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At The Beginning There Was Light

Updated: Jun 21, 2022

By: Rodrigo Pereyra, Sr. Consulting Engineer and Gro-Doctor

Light is one of the fundamental drivers of plant growth and development. Light is quite literally etched into a plant’s basic alchemy: photosynthesis. A plant’s response to light is mediated by three separate, but interlinked, dimensions: light intensity (amount of light), spectrum (what kind of light) and duration (how long light is applied). Light intensity is typically associated with plant biomass; spectrum affects plant morphology; and light duration, or photo-period, is associated with flowering. The response to these three dimensions will naturally vary by plant type and in many cases within genotype/phenotype within the plant.

For controlled environment agriculture (CEA), whether indoor cannabis, greenhouse vegetables or ornamentals, understanding light and its interplay with other environmental parameters (temperature, relative humidity, CO2, fertigation, etc.) is a central part of stable cultivation. Lighting applications will vary by facility: sole source (indoor), supplemental (greenhouse) or photo-periodic (indoor/greenhouse). Decisions related to lighting will affect a facility’s investment and profits: for indoor cannabis, for example, lighting can be up to 30% of a facility’s equipment expense and approximately 40% of its energy expenditure. Making the right choices by understanding the implications of a lighting strategy is a foundational component of a successful operation.

These choices involve having clarity on the technical terminology and its practical use in the design and running of a grow operation However, when it comes to horticultural lighting it can be an alphabet soup: PAR, PPF, PPFD, DLI, SPD, LED, HPS, micromoles etc. The horticultural lighting industry has come a long way in demystifying these terms, but it’s important to continuously reinforce the message so that lighting companies, cultivators, operators and C-suite all speak the same language. So let’s focus on defining the critical parameters on the cultivation side that everyone in the industry should know and be comfortable with: PAR, PPF, Efficiency, PPFD, uniformity and DLI.

Photosynthetically Active Radiation (PAR)

Electromagnetic energy in the spectral range between 400 and 700 nanometers that stimulates plants’ photosynthetic process. Although most grow lights have wavelengths within the PAR range, there may be components of the spectrum that fall outside the range and should be taken into account as radiation outside of PAR (e.g. UV or Far Red).

Photosynthetic Photon Flux (PPF):

The amount of light emitted by a light fixture in the PAR range, measured in µmol/s. PPF is dependent on fixture power and output of the respective light source including spectrum, whether High Pressure Sodium (HPS) or Light Emitting Diode (LED). PPF will give you information on the quantity of light emitted by a source. From PPF it is possible to calculate the Efficiency of the fixture, which is a measure of how efficient a fixture is in converting electrical watts to µmol/s. Efficiency is calculated by dividing PPF over fixture power and measured as µmol/J. Efficiency typically ranges from ~1.9 µmol/J for HPS up to 3.0 µmol/J for certain LED fixtures. This information is provided by most lighting manufacturers

Photosynthetic Photon Flux Density (PPFD):

The amount of light that reaches an area at a moment in time, measured in µmol/m2s. It’s also referred to as irradiance. PPFD not only depends on fixture PPF, but also on fixture spacing, distance to the crop and the optical system on the fixture. PPFD will have spatial variation, so the value you measure below the fixture will vary from the measurement a couple of feet away. That variation will depend strongly on the optics and overlap of the light beams. PAR or Quantum sensors measure PPFD at a single point; it’s important to measure over a grid of points to understand the uniformity of light over the canopy.

Daily Light Integral (DLI)

The total amount of PPFD received over a day or over a prescribed photoperiod, measured in mol/m2day. Photoperiod is the total amount of time of lights on vs. lights off over 24 hours, which will vary depending on crop and stage of growth cycle within the crop. DLI is a critical metric of overall crop light dosage, in particular for supplemental lighting in greenhouses. DLI has to be balanced with the contribution from sunlight over the course of the day and account for the change in seasonal light level. A light dosage which is below the prescribed DLI for the crop will likely result in sub-optimal plant yield and/or quality.

As important as knowing the terminology above, cultivators and facilities should develop techniques to constantly measure and quantify the effects of lighting on the crop, environment and operations. If these parameters are not measured and their effects quantified, operators won’t be able to take corrective actions or ultimately optimize to increase yields and achieve consistent product. So keep a PAR meter in hand, measure, quantify and find ways to aggregate data to know how all those photons are helping your crop and facility achieve its goals.

As always, to measure results & increase your yields, think Gro iQ! Happy Growing!

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