings for “sunlight”, “fluorescent” and “incandescent” lamps. But because lux meters were made for measuring the amount of light usable by humans, they don’t tell us anything about how plants will respond to the lamp.

Plant biologists define light in the 400nm to 700nm spectral region as “photosynthetically available radiation” or PAR. The unit for measuring PAR, micro-mols per second (μmol/s), indicates how many photons in this spectral range fall on the plant per second. Inexpensive PAR meters use sensors that respond over the entire 400-700nm spectrum and have their own sensitivity curves that need different calibration for sunlight, fluorescent and HID lighting.

All these systems are too broadly responsive to measure an LED’s narrow emission spectrum. It makes HID light seem brighter by over-measuring yellow-green light and make LED light seem dimmer by measuring the red and blue light.
To properly measure the amount of energy present for photosynthesis we must use a spectroradiometer. It measures energy in watts at each specific wavelength over a range of wavelengths. A spectroradiometer can provide a direct comparison of each lamp’s ability to produce light that plants can use for photosynthesis. Spectroradiometers are expensive instruments, not usually found outside laboratories. A more common instrument, called a spectrometer, can show relative light output over a spectral range, but does not measure energy in watts.

Lighting manufacturers should be publishing spectroradiometric data showing the energy per wavelength produced by their lamps. That data would allow growers to know how much usable light their plants will receive from each system and you could accurately compare different lighting technologies, whether it be HPS vs. LED or different LED horticultural lights.

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Almari van der Loo - August 20, 2019


Could you please help me to find a supplier for a hand held par meter?

Thank you

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