Light sources emit light at a certain wavelength in nanometers (nm), which is detected at the other end of a fiber by a photo detector calibrated to the same wavelength. The center wavelength of the light source is measured at the point of peak intensity. Light sources emit a range of wavelengths, not just one. The difference between the lowest and the highest frequency of light is called the spectral width. For practical purposes, spectral width is measured at Full Width-Half Maximum (FWHM); the width (Full Width) is measured at the point exactly halfway between zero power and peak power (Half Maximum). Careful use of this source will provide years of reliable service.