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Ocean Optics
Worldwide Headquarters
Largo, Florida, USA

+1 727-733-2447

[email protected]

Sales, Service
& Support Facility
Duiven, The Netherlands

+31 26-319-0500
+33 442-386-588

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Ocean Optics GmbH Sales,
Service & Support Facility
Ostfildern, Germany

+49 711-34-16-96-0

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Sales Support
for the
United Kingdom

+44 1865-819922

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Sales, Service
& Support Facilities
Shanghai, PRC – Beijing, PRC

+86 21-6295-6600

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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).