PAR Lighting Calculator

Feb 14, 2016 @ 2:05am

Plants require light to function, but they are sensitive to light at different wavelengths than humans, and that makes calculating the lighting required for indoor plant growth difficult, especially when all the light bulbs out there are rated in lumens, a measure of how bright they appear to humans. So I made a calculator to determine the actual PAR rating of a light setup.

So why do this? Lights are routinely measured in lumens, lumens is a unit that describes the brightness of a light as we would see it, so a bright UV light is zero lumens, a dim green light can be many lumens, while a light that puts out less total light, but is red, may actually be less lumens. This is because the human eye is more sensitive to green than red.

Similarly, plants use light via chlorophyll to grow. chlorophyll is sensitive mostly red and green light. Studies have been done to determine how well plants grow under various wavelength of light. These studies can be used to develop a photosynthetically active radiation spectrum that describes how well a plant grows under different wavelengths of light. This spectrum is known as the PAR action spectrum and is used to determine the intensity of a light as a plant "sees" it. This is usually expressed as the photosynthetic photon flux in µmol/s, or how many photons a plant can use from the light (chlorophyll cares about the number of photons received, not watts). The PPF of a light is similar to it's lumens, it's the total light given off considering how usable it is for a plant or human respectively.

The intensity that a plant receives is dependent on the fixture and distance from the light. Normally we see this expressed in lux or foot-candles. It is a measure of how much light hits an area. For plants we use PPFD, photosynthetic photon flux density. This is usually expressed in µmol/m^2*s, various plants require different levels. When someone says their PAR light level is 200, they mean the PPFD is 200µmol/m^2*s.

When growing plants indoors PPFD is not all that matters, it's also important to determine the daily light integral (DLI), this is a measure of the total PPFD received over the course of the day, when outside this accounts for things such as clouds, shade at various times of the day, and the fact that the sun appears dimmer in the morning and evening and brighter at noon. The DLI is one of the best measurements for determining if a plant is getting the proper amount of light over the course of a day for proper growth. For artificial lighting, the DLI can be increased by increasing the photoperiod, or how long the lights are on per day. Many plants use the photoperiod of the light to detect the season, and they will flower in response to the photoperiod. Many temperate species require that the photoperiod is varied to simulate the seasons. Ideal values of DLI vary by plant, Measuring Daily Light Integral in a Greenhouse, but for a low light plant 5mol/day may be sufficient while a full sun plant may require 20+mol/day.

My calculator calculates all of the above numbers, it can even convert between lumen and PPF and it can convert between lux and PPFD. I created it because with all the LED lights coming out the industry has come out with LED grow lights, these use high efficiency single wavelength LEDs picked to coincide with the peaks on the PAR spectrum. Unfortunately their advertising states what LEDs they have and talks all about their spectrum, but they skip any talk of actual brightness, probably because of how hard it is to explain it. My calculator can calculate the PAR ratings for these lights, as an example this is the light I got, they list that is has 12 LEDs, and is 12W and has three each of 660nm, 630nm, and 460nm LEDs. That defines the spectrum for that specific light. On my calculator you can enter a custom spectrum of "660:3, 630:3, 460:3" to specify a light with three equal brightness LEDs of each of those wavelengths. Then you can input the radiant flux (typically 20-30% of the power consumed for an LED) and get the PPFD for your setup, or you can measure the light in Lux with an accurate light meter and it will give you the PPFD (in practice this probably won't work, most light meters are NOT accurate when measuring colored light, if your light meter is accurate for all colors this will work). Hopefully this helps people trying to figure out the lighting levels required for their brand new LED grows lights (or whatever light they happen to be using).

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