The Physics of Light and Light Measurement


Mar 12, 2020 | 1 comment


Light is the visible portion of the ELECTROMAGNETIC RADIATION emitted from the sun and other sources. PHOTONS are the particles that make up light. Because they are small, they are measured using units of moles (mol).  One mol = 6.02 x 1023 photons. Sometimes, converting the number of photons per one mol is not small enough, so it is further converted to µmol (one mol = 1 x 106 µmol).

Instead of using large numbers of photon particles, we can describe recommended light intensities (PPFD, defined below) using a reasonable value.  For example, a flowering crop may need 600-900 µmol/m2·s for optimal productivity.

PAR Spectrum

Not all photons are the same. Photons that appear blue have a higher energy and shorter wavelength than photons that appear red. PHOTOSYNTHETICALLY ACTIVE RADIATION (PAR) is the region of the spectrum from 400 to 700 nm. This is the range in which photons optimally contribute to plant growth and development.

Graphic showing average plant photosynthetic response to light (PAR/Relative Response)


PHOTOSYNTHETIC PHOTON FLUX (PPF) is the amount of PAR spectrum light emitted from a light source over time, usually presented in units of µmol/s. The PHOTOSYNTHETIC PHOTON FLUX DENSITY (PPFD), on the other hand, is the instantaneous of PAR spectrum light hitting a defined area, usually presented in units of µmol/m2·s.


In CONTROLLED ENVIRONMENT AGRICULTURE (CEA), it is extremely important to take accurate light measurements of PPFD. This allows optimization of light levels for different crops and stages of development. The compounding effect of multiple fixtures and wall reflectivity (especially in shallow racking found in VERTICAL FARMING applications) emphasizes the need for accurate light measurements over and throughout the canopy.


PHOTOBIOLOGISTS use light measurement data to determine ideal light levels and amount of light to supply different crops per day. This term is known as the DAILY LIGHT INTEGRAL (DLI), and is essentially PPFD x Time . For greenhouses, it is important to obtain accurate natural light readings inside, in order to determine how much additional light is needed to achieve the best growing conditions.

PAR Light Meter

PAR Light Meter

To find out more, view this video of Dr Bruce Bugbee, the President of Apogee Instruments and a professor at Utah State University, explaining PAR, PPF, PPFD:


Controlled Environment Agriculture (CEA) A technology-based approach toward food and medicine production. The aim is to provide protection and maintain optimal growing conditions throughout the development of the crop. Production takes place within an enclosed growing structure such as a greenhouse or building. CEA optimizes the use of resources such as water, energy, space, capital, and labor.
Daily Light Integral (DLI) The number of PAR photons received per square meter of canopy over the course of a day.
Electromagnetic Radiation This includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays, characterized by photon wavelength.  Visible light and PAR light are defined as 400-700 nanometers.
Mole (mol) A standard scientific unit to measure large quantities of small entities, such as molecules or photons.  One mol = 6.02 x 1023 photons.
Photobiologist A scientist who studies how light interacts with biology/life.
Photosynthetically Active Radiation (PAR)
The wavelength range that plants are able to utilize for photosynthesis and is defined as photons between 400 and 700 nm.
Photosynthetic Photon Flux (PPF) The number of micromoles of photosynthetically active radiation (PAR) emitting from a light source every second.
Photosynthetic Photon Flux Density (PPFD) The number of micromoles of photosynthetically active radiation (PAR) irradiating a square meter every second.
Vertical Farming the practice of growing food and/or medicine in vertically stacked layers, vertically inclined surfaces and/or integrated in other structures.


1 Comment

  1. Judy

    hello, i have one question for PPFD


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