The value of the ratio of UVA to UVB in sunlight

Photochem Photobiol. Nov-Dec 2011;87(6):1474-5. doi: 10.1111/j.1751-1097.2011.00980.x. Epub 2011 Aug 25.


The objective of this communication is to present the calculated ratio between UVA and UVB irradiance from sunrise to sunset and under a number of weather conditions. UVA plays an important role in the sun spectrum and a lot of attention has been paid lately regarding the protection of people from UVA. Solar spectra were collected in Kuwait City located at 29.3° North latitude (similar to that of Houston, TX) over a period of 8 months and under various weather conditions. Spectra were collected from 260 nm to 400 nm in 2 nm increments for solar elevation angles from 10° to 90° using a calibrated Optronics Laboratories OL-742 Spectroradiometer. The measurements reported in this study the ratio of UVA (320-400 nm) to UVB (280-320 nm) in solar terrestrial radiation remains essentially constant and equal to 20 for the part of the day when the solar elevation is greater than 60°. Consequently the value of the ratio of solar UVA/UVB should be considered as equal to 20 for studies in photobiology and photomedicine. When the wavelength limiting the range of UVA and UVB is 315 nm (i.e. UVB: 280-315 nm and UVA: 315-400 nm) the ratio of UVA to UVB becomes equal to 41.

Publication types

  • Letter