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QUAKTZ PLATE
Fig. 1. Schematic of optical system used on interferometer gauge.
2.7 X 1014 cycle/sec. The gain of this amplifier is as constant as the wavelength of light. This, of course, is as good as any quantity we know of and is actually used as the fundamental standard of length measurement.
With such a satisfactory amplifier the characteristics of a pressure measuring device depend entirely on the mechanics of the diaphragm and the recording system used.
A photographic and optical system designed to use such an amplifier was first described in October 1948,1-2 but is briefly described here again to clarify the remainder of this paper for those who are not familiar with the interferometer gauge.
Optical System
In Fig. 1, a steel diaphragm is receiving a transient pressure as represented by the hammer blow. The diaphragm deflects slightly in response to the pressure, and it is this slight deflection that we wish to record on the moving film. To do this a quartz backing plate, with one face ground and polished spherically concave on a large radius, is placed next to the flat side of the diaphragm. The outer edges of the plate are ground and polished flat
to make a highly stable reference with respect to the steel diaphragm.
The spherical cavity in the quartz is coated with a half-reflecting film of aluminum so that when the assembly is viewed in monochromatic light a set of sharply defined interference fringes, or Newton's rings, is formed. If the monochromatic light is admitted through a glass prism as shown in the diagram, however, only a narrow strip of this set of rings is formed. The rings then appear as short sections of arcs and may be photographed on a moving film as distinct parallel lines. Any movement of the diaphragm, however, causes a change in the air space between the quartz plate and the steel, which makes an amplified movement at the sections of arc and a corresponding change in the lines recorded on the moving film.
In practice, the quartz backing piece is ground to produce approximately 50 fringes. Since a deflection of about one-tenth of a fringe can be measured on the film, the displacement of the diaphragm can be measured to about one part in 500.
As long as the deflection of the diaphragm stays well below the elastic limit of the steel or quartz used, it is strictly proportional to the pressure applied. The number of fringes from
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November 1952 Journal of the SMPTE Vol. 59