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Before entering into the different techniques of optical metrology some basic terms and definitions have to be established. Optical metrology is about light and therefore we must develop a mathematical description of waves and wave propagation, introducing important terms like wavelength, phase, phase fronts, rays, etc. The treatment is kept as simple as possible, without going into complicated electromagnetic theory. | Optical Metrology. Kjell J. Gasvik Copyright 2002 John Wiley Sons Ltd. ISBN 0-470-84300-4 1 Basics INTRODUCTION Before entering into the different techniques of optical metrology some basic terms and definitions have to be established. Optical metrology is about light and therefore we must develop a mathematical description of waves and wave propagation introducing important terms like wavelength phase phase fronts rays etc. The treatment is kept as simple as possible without going into complicated electromagnetic theory. WAVE MOTION. THE ELECTROMAGNETIC SPECTRUM Figure shows a snapshot of a harmonic wave that propagates in the --direction. The disturbance z t is given as z t U cos hn - L u vt 5 1-1 The argument of the cosine function is termed the phase and 8 the phase constant. Other parameters involved are U the amplitude k the wavelength v the frequency the number of waves per unit time k In the wave number The relation between the frequency and the wavelength is given by kv v 1-2 where v the wave velocity z t might represent the field in an electromagnetic wave for which we have v c 3 x 108 m s 2 BASICS Table The electromagnetic spectrum From Young 1968 z E Wavelength Photon energy meters electro n volts 10s IO 12 104 10 102 10 8 1 10 6 IO 2 10 4 10 4 IO-2 IO 6 1 IO 8 102 IO 10 104 10 12 IO6 10- 4 108 io- 6 io10 IO 8 io12 Power transmission A M radio FM radio television Microwaves radar Infrared I Visible light--------- Ultraviolet X - rays Gamma roys cosmic rays o . À A Red 6600 Orange 6100 Yellow 5700 Green 5200 Blue 4700 Violet 4100 The ratio of the speed c of an electromagnetic wave in vacuum to the speed v in a medium is known as the absolute index of refraction n of that medium c n v The electromagnetic spectrum is given in Table . THE PLANE WAVE. LIGHT RAYS 3 Although it does not really affect our argument we shall mainly be concerned with visible light where X 400-700 nm 1 nm 10 9 m v x 1014 Hz THE PLANE WAVE. LIGHT .