Optical Profilometer - How It Works And Its Advantages

By Author: Vishal Khosla
Total Articles: 19
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When the distance of the reference mirror to the beam splitter is the same as that from the splitter to the test surface, interference is observed in the combined beam whenever the beam lengths differ. Light and dark bands are observed in the recombined beam.

Optical profilometry is a technique that allows for non-destructive, noncontact and rapid surface metrology. It refers to the measurement of small-scale features on a surface including surface roughness, waviness and primary form of the surface. Surface metrology is important when two surfaces are mated or during machining of precision parts because of its influence on behaviors such as friction and adhesion and also their manufacture, fracture, wear and tear, etc. Whereas the earlier profilometers measured the surface parameters as it was moved against the profilometer's stylus that was in contact, modern profilometers are primarily of the noncontact type. They use the principle of optical interference profiling. The principle of optical profiling uses light's wave properties to compare the path difference between reference and test surfaces for accurate surface ...
... measurements.

An optical profilometer is actually a type of microscope in which a beam splitter splits the light from a lamp source. One path takes the light to the surface to be tested and the other path directs light to a reference plane. The reflections that emanate from the two surfaces are recombined. When the distance of the reference mirror to the beam splitter is the same as that from the splitter to the test surface, interference is observed in the combined beam whenever the beam lengths differ. Light and dark bands (interference fringes) are observed in the recombined beam. As the reference mirror surface is close to perfect flatness, the light and dark bands are due to variances on the test surface.

A digital camera sees the interference fringes as lighter and darker areas when the recombined beam is projected on to it. In the picture or interferogram, the transition from each of the lighter to the darker bands represents one half of wavelength difference between the test and reference paths. The height differences over an entire surface can be calculated if the wavelength is known. A 3D map of the surface can be drawn out if the height differences are known.

In practice, optical profilers scan the surface vertically and interference is created when the surface lying in the field of view passes through the focal plane. Points at different heights on the surface reach the optimal focus at different points in time. The greatest contrast occurs when each height level is in optimal focus. With optical profilers that are well calibrated, vertical resolution of the order of nanometers (1 nanometer = 10 Angstroms) is possible. An optical profilometer is used in applications when accurate measurement of surface variations is required. For example, the data storage industry uses highly polished surfaces whose surface variations need to be measured with these instruments. They are used to study the effects of polishing and grinding and other material processing procedures. They are used extensively in wear analysis to calculate the volumes of scratches and voids on surfaces.

The primary advantage of using an optical profiler is that it is noncontact and is three dimensional whereas stylus profilometers are two dimensional. Optical profilers have high sampling resolutions and more data is obtained with each measurement. Maintenance costs are low as there is no need to replace the stylus.

Author of this article is associated with Rtec Instruments that offers instruments such as pin on disk tribometer, atomic force microscope, chemical mechanical polisher, multi-functional tribometer, stress tester and many more.