PAR, PPF, PPFD, DLI - Understand the units of measurement related to the useful light for plants

This page provides an overview of the units of measurement of light radiation that activate photosynthesis and other biochemical processes in plants.

The best known and most widespread light intensity measurement system is based on a quantity called lumen which consists of the reaction curve of the eye to the stimulus while its derivative lux is the quantity of light coming from a refraction of the surface hit by photons ( lumen / sqm).

Light is measured with photometric quantities derived from the sensitivity of the human eye to colors, or with radiometric quantities referred to the energy transported and independent of the wavelength. Lumen and lux are photometric units of measurement referring to the light perceived by the human eye. Since plants use wavelengths different from those of human vision for photosynthesis, photometric measurements of the light intensity of a lamp do not indicate how many photons useful for photosynthesis are actually projected on the plant.

The chemical activity of plants shows peaks around 400-450nm (nanometers) and 650-670nm for the production of chlorophyll A and on 450-500 nm and 600-640nm for chlorophyll B. In the germination and growth phase, plants use mainly blue light at 450-480nm, while in bloom they use red light at 640-660nm. A mixture of red and blue radiations is necessary in all phases of life even if the maximum efficiency of a lamp is obtained on 650nm, in the “deep red” area. The flowering of many plants also requires wavelengths beyond deep red, up to 700nm “far red”.

Hence the light radiations that activate photosynthesis and other biochemical processes in plants are called PAR (Photosynthetically Active Radiation) and are found in the wavelength range of 400 to 700 nm.

Now let's see what the basic terms are in the agriculture industry:

PAR region
Photosynthetic active radiation is the bandwidth of 400 to 700nm, which is the light mainly used by plants.
Different plants require different combinations of wavelengths within the PAR region.

PPF (Photosynthetic Photon Flux) - Flux of photosynthetic photons per second

  • Measured in μmol / s
  • Total number of photons emitted per second in the PAR region.
  • But how many will reach the plant and at what distance?

PPFD (Photosynthetic Photon Flux Density) - Density of the photosynthetic photon flux per m2 per second

  • Measured in μmol / m2
  • It represents the number of photons that reach the plant within the PAR region in a given area.
  • It decreases exponentially as the distance between the light source and the implant surface increases.

The ideal PPFD values for fruits and vegetables range from 400 to 1000, with strawberries and salads between 250 and 450, while tomatoes require 450 to 750 µmol / m2 / s. At our latitude the midday sun emits 1200 and 2000 PPFDs depending on the seasons.

DLI (Daily Lighting Integral) - Daily lighting integral

  • Plants need a minimum amount of light per day to meet their basic biological needs, this varies depending on the species.
  • For flowering and fruiting, high light levels can show significant increases in both quality and quantity.
  • DLI = PPFD (μmol / m2) x 3,600 (s / h) x photoperiod (hours / day).

We can imagine the behavior of photons as if they were water droplets. The instantaneous light level in μmol / mq s is the number of droplets collected in one square meter of surface per second. In the same way, however, we can consider the number of total droplets that fall in a day or in 18 hours of light, measuring the water level in the evening, calling it the 'daily integral of light' μmol / m2 day.

Plant Lighting Efficacy - Electrical Efficiency

  • measured in μmol / J
  • It refers to how efficient a horticultural lighting system is at converting electrical energy into photons of PAR

Therefore the main unit of measurement used for the measurement of active photosynthetic radiation (PAR) is the photosynthetic photonic flux density (PPFD), i.e. the number of photons that reach plants to stimulate photosynthesis.

However, a single PPDF measurement is not sufficient to assess the entire growing area, depending on multiple factors that can affect photosynthesis, photo-morphogenesis.

Interesting to try this DLI Calculator, to give it a try Click here

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Calculate the DLI (Daily light integral) of the greenhouse you have in mind. Starting from the Italian geographical area concerned and knowing the limiting factors, in terms of the reduction of light radiation determined by the structure and material chosen as roofing, you can calculate the DLI of your project.

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