ultraviolet radiation

The boundary between visible light and ultraviolet begins at around 400 nanometers, and wavelengths smaller than 100 nm are considered X-rays. The earth’s atmosphere absorbs the ultraviolet with very short wavelengths; the shortest ultraviolet wavelengths that reach earth’s surface are about 286 nm.

The spectrum of ultraviolet radiation has been divided in a number of ways:

A division often used by physicists:

Name of Band	Range of Wavelengths
near UV	400 to 300 nanometers
middle UV	300 to 200 nm
far UV	200 to 100 nm
extreme UV	100 nm to around 40 – 10 nm

A division based on the radiation’s biological effects¹:

Name of Band	Range of Wavelengths
near UV	400 to 315 nanometers
actinic UV	315 to 200 nm
vacuum UV	less than 200 nm

The International Commission on Illumination (the CIE) defined three UV bands², again on the basis of biological effects. This classification is the one encountered in advertisements for sunscreens:

Name of Band	Range of Wavelengths
UV-A	400 to 315 nanometers
UV-B	315 to 280 nm
UV-C	280 to 100 nm

Taking the lower limit of UV-A as 320 nm instead of 315, and the upper limit as 290 instead of 280, is a common practice.

1. A. F. McKinlay
Ultraviolet radiation: Potential hazards.
In Physics in Medicine and Biology Encyclopedia. Vol. 2
Oxford: Pergamon, 1986.

2. Commission Internationale de l’Eclairage.
International Lighting Vocabulary. Third Edition.
Publication CIE 17 nr (E-1.1)
Paris: Commission Internationale de l’Eclairage, 1991.