Optical Techniques for Measurement of Chamber Spacing

Two optical methods were investigated for accurately measuring the gap, or chamber spacing, that separates two closely spaced transparent plates. The first method uses a special microscope in which the main feature is a unit-magnification catadioptric system that gives an aberration-free image of the chamber outside the plates, where it is accessible to a high-power objective. The second method is based upon the light section principle, whereby the image of a slit is projected onto the boundaries of the chamber and is thus doubled. The reflected images are observed with a microscope and the degree of separation, which is proportional to the chamber spacing, is measured. Accuracy better than 2 µm is obtained for the two techniques. The choice of the appropriate method depends on the surface quality of the chamber boundaries.