Lapped

Flatness

Another method that is commonly used with lapped parts is the reflection and interference of monochromatic light. A monochromatic light source and an optical flat are all that are needed. The optical flat – which is a piece of transparent glass that has itself been lapped and polished on one or both sides – is placed on the lapped surface. The monochromatic light is then shone down through the glass. The light will pass through the glass and reflect off the workpiece. As the light reflects in the gap between the workpiece and the polished surface of the glass, the light will interfere with itself creating light and dark fringes called Newton's rings. Each fringe – or band – represents a change of one half wavelength in the width of the gap between the glass and the workpiece. The light bands display a contour map of the surface of the workpiece and can be readily interpreted for flatness. In the past the light source would have been provided by a helium-neon lamp or tube, using the neon 632.8 nm line, or mercury vapor green line but nowadays a more common source of monochromatic light is the low pressure sodium lamp. Today, laser diodes and LEDs are used, both being inexpensive and narrow-band light sources. With semiconductor light sources, blue is an option, having a smaller wavelength than red.