



Wm^{2} (CIE) 



Js^{1}m^{2} (CIE) 



UVindex 



Example: 75 mWm^{2 } CIEweighted UV equals (75/1000)*40 = 3 on the UVindex scale.





Whm^{2} (CIE) 




Jm^{2} (CIE) 




MED* 




SED 




* 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 } CIEweighted UV.
There is an international recommendation to use the so called CIEaction 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 CIEaction spectrum. This gives the CIEweighted spectral irradiance. Unit e.g. Wm^{2}/nm.
If the CIEweighted spectral irradiance is integrated over wavelength one will get the CIEweighted irradiance. Unit e.g. Wm^{2}.
If the CIEweighted irradiance is integrated over time one will get the CIEweighted irradiation. Sometimes referred to as dose. Unit e.g. Whm^{2 }or Jm^{2}.
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).
A special dose concept the MED has been widely used. One MED (Minimum Erythemal Dose) is the CIEweighted irradiation of 210 Jm^{2}.
Please, note that other doseamounts 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.
The UVindex is a nondimensional unit defined as the CIEweighted UVirradiance (Wm^{2}) multiplied by the factor 40 (W^{1}m^{2}).
Sometimes the irradiance is directly given in energy (J) per time (s) per area (m^{2}). In this case it is necessary to present the wavelength interval and eventual weigthing for each wavelength.
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 wavelength 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 × 10^{18} 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/m^{2} PAR equals 2.77 × 10^{18} / 6.022141 × 10^{23} ≈ 4.6 µmol/m^{2}/s. The other way round; 1 µmol/m^{2}/s ≈ 0.22 W/m^{2} PAR.
ICNIRP (1995), Global Solar UV Index, A joint recommendation of WHO, WMO, UNEP and ICNIRP, ICNIRP1/95, ISBN 3980478904
McKinlay A.F. and Diffey B.L. (1987), A reference spectrum for ultraviolet induced erythema in human skin, CIEJournal 6, pp.1722.
Morel, A., and R.C. Smith. (1974). Relation between total quanta and total energy for aquatic photosynthesis, Limnol. Oceanogr., 19(4), 591  600.