How do I calculate PAR?
The amount of energy per photon at a given wavelength is computed as:
E = hc/L where: E is the total energy measured in Joules h is the Planck constant (6.626E-34 J*s) c is the speed of light (299792458 m/s) L is the wavelength of the photon measured in meters
Given that absolute irradiance is in units of uWatts/cm^2/nm, the conversion is as follows.
- Multiply by collection area (cm^2): [µWatts/cm^2/nm] * cm^2 = µWatts/nm
- Multiply by integration time (in seconds): [µWatts/nm] * s = µJ/nm
- Divide by E (as computed above for this wavelength): [µJ/nm] / E = (photons * 10^6)/nm
- Divide by Avagadro’s constant (A = 6.022 * 10^23): [(photons * 10^6)/nm] / A = µmol photons/nm
- Integrate from 400nm to 700nm = µmol photons
- Divide by collection area in m^2 = µmol photons / m^2
- Divide by int. time in seconds = µmol photons / m^2 / seconds
For additional background, see Wikipedia’s entry on Photosynthetically Active Radiation.
You can set up a PAR measurement using the Schematic feature in the OceanView software. An example is demonstrated in the pictures below (click to expand).