Precise topography of optical surfaces

The unit of length usuall y used in measuring opt ical s urface fea t ures, by interference of light, is one-half t he wavelengt h of t he monochromatic light that is used. A method is des cribed in whi ch the uni t of lengt h is a much s maller fract ion of the wavelengt h. Topographi c maps of optical s urfaces are made in which the contour inter val is less t han onet hirtieth t he wavelengt h. The smallness of the unit, depends upon t he quality of the surfaces. Irregulari t ies, t o o s mall t o be detected with t he Fizeau or t wo-beam fringes, are made to appear very pro minen t. Surface markings, caused b y t he final polishin g actions, can be made t o stand o u t in bold relief.