Light Sources and Detectors The most important ‘hardware’ in optical metrology is light sources and detectors. To appreciate the various concepts of these devices, we first introduce the different units and terms for the measurement of electromagnetic radiation. Then the laser is given a relatively comprehensive treatment. The description of detectors involves some understanding of semiconductor technology. Therefore a brief introduction to semiconductors is given in Appendix E. Because of the increasing use of the CCD camera in optical metrology, this device is described separately in Section . . | Optical Metrology. Kjell J. Gasvik Copyright 2002 John Wiley Sons Ltd. ISBN 0-470-84300-4 5 Light Sources and Detectors INTRODUCTION The most important hardware in optical metrology is light sources and detectors. To appreciate the various concepts of these devices we first introduce the different units and terms for the measurement of electromagnetic radiation. Then the laser is given a relatively comprehensive treatment. The description of detectors involves some understanding of semiconductor technology. Therefore a brief introduction to semiconductors is given in Appendix E. Because of the increasing use of the CCD camera in optical metrology this device is described separately in Section . RADIOMETRY. PHOTOMETRY To compare light sources we have to make a brief introduction to units and terms for the measurement of electromagnetic radiation Slater 1980 Klein and Furtak 1986 Longhurst 1967 . Below we present the most common radiometric units. Radiant energy Q is energy travelling in the form of electromagnetic waves measured in joules. Radiant flux dQ dt is the time rate of change or rate of transfer of radiant energy measured in watts. Power is equivalent to and often used instead of flux. Radiant flux density at a surface M E d QA is the radiant flux at a surface divided by the area of the surface. When referring to the radiant flux emitted from a surface it is called radiant exitance M . When referring to the radiant flux incident on a surface it is called irradiance E. Both are measured in watts per square metre. Note that in the rest of this book we use the term intensity which is proportional to irradiance. Radiant intensity I 3 3Q of a source is the radiant flux proceeding from the source per unit solid angle in the direction considered measured in watts per steradian. Radiance L d2 dQdA cos 0 in a given direction is the radiant flux leaving an element of a surface and propagated in directions defined by an elementary cone containing the given
