that radiant energy exhibits trated into a small range of wavelengths. If light from a discharge primarily wave properties while at the shorter wavelengths, x-ray lamp such as sodium is examined by some sort of spectrum analyser and shorter, radiant energy primarily exhibits ray and quantum it is possible to observe the various wavelengths of the components properties. In the region of the optical spectrum ray, wave, and which it contains. Typically these would be a number of fairly quantum properties will have their importance to varying degrees. narrow wavelength ranges which contain most of the energy and which are separated by much wider regions of darkness. Each narrow band of energy constitutes what is known as a spectral line which can be termed monochromatic. However it should be noted The optical spectrum that the spectral lines are never infinitely sharp. They always consist of a small band of frequencies however narrow which make up what Light is electromagnetic radiant energy and makes up part of the is called the linewidth. A laser line is monochromatic but again will electromagnetic spectrum. The term optical spectrum is used to have a finite linewidth. describe the light portion of the electromagnetic spectrum and embraces not only the visible spectrum (that detectable by the eye) but also the important regions in optoelectronics of the ultraviolet and infrared. 7.3 Radiometry and photometry The optical spectrum forms only a very narrow region of the complete electromagnetic spectrum. Figure 7.1 is an expanded Radiometry is the science and technology of the measurement of diagram showing more detail of the ultraviolet, visible and infrared radiation from all wavelengths within the optical spectrum. The regions. By convention, optical radiation is generally specified basic unit of power in radiometry is the watt (W). according to its wavelength. The wavelength can be determined Photometry is concerned only with the measurement of light from a specific electromagnetic frequency from Equation 7.1, det