AN11General: PAR Light Sensors
Sea-Bird has several application notes dealing with PAR sensors from various manufacturers; this application note provides an overview of PAR measurements and units, and is applicable to all PAR sensors.
PAR is an abbreviation for Photosynthetically Available Radiation (also called Photosynthetically Active Radiation). Solar radiation reaching Earth’s surface is a mixture of ultraviolet light, visible light, and near-visible infrared radiation. All of this radiation conveys heat; the portion between approximately 400 and 700 nm wavelength can be captured and used by photo-autotrophs (organisms capable of obtaining energy directly from sunlight), and is called PAR.
Irradiance is the flux of solar radiation incident on a surface per unit time per unit area and is reported in units of energy content (Watts/m2) or photon content (quanta/m2•sec or μEinsteins/m2•sec). Conversion from energy to photon content can be made with Planck’s equation, provided that the light wavelength is known. The energy of a photon is related to its wavelength as follows:
E = hc / λ
- h = Planck’s constant (6.626 x 10 -34 Joules sec)
- c = speed of light (2.998 x 10 8 m/sec)
- λ = wavelength (m)
This equation provides the energy for a single wavelength. For a broad spectrum PAR sensor, a wavelength of approximately 550 nm (550 x 10 -9 m) is typically used for the conversion.
“For marine atmospheres with sun altitudes above 22 degrees, the quanta/watt ratio for the region 400 to 700 nm is 2.77 x 10 18 quanta/sec/Watt to an accuracy of plus or minus a few percent.” This quote and further discussion of the relationship of quanta to Watts in the water column is found in Smith and Morel (1974) Limnol. Oceanogr. 19(4):591-600.
E (at 550 nm) = hc / λ = (6.626 x 10 -34 Joules sec) * (2.998 x 10 8 m/sec) / (550 x 10 -9 m)
= 3.61 x 10 -19 Joules
(Note: 1 / 3.61 x 10 -19 = 2.77 x 10 18 quanta/sec/Watt, the value quoted in the above reference.)
Application notes for underwater PAR sensors (11Chelsea, 11Licor, 11QSP-L, and 11QSP-PD) and surface PAR sensors (11S and 47) describe how to enter coefficients from the manufacturer’s calibration in the CTD configuration (.con or .xmlcon) file to provide Seasoft output in μEinsteins/m2•sec. To calculate irradiance in other units:
|To convert to:||For Underwater PAR Sensors,
set Multiplier to:
|For Surface PAR Sensors,
multiply calculated Conversion factor by:
|μEinsteins/cm2•sec||(1.0) / (100 cm/m) 2 = 1 x 10 -4|
|Einsteins/m2•sec||(1.0) / (1 x 10 6 μEinsteins/Einstein) = 1 x 10 -6|
|Einsteins/cm2•sec||(1 x 10 -6) / (100 cm/m) 2 = 1 x 10 -10|
|quanta/m2•sec||(1 x 10 -6) * (6.022 x 10 23 quanta/Einstein) = 6.022 x 10 17|
|quanta/cm2•sec||(6.022 x 10 17) / (100 cm/m) 2 = 6.022 x 10 13|
|Watts/m2||(6.022 x 10 17) / (2.77 x 10 18 quanta/sec/Watt) = 0.2174|
|Watts/cm2||(0.2174) / (100 cm/m) 2 = 2.174 x 10 -5|
|μWatts/m2||(0.2174) * (1 x 10 6 μWatts/Watt) = 2.174 x 10 5|
|Note: 1 Einstein = 1 mole (6.022 x 10 23) of photons 1 Watt = 2.77 x 10 18 quanta/sec|
- In our Seasoft V2 suite of programs, edit the CTD configuration (.con or .xmlcon) file using the Configure Inputs menu in Seasave V7 (real-time data acquisition software) or the Configure menu in SBE Data Processing (data processing software).
- Multiplier can also be used to scale output for comparing the shape of data sets taken at disparate light levels. For example, a multiplier of 10 would make a 10 μEinsteins/m2•sec light level plot as 100 μEinsteins/m2•sec.
Application Note Revision History
|- Initial release.|
|May 2007||- Incorporate Seasave V7.
- Eliminate discussion of DOS software.
|February 2010||- Change Seasoft-Win32 to Seasoft V2.
- Add information on .xmlcon configuration file.
- Update address.
|February 2011||- Correct units for h = Planck’s constant (6.626 x 10 -34 Joules/sec corrected to Joules sec)|