Whether you are using horticulture lighting systems to provide supplemental lighting in a greenhouse or sole-source lighting indoors, it is important that you know the quality and quantity of light that your fixtures are providing to your plants. There are numerous metrics used to measure light; however, certain ones were developed for applications related to human vision, while others were developed specifically for measuring light that influences plant growth and development.
Humans use Lumens
It goes without saying that plants perceive light very differently than humans. Where plants use light to drive photosynthesis and other light mediated physiological responses, humans and many other animals use something called photopic vision in well-lit conditions to perceive color and brightness of light. Lumens are a unit of measurement based on a model of human eye sensitivity in well-lit conditions, which is why the model is called the photopic response curve (Figure 1). As it shows, the photopic response curve is bell shaped and shows how humans are much more sensitive to green light than blue or red light. LUX and foot candle meters measure the intensity of light (using lumens) for commercial and residential lighting applications, with the only difference between the two being the unit of area they measure (LUX uses lumen/m2 and foot candle uses lumen/ft2).
The fundamental problem with using LUX or foot candle meters when measuring the light intensity of horticulture lighting systems is the underrepresentation of blue (400–500 nanometers) and red (600–700 nm) light in the measurement. Humans may not be efficient at perceiving light in these regions, but plants are highly efficient at using blue and red light to drive photosynthesis.
Plants use PAR
Plants primarily use wavelengths of light between 400-700 nm to drive photosynthesis (Figure 1), which is why this range is called photosynthetically active radiation (PAR). PAR is a much used (and often misused) term related to horticulture lighting. PAR is not a measurement or metric like feet, inches or kilos. Rather, it defines the type of light needed to support photosynthesis. The amount and spectral light quality of PAR are the important metrics to focus on. Quantum sensors are the primary instrument used to quantify the light intensity of horticulture lighting systems. A quantum sensor measures the total amount of PAR landing on a specific location of your plant canopy and is measured in micromoles per square meter per second (µmol/m2/s). These sensors work by using an optical filter to create a uniform sensitivity to PAR light, and can be used in combination with a light meter to measure instantaneous light intensity or a data logger to measure cumulative light intensity (Figure 2).
If you used a LUX or foot candle meter to measure the intensity of a horticulture lighting system, you would end up receiving different values depending on the spectrum of the fixture, even if the fixtures were providing the same PAR intensity to your crop. For example, if you took a measurement using a LUX or foot candle meter under a fixture using broad spectrum (i.e. white) light emitting diodes (LEDs) you would receive a much higher value than if you took the same measurement under a fixture with red and blue LEDs, even if both fixtures were providing the same amount of PAR. This will cause growers to miscalculate the actual amount of PAR they are delivering to their crop, which can lead to unfavorable growth and development. This is why lumens, LUX, and foot candles should not be used as metrics for horticulture lighting.
When you are looking to optimize growing conditions by measuring and fine tuning light intensity in a controlled environment, it is extremely important to use the correct metrics and instruments. The amount of PAR delivered to a crop has a direct correlation to the amount of water, CO2, and nutrients a plant requires, in addition to other environmental factors (e.g. temperature and humidity) that need to be adjusted based on light intensity.
Measuring Lumens instead of PAR is just one example of using incorrect metrics for horticulture lighting that can have an adverse effect on your crop.
For more information regarding this topic, please see Horticulture Lighting Systems: Understanding The Numbers.