Environmental robust interferometry for online/in-process surface inspection

The applications of optical interferometry are very extensive. It can be used to measure displacement, vibration, angle, distance, pressure, refractive index and temperatures, to just name a few. As a high accuracy, high sensitivity measurement method, interferometers are normally operated in optical laboratories in which operational environments are well controlled. Any variations in temperature, air flow, air pressure and vibrations in the environment will induce errors to the measurement results, and even make a measurement impossible to conduct. In this paper, we will focus on discussions of the research conducted in our research center on the applications of optical interferometry at a shop floor environment, which is normally subjected to environmental disturbances and vibrations. There are a number of methods to reduce the influence of these noises. While controlling the measurement environment by using a vibration isolation stage and retaining a stable temperature is an effective way to reduce noise for laboratory and off-line applications, it may not be practical in manufacturing conditions, for instance, when a measurement part is too large to be mechanically insulated. In order to extend the application of interferometry to shop floor inspection, two methods can be adapted. One method is by introducing a reference interferometer and vibration compensation system to the main interferometer to compensate the environmental disturbance. Complete common-path interferometers such as the scatterplate interferometer are insensitive to noises as well. These noise reduction approaches are generally used for laser based phase shifting interferometry, for which the applications are limited to measurement of relatively smooth surfaces due to the well-known 2π phase ambiguity problem of monochromatic interferometry. The other method is to collect all the necessary measurement data in just one data sampling. This kind of so called one shot interferometer is immune to the environmental noise and vibrations. We will discuss the above two kinds of interferometry through three case studies: the common path wavelength scanning interferometer, and the single shot line-scan dispersive interferometer and single point dispersed reference interferometer. These researches mainly focus on surface measurement and inspection. The methods discussed here may also be applied to other application areas depending on the optical setup of the interferometer system and the measurement objects. Distance and displacement measurement using interferometry on shop floor environment are also discussed.