Blogs Apr 20, 2017

Demystifying Lumens, Lux and PAR

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LED lights require new thinking about the way we measure light spectrum for use by plants. The typical rating most growers are familiar with is the “lumen”. 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 energy over the spectrum, and most importantly, it doesn’t tell us how much is useful for plants.

The problem with lumens is especially pronounced when measuring light at the far ends of the human visual response curve. Consider three lamps—red, green and blue—each emitting the same number of watts of optical energy. The red and blue lamps 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 the highest at green. That’s why a high lumen rating does not necessarily make a lamp better suited to growing plants. Similarly, light meters that measure in “lux” tell us very little about a lamp’s plant-growing power. The light sensors in lux meters have their own spectral response curves which may over- or under-measure light at various colors. This is why lux meters usually have different settings for “sunlight,” “fluorescent” and “incandescent” lamps. Again, because lux meters are meant for measuring the amount of light usable by humans, they don’t tell us anything about how plants will respond.

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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 each second. Inexpensive PAR meters use sensors that respond over the entire 400-700nm spectrum and have their own sensitivity curves that require different calibration for sunlight, fluorescent and HID lighting. All these systems are too broadly responsive to measure an LED’s narrow emission spectrum. They make HID light seem brighter by over-measuring yellow-green light and make LED light seem dimmer by under-measuring red and blue light.

To properly measure the amount of energy present for photosynthesis we must use a spectroradiometer. This instrument 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.)

Manufacturers should publish spectroradiometric data showing the energy per wavelength produced by their lamps. This data will allow growers to accurately compare different lighting technologies—whether HPS vs. LED or different LED horticultural lights—and know how much usable light their plants will receive from each system.

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About LumiGrow, Inc.

LumiGrow revolutionized horticulture in 2008 with the introduction of the first smart LED grow lights in North America.  Today, LumiGrow leads the world forward in grow light innovation with their TopLight and BarLight smart fixtures designed to maximize a growers’ profits.  The LumiGrow lights are wirelessly controlled by their smartPAR software to optimize yield, quality, and custom plant traits.  LumiGrow lights can be fine-tuned for greenhouse environments by pairing with their award-winning smartPAR Light Sensor to ensure consistent crop production year-round at the lowest energy cost.

LumiGrow has the largest install-base of smart LED grow lights in North America with installations worldwide.

Designed for Greenhouse & High Bay Applications
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