The length of the light wave, usually measured from crest to crest, which determines its color. Common units of measurement are the micrometer (micron), the nanometer, and (earlier) the Angstrom unit
US Department of Labor, Eye and Face Protection eTool: Glossary, Occupational Safety & Health Administration OSHA, Washington, D.C., USA 2010. https://www.osha.gov/SLTC/etools/eyeandface/glossa ry.html, (OSHA/ Eye and Face Prot - Amer) {622}