Photosynthetically Active Radiation (PAR)

Photosynthetic organisms such as macro algae, cyanobacteria, and terrestrial plants utilize energy from radiation that has wavelengths ranging from 400 to 700 nm. Ratiation in this range is typically referred to as Photosynthetically Active Radiation or Photosynthetically Available Radiation (PAR). Pigments in the cells of the organism vary, with each type absorbing certain wavelengths of light more effectively than others. For example, Chlorophyll a is an important photosynthetic pigment for both eukaryotic algae and cyanobacteria, and it is most effective at absorbing radiation with wavelengths of approximately 450 nm [1].

PAR measurements are a metric of the number of photons that impact the sensor over a period of time, or photons per unit time per unit surface are [2]. Common units of measurement for Photosynthetic Photon Flux Density (PPFD) are micromoles of quanta per second per square meter (µmol s-1  m-2 ) [2].

How it works


LI-COR is one major manufacturer of PAR sensors, and all of their instruments use silicon photodiodes with colored glass filters and interference filters [2]. A photodiode is an application of the photoelectric effect,an phenomenon whereby an photon impacts an atom and causes the excitation of a valence electron, with a subsequent release of energy that can be corellated to the wavelength of the initial photon. A diode translates the released energy into an electrical signal, the voltage of which is ultimately converted back into a measure of the incoming photons of light (µmol s-1  m-2 ) [3].



The material that the diode is constructed of dictates the sensitivity of its response to incoming photons, and silicon is used in LI-COR PAR sensors and many cameras because of its strong response across the full range of visible light [4].

However, silicon diodes respond to wavelengths of light that are outside of the desired range for PAR (400-700 nm), so interference filters are used to prevent photon with wavelengths above 700 nm from impacting the sensor [2].

Sensor Variations
There are two primary variants of PAR sensors that would be used by an oceanographer:


 * Standard quantum - a standard PAR sensor only measures incident radiation that impacts the sensor from directly above, PPFD. For terrestrial application a sensor such as the LI-190 is sufficient [2], but it is inadequate for measuring diffuse multidirectional light in an underwater environment.




 * Spherical quantum  - a spherical PAR sensor such as the LI-193SA is designed to capture incident light from all directions. This metric is referred to as Photosynthetic Photon Flux Fluence Rate (PPFFR), and is particularly relevant when studying planktonic organisms. A portion of the bottom of the sensor is blocked by the detector housing. The decreased response is not considered to be significant in most cases because of the relatively low level of radiation from below the sensor as compared to total radiation.
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