The optical spectrum
 
We have seen earlier that the Earth’s atmosphere has a window which allows transmission of electromagnetic radiation with a range of frequencies, the centre of the band being close to the peak sensitivity of the eye. The eye is not sensitive to all of the frequencies which arrive at the bottom of the atmosphere in this band. Energy in the form of near ultraviolet and infrared radiation is arriving from space and penetrates down to ground-level but is not sensed by the eye. It can, however, be collected by ordinary optical telescopes and made available for measurement by other detectors. There are also telescopes which are designed with high efficiency for infrared measurements.
For convenience in the following discussion, we shall call this window which includes the visible wavelengths the optical window and the range of frequencies which it covers the optical spectrum. The term visible spectrum is reserved to describe the range of frequencies which can be detected by eye.
Before discussing three important types of detector which are used for making measurements in the optical spectrum, it will be useful to consider the concepts of spectral sensitivity.