APPLICATION NOTE NO. 11 General

PAR Light Sensors
June 2005

Application note in pdf format

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 the Earth’s surface comprises a mixture of ultraviolet light, visible light, and near-visible infrared radiation. All of this radiation conveys heat; the portion between approximately 400 nm and 700 nm wavelength can be captured and used by photo-autotrophs (organisms that are capable of obtaining their 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/m^2.sec or μEinsteins/m^2.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

where

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) file to provide SEASOFT output in μEinsteins/m^2.sec. To calculate irradiance in other units:

Notes:

• Edit the CTD configuration (.con) file using the Configure menu (in SEASAVE or SBE Data Processing in our SEASOFT-Win32 suite of programs) or SEACON (in SEASOFT-DOS).
• 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.

Last modified: 06-Apr-2007

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