Interferometry notes - Summary Maths for Engineers Custom Book Source Book

Interferometry notes - Summary Maths for Engineers Custom Book Source Book Engineering () Interferometry Chapter Outline  Interferometer  Interferometry  Optical flats  Autocollimator Interferometers: They are optical instruments used for measuring flatness and determining the length of the slip gauges by direct reference to the wavelength of light. It overcomes the drawbacks of optical flats used in ordinary daylight. In these instruments the lay of the optical flat can be controlled and fringes can be oriented as per the requirement. An arrangement is made to view the fringes directly from the top and avoid any distortion due to incorrect viewing. Autocollimators This is an optical instrument used for the measurement of small angular differences. For small angular measurements, autocollimator provides a very sensitive and accurate approach. Auto-collimator is essentially an infinity telescope and a collimator combined into one instrument. The principle on which this instrument works is given below. O is a point source of light placed at the principal focus of a collimating lens in Fig. 8.30. The rays of light from O incident on the lens will now travel as a parallel beam of light. If this beam now strikes a plane reflector which is normal to the optical axis, it will be reflected back along its own path and refocused at the same point O. If the plane reflector be now tilted through a small angle 0, [Refer Fig] then parallel beam will be deflected through twice this angle, and will be brought to focus at O’ in the same plane at a distance x from O. Obviously OO’=x=2θ.f, where f is the focal length of the lens. There are certain important points to appreciate here : The position of the final image does not depend upon the distance of reflector from the lens, ation x is independent of the position of reflector from the lens. But if reflector is moved too much back then reflected rays will completely miss the lens and no image will be formed. Thus for full range of readings of instrument to be used, the maximum remoteness of the reflector is limited. For high sensitivity, i.e., for large value of x for a small angular deviation θ, a long focal length is required. 1. Principle of the Autocollimator. A cross-line “target” graticule is positioned at the focal plane of a telescope objective system with the intersection of the cross-line on the optical axis, i.e. at the principal focus. When the target graticule is illuminated, rays of light diverging from the intersection point reach the objective via a beam splitter and are projected-from the objective as parallel pencils of light. In this mode, the optical system. is operating as a “collimator” A flat reflector placed in front of the objective and exactly normal to the optical axis reflects the parallel pencils of light back along their original paths. They are then brought to focus in the plane of the target graticule and exactor coincident with its intersection. A proportion of the returned light passes straight through the beam splitter and the return image of the target crossline is therefore visible through the eyepiece. In this mode, the optical system is operating as a telescope focused at infinity.