Figure 1: Photosynthetic light response curves
Plants and people perceive light very differently from one another. Humans and many other animals use something called photopic vision in well-lit conditions to perceive color and 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 you can see, 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 are measured over (LUX uses lumen/m2 and foot candle uses lumen/ft2).Using LUX or foot candle meters to measure the light intensity of horticulture lighting systems will give you varying measurements depending on the spectrum of the light source, even if you are measuring the same intensity of PAR.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 nm) and red (600 – 700 nm) light in the visible spectrum. Humans may not be efficient at perceiving light in these regions, but plants are highly efficient at using red and blue light to drive photosynthesis. This is why lumens, LUX, and foot candles should never be used as metrics for horticulture lighting.
PAR is photosynthetic active radiation. PAR light is the wavelengths of light within the visible range of 400 to 700 nanometers (nm) which drive photosynthesis (Figure 1). 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 light are the important metrics to focus on. (To find out more about spectral light quality click here). Quantum sensors are the primary instrument used to quantify the light intensity of horticulture lighting systems. These sensors work by using an optical filter to create a uniform sensitivity to PAR light (Figure 1), and can be used in combination with a light meter to measure instantaneous light intensity or a data logger to measure cumulative light intensity.
- How much PAR the fixture produces (measured as Photosynthetic Photon Flux)?
- How much instantaneous PAR from the fixture is available to plants (measured as Photosynthetic Photon Flux Density)?
- How much energy is used by the fixture to make PAR available to your plants (measured as Photon Efficiency).
PPFD is photosynthetic photon flux density. PPFD measures the amount of PAR that actually arrives at the plant, or as a scientist might say: “the number of photosynthetically active photons that fall on a given surface each second”. PPFD is a ‘spot’ measurement of a specific location on your plant canopy, and it is measured in micromoles per square meter per second (μmol/m2/s). If you want to find out the true light intensity of a lamp over a designated growing area (e.g. 4’ x 4’), it is important that the average of several PPFD measurements at a defined height are taken. Lighting companies that only publish the PPFD at the center point of a coverage area grossly overestimate the true light intensity of a fixture. A single measurement does not tell you much, since horticulture lights are generally brightest in the center, with light levels decreasing as measurements are taken towards the edges of the coverage area. (Caveat Emptor: Lighting manufacturers can easily manipulate PPFD data. To ensure you are getting actual PPFD values over a defined growing area, the following needs to be published by the manufacturer: measurement distance from light source (vertical and horizontal), number of measurements included in the average, and the min/max ratio). Fluence always publishes the average PPFD over a defined growing area at a recommended mounting height for all of our lighting systems.
Figure 2: PPFD measurements | 120 x 120 cm single canopy @ 60 cm | URSA Optilux 640
Photon Efficiency refers to how efficient a horticulture lighting system is at converting electrical energy into photons of PAR. Many horticulture lighting manufacturers use total electrical watts or watts per square foot as a metric to describe light intensity. However, these metrics really don’t tell you anything since watts are a measurement describing electrical input, not light output. If the PPF of the light is known along with the input wattage, you can calculate how efficient a horticulture lighting system is at converting electrical energy into PAR. As a reminder, the unit for PPF is μmol/s, and the unit to measure watts is Joule per second (J/s), therefore, the seconds in the numerator and denominator cancel out, and the unit becomes µmol/J. The higher this number is, the more efficient a lighting system is at converting electrical energy into photons of PAR.
In order to invest in the proper horticulture lighting system to meet your cultivation and business goals, you need to know the PPF, PPFD, and photon efficiency to make informed purchasing decisions. However, these three metrics should not be used as sole variables to base a purchasing decisions. There are several other variables such as form factor and coefficient of utilization (CU) that need to be considered as well.
All factors need to be used in combination to select the most appropriate systems based on your cultivation and business goals, and the take home message is that PPF, PPFD, and photon efficiency are the proper metrics used by scientists and industry leading horticulture lighting companies. If a company does not provide you with the correct metrics used for horticulture lighting, they should not be selling horticulture lighting systems, and you will not be able to verify the true efficacy of their system. Fluence Bioengineering always publishes these metrics in product literature and is one of the leaders in photosynthetic photon efficiency as verified by Rutgers and Utah State University.