Solar UV radiation may be measured in radiometric units (irradiance, W.m-2) or spectrally weighted to account for biologically effective UV radiation. The response function considered is the CIE erythema reference action spectrum. This action spectrum represents the ability of UV radiation, as a function of wavelength, to produce just perceptible erythema in human skin. Erythema is the reddening of the skin, an inflammatory response caused by the actinic effect of solar radiation or artificial optical radiation.
The interaction of UV radiation with human skin is considered a photochemical interaction. According to the Bunson-Roscoe law of reciprocity- high level exposure for a short period having the same effect as low-level exposure for a prolonged time. In the consideration of erythema, radiant exposure, the product of irradiance (W.m-2) and exposure time (s) is therefore important.
Minimum Erythemal Dose
The minimum erythemal dose (MED, J.m-2) is defined as the erythemal radiant exposure that produces a just noticeable erythema on a single individual's previously unexposed skin. This measure is subjective with regards determination of the reddening of the skin and depends on many variables including individual skin pigmentation and exposure site. Where MED is used in populations of different skin types, it must be defined for skin type, it use is therefore limited.
Standard Erythemal Dose
The standard erythemal dose (SED, J.m-2) is equivalent to an erythemal radiant exposure of 100 J.m-2. The SED is independent of skin type and a particular exposure dose in SED may cause erythema in fair skin but none in darker skin.
The global solar UV index (UVI) was developed as an easy-to-understand measure of biologically effective UV radiation with a view to promote public awareness of the risks of UV radiation exposure and sun protection. In the initial work done on UVI in Canada, a typical midday summer erythemal irradiance of 250 mW.m-2 was found leading to the (arbitrary) definition of one UV index equal to an erythemally weighted irradiance of 25 mW.m-2. This gives a typical range of UVI 0 to 11+. The attached spreadsheet demonstrates the computation of UVI and the relation to SED.
Measurement of Erythemal Irradiance
The evaluation of erythemal irradiance requires a suitable spectroradiometer configured to measure spectral irradiance over the spectral range 280-400nm in steps of 1nm in the case of the measurement of solar irradiance and 250-400nm in steps of 1nm in the case of artificial sources of light.
A typical configuration might include the DMc150 double monochromator fitted with 2400g/mm diffraction gratings configured to measure 250(280)-400nm with 1nm bandwidth having as global irradiance input optic a D-series cosine corrected input optic coupled to the monochromator using a FOP_UV quartz fibre bundle. The spectroradiometer is typically calibrated using the CL6_H standard of spectral irradiance.
Spectral integrals can be implemented in the following manner. Save the two action spectra attached to this note in the Benwin+ action spectra folder. In Benwin+, go to analysis/ set spectral integrals. Hit "+" to add new lines. Provide a name, spectral range (250-400nm or 280-400nm) and from the pull down list select the pertinent action spectrum. Erythema.action will report the erythemally weighted irradiance. Erythema_UVI will report the UV index.
Henceforth, to the right of the measured result will be reported these two integrals.
WHO, 2006, “Solar Ultraviolet Radiation Global burden of disease from solar ultraviolet radiation” (http://www.who.int/uv/health/solaruvradfull_180706.pdf)
ISO 17166:1999(E)/CIE S 007/E-1998, “Erythema Reference Action Spectrum and Standard Erythema Dose”
CIE International Lighting Vocabulary- http://eilv.cie.co.at/