UV LIGHT:

UV light is the electromagnetic energy at wavelengths between X-rays and visible light. The electromagnetic spectrum is illustrated below.

Electromagnetic Spectrum

UV light is broken down into three regions: UV-A, UV-B, and UV-C. UV-A consists of wavelengths from 315 – 400 nm. These are the wavelengths that cause skin to tan. UV-B consists of wavelengths from 280 – 315 nm. These are the wavelengths that cause sunburn. UV-C consists of wavelengths from 100 – 280 nm. These are the wavelengths of interest in water disinfection because they damage DNA and RNA in cells. The germicidal range is where DNA absorbance is the highest, 240 – 290 nm with the peak at 265 nm. The figure below illustrates the DNA absorbance spectra.

UV Absorbance of Nucleotides and Nucleic Acid at pH 7 (USEPA, 2003)

This germicidal UV does not immediately inactivate the organism but introduces genetic interference that inhibits replication of the organism.

Key measurements assessing the amount of light absorbed by the transmitting medium (e.g. water) are UV absorbance and UV transmittance. The UV absorbance is determined by the following equation:

A = log10(Io / It) = log10(1 / T)
Where:
A = absorbance
Io = initial UV intensity
It = measured UV intensity at receptor

The percent transmittance is determined by the following equation:

%T = (It / Io) X 100% or %T = 10^-A
Where T = percent transmittance