Units and conversions

Units:

  • CIE-weighted irradiance
  • SED
  • MED
  • UV-index
  • Wm-2
  • PPFD

    • Conversion factors

    CIE-weighted irradiance (power density)

    From \ To
    Wm-2 (CIE)
    Js-1m-2 (CIE)
    UV-index
    Wm-2 (CIE)
    1
    1
    40
    Js-1m-2 (CIE)
    1
    1
    40
    UV-index
    1/40
    1/40
    1

    Example: 75 mWm-2 CIE-weighted UV equals (75/1000)*40 = 3 on the UV-index scale.

    Irradiation (energy density)

    From \ To
    Whm-2 (CIE)
    Jm-2 (CIE)
    MED*
    SED
    Whm-2 (CIE)
    1
    3600
    3600/210
    36
    Jm-2 (CIE)
    1/3600
    1
    1/210
    1/100
    MED*
    210/3600
    210
    1
    2.1
    SED
    100/3600
    100
    1/2.1
    1

    * There are different MEDs. The MED used in this case is defined below.

    Example: 2.5 MEDs equal 2.5*210/3600 = 0.1458 Whm-2 = 145.8 mWhm-2 CIE-weighted UV.

    CIE-weighted irradiance

    There is an international recommendation to use the so called CIE-action spectrum, McKinlay and Diffey (1987), for to mimic the erythemal effect of UV radiation.

    The irradiance of each wavelength in the UV is weighted by the weighting factor given by the CIE-action spectrum. This gives the CIE-weighted spectral irradiance. Unit e.g. Wm-2/nm.

    If the CIE-weighted spectral irradiance is integrated over wavelength one will get the CIE-weighted irradiance. Unit e.g. Wm-2.

    If the CIE-weighted irradiance is integrated over time one will get the CIE-weighted irradiation. Sometimes referred to as dose. Unit e.g. Whm-2 or Jm-2.

    SED

    One standard erythema dose (SED) is equivalent to an erythemal effective radiant exposure of 100 Jm-2. Further reference to the SED can be found in CIE (1998).

    MED

    A special dose concept the MED has been widely used. One MED (Minimum Erythemal Dose) is the CIE-weighted irradiation of 210 Jm-2.

    Please, note that other dose-amounts have been in use. To avoid confusion it should be clearly stated which action spectrum and which MED that is used.

    Another factor that must be given is the time period over which the integration is done, e.g. hour, day, year etc.

    UV-index

    The UV-index is a non-dimensional unit defined as the CIE-weighted UV-irradiance (Wm-2) multiplied by the factor 40 (W-1m2).

    Wm-2

    Sometimes the irradiance is directly given in energy (J) per time (s) per area (m2). In this case it is necessary to present the wavelength interval and eventual weigthing for each wavelength.

    PPFD

    Quantum units take into account that radiation is made up of discrete parcels of energy called photons or quanta. Conversion of radiometric units to quantum units is wave-length dependent because the energy of a photon is determined by its wavelength. Hence it is not straightforward to convert between irradiance and quantum units. Quantum units are often used for photosyntetically active radiation to quantify the amount of light available for photosynthesis. One such unit is the Photosynthetic Photon Flux Density (PPFD). It is defined as number of photons in the 400 - 700 nm wavelength interval arriving per unit time on a unit area of the flat receiver. Morel and Smith (1974) studied the relationship between total quanta and total energy for aquatic photosynthesis and concluded that 1 W of PAR corresponds to approximately 2.77 × 1018 quanta per second for marine atmospheres above the water surface with Sun altitudes above 22 degrees (solar zenith angle smaller than 68 degrees). This rule of thumb can be helpful to provide a rough estimate of PAR if only measurements in terms of energy are available.

    Example: 1 W/m2 PAR equals 2.77 × 1018 / 6.022141 × 1023 ≈ 4.6 µmol/m2/s. The other way round; 1 µmol/m2/s ≈ 0.22 W/m2 PAR.

    References

    ICNIRP (1995), Global Solar UV Index, A joint recommendation of WHO, WMO, UNEP and ICNIRP, ICNIRP-1/95, ISBN 3-9804789-0-4

    McKinlay A.F. and Diffey B.L. (1987), A reference spectrum for ultraviolet induced erythema in human skin, CIE-Journal 6, pp.17-22.

    Morel, A., and R.C. Smith. (1974). Relation between total quanta and total energy for aquatic photosynthesis, Limnol. Oceanogr., 19(4), 591 - 600.