Profilometer is most commonly used in U.S.A. It is used for direct measurement of surface quality. It is a dynamic instrument and principle is similar to gramophone pick-up.

The surface roughness is concerned with both size and shape of irregularities. Both size and shape i.e. height of irregularities, ir spacing and form are important for specifying surface roughness.

Microscopic inspection method have some limitation that at a time only a small portion of surface can be inspected. So for inspection, several readings are required in this method to get an average value.

Second order Irregularities caused by vibrations due to any reason such as chatter marks. If machine is perfect and free of vibrations, some irregularities can be caused by machining itself due to characteristic of process, such type of irregularities include third order. Fourth order includes irregularities due to rupture of material during chip removal.

There are difficulties in measurement of surface roughness in three dimensional geometry, but for simplification purposes, it is reduced into two dimensional geometry. To measure, direction is generally perpendicular to direction of predominant surface markings or ?lay?.

Mean line of profile is line which have form of geometrical profile. Mean line of profile divides effective profile so that sum of squares of distances between effective points and mean line is a least. When waveform is repetitive, centre and mean line are equivalent.

In one method of microscopic inspection, a straight edge is placed on surface to be inspected and a beam of light is projected at about 60 degree to work. The shadows cast into surface scratches are magnified and studied.

If any surface has large number of irregularities i.e. hills and valley n surface will obtain good bearing properties. Rate of wear is proportional to load per unit area and surface area in contact.

The error or irregularity falls under first and second order are also termed as macro-geometrical errors and irregularities or error of third and fourth order are termed as micro-geometrical errors.

In method of inspection of surface by using micro interferometer, an optical flat is used which is placed on surface to be examined and illuminated by using monochromatic light. Interference bands are studied by using a microscope. Defects or scratches appear as interference lines extending from dark bands into bright bands.