The purpose of the classic Lloyd mirror was to duplicate a point source by specular reflection into a second source (which thereby becomes coherent to the first) and to then observe interference. Figure 9.1(a) illustrates the scheme. In fact, interference can be observed at the end of a flat component (not to say mirror), covering the entire length of the component, and on some surface areas to the right and left of the plane of incidence. Fringes first run parallel to the surface and soon take on a hyperbolic slope to both sides of the incident plane. Therefore, a meaningful evaluation is restricted to zones near the incident plane. There is a temptation to use this evaluation for metrology; however, due to the variable angle of incidence, the applications would be restricted to very undemanding tasks.
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