A unit of erythemal flux density. One finsen is one unit of erythemal flux per square centimeter. Symbol, FU. “Erythemal” means “skin reddening,” and the finsen was used to measure the intensity of ultraviolet radiation in terms of its capacity to cause reddening of the skin.

The “unit of erythemal flux” (the names E-viton and erytheme have been suggested for it) is the amount of radiant flux that causes the same amount of erythemal effect as 10 microwatts of radiation of wavelength 296.7 nanometers.¹

The finsen is named for Niels Ryberg Finsen (1860-1904), a Danish physician who won the Nobel Prize for Physiology in 1903 for his work on light in the treatment of disease. Ryberg discovered that exposing smallpox patients to light rich in red and deficient in violet wavelengths tended to prevent scarring,² (a treatment soon pooh-poohed³), and he developed successful methods for treating a form of skin tuberculosis with ultraviolet light.

1. American Standard Definitions of Electrical Terms.
New York: American Institute of Electrical Engineers, 1941.

See references 55.15.015 and 55.15.020.

2. Niels Ryberg Finsen.
The chemical rays of light and smallpox.
In Niels Ryberg Finsen, Phototherapy, J. H. Sequiera, translator.
New York: Arnold, 1901.

3. T. F. Ricketts and J. B. Byles.
The red light treatment of smallpox.
Lancet, volume 2: 287-290. (1904)

J. F. Schamberg.
The passing of the red light treatment of smallpox.
Journal of the American Medical Association, volume 43: 1641-1642. (1904)

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