Thorough knowledge of the physical nature of light and light perception provides the foundation for a comprehensive understanding of optical measurement techniques. Yet, from a practical point of view there is little necessity to fully understand formation and propagation of light as an electromagnetic wave as long as the reader accepts wavelength as the most important parameter describing the quality of light. The human eye perceives light with different wavelengths as different colors (figure II.1.), as long as the variation of wavelength is limited to the range between 400 nm and 800 nm (1 nm = 1 nanometer = 10^-9 m). In the optical range of the electromagnetic spectrum, wavelength is sometimes also given in Ångstrøm (Å= 10^-10 m). Outside this range, our eye is insensitive to electromagnetic radiation and thus we have no perception of ultraviolet (UV, below 400 nm) and infrared (IR, above 800 nm) radiation.



Fig. II.1. Monochromatic electromagnetic radiation of different wavelengths between 400 nm and 800 nm causes the impression of different colors. Outside this wavelength range, the human eye is insensitive.