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Home > Frequently Asked Questions > Measurements > Irradiance > How do I calculate PAR?

How do I calculate PAR?

The amount of energy per photon at a given wavelength is computed as:

E = hc/L

    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.

  1. Multiply by collection area (cm^2): [µWatts/cm^2/nm] * cm^2 = µWatts/nm
  2. Multiply by integration time (in seconds): [µWatts/nm] * s = µJ/nm
  3. Divide by E (as computed above for this wavelength): [µJ/nm] / E = (photons * 10^6)/nm
  4. Divide by Avagadro’s constant (A = 6.022 * 10^23): [(photons * 10^6)/nm] / A = µmol photons/nm
  5. Integrate from 400nm to 700nm = µmol photons
  6. Divide by collection area in m^2 = µmol photons / m^2
  7. 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).





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