For our controlled environment agriculture (CEA) lighting we use a combination of CMH and HPS lamps. Learn about correct spectrum and methods to measure light for growing cannabis such as PAR, PPF, PPFD, and DLI!
When growing indoors in a controlled environment agriculture (CEA) environment the aim is to have a technology-based approach toward cannabis production. The goal of CEA is to provide plant protection and maintain optimal growing conditions throughout the development of the crop on a consistent basis. Production takes place within an enclosed growing structure that requires lighting to drive plant photosynthesis.
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. Humans are much more sensitive to green light, than blue or red light.
LUX, and foot candle meters are most often the measurement taken when quantifying the intensity of light (using lumens) for commercial and residential lighting applications. 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.
This is why we use PAR.
What is PAR?
Plants primarily use wavelengths of light within the visible range of 400 to 700 nanometers (nm) to drive photosynthesis, which is why this range is also 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. Humans use Lumens – plants use PAR.
How do we measure light?
As mentioned PAR is not a measurement – it’s just the spectrum of light plants use for photosynthesis. So how do we measure the amount of light within PAR?
What really counts are the three measurements: PPF, PPFD and DLI.
When discussing lighting systems that will trigger and promote photosynthesis and whether or not the light or amount of light is correct for a growing application, there are three measurements that are important:
• How much light a light source produces
• How much of that light is available to the plants from said light source
• How much light the plant receives during the photoperiod
Photosynthetic Photon Flux (PPF)
The first measurement is Photosynthetic Photon Flux or “PPF” which measures the total amount of light that is produced by a light source each second. Put another way, PPF tells us how much PAR is emitted by a light source per second. More technically, PPF measures the “photosynthetically active photons emitted by a lighting system per second”. This measurement is expressed in “micromoles per second” (μmol/second). Note though, that PPF does not tell us how much of the measured light actually lands on the plants or any other surface.
Photosynthetic Photon Flux Density (PPFD)
The second measurement is PPFD which measures the light that actually arrives at the plant. Photosynthetic Photon Flux Density or “PPFD” is a measurement of the amount of light that actually reaches your plants 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. This measurement is expressed by scientists and light engineers as: μmol/m2/s.
A basic quantum meter can be used to measure the instantaneous photosynthetic photon flux density (PPFD) in micromoles of photons per meter squared per second (μmol/m2/s). One such quantum meter is the MQ-501: Handheld Full-spectrum Quantum Meter.
Daily Light Integral (DLI)
The third measurement is DLI, which measures the total amount of light that is delivered to a plant every day. The grower can think of DLI as the plant’s daily “dose” of light, though a scientist would probably say DLI is a cumulative measurement of the total number of photons that reach the plants during the daily photoperiod. DLI measures the number of “moles” of photons per square meter per day and is expressed as: μmol/m2/d.
If one were to leave grow lights on longer for a longer period of time for a day, the plants would absorb more light. Or, put in more technical language, a lighting system with a lower PPF can deliver the same DLI to a plant compared to a lighting system with a higher PPF if the photoperiod is extended. DLI is analogous to the total amount of rain that falls during a storm, as opposed to how fast the rain fell (which would be PPFD). DLI is the most important metric for determining the overall growth rate of plants.
For cannabis, we are looking for a DLI value of 48 – 60 DLI.
We can use PPFD to find DLI (Daily Light Integral), and use DLI to find PPFD:
DLI (mol/area^2/day) = (PPFD*3,600*daylength)/1,000,000
PPFD (umol/area^2/second) = ((DLI*1,000,000)/daylength)/3,600
The following is a breakdown of how PPFD and DLI numbers can vary by simply changing the length of time of a light cycle:
60 DLI for 20 hour daylength = ~833 PPFD
60 DLI for 18 hour daylength = ~926 PPFD
60 DLI for 12 hour daylength = ~1,389 PPFD
48 DLI for 20 hour daylength = ~667 PPFD
48 DLI for 18 hour daylength = ~741 PPFD
48 DLI for 12 hour daylength = ~1,111 PPFD
Light Measurement Summary:
PPF tells us how many photons of light are emitted by a light source each second.
PPFD tells us how densely the fixture distributes the light photons on a one meter square target (i.e. your plants) in one second.
DLI tells us how many photons were delivered to a one meter square target in a full photoperiod (i.e. a day).
Light Types in Use
Here, at Golden Apple Cannabis Co. we primarily use the Philips brand MasterColor CDM Elite Agro 315W T12 metal halide lamps which are specifically designed for the horticulture market. These lamps support excellent Photosynthetic Photon Flux or PPF with a value of 1.9 micromole per second. This makes these lamps some of the most efficient on the market for any High Intensity Discharge (HID) lamp. In addition to being efficient which helps environmental offset, the Elite Agro lamps provide the most complete light spectrum within PAR per watt of any HID lamp on the market.
In addition, we also use a few more traditional High Pressure Sodium lamps to bolster the far-red end of the PAR spectrum. The particular type of lamps we use for this purpose are Philips Master GreenPower 600W 400V EL lamps.
We use all of these technologies to provide the best spectrum of light in the most efficient manner as possible for our cannabis plants.