Self-Calibration: Reversal, Redundancy, Error Separation, and ‘Absolute Testing’

Refractive indices of zinc sulfide in the 0.405–13-μm wavelength range

We introduce a technique to measure transparent glass slab thickness. It employs a very simple setup combining two interferometers: a forward-going beam scheme and a self-mixing readout of the beam reflected back to the laser source. Interestingly, the difference of the two readouts provides a quantity related to thickness measurement, irrespective of refractive index. We demonstrate this method using samples on a range of thickness from 30 to 1000μm.

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Thickness measurement of transparent plates by a self-mixing interferometer

We report a simple method of measuring the absolute values of the phase refractive index of an optical material of a flat plate shape over a wide spectral range at a single measurement run A white-light interferometric technique with angle rotation of the optical plate sample located in one of the interfer ometer arms was used in this method The validity of this method was proved by measuring the absolute phase refractive indices of flat plate samples of fused silica and BK7, and by comparing them with calculated values from their well-known Sellmeier dispersion formulas The accuracy of this refractive index measurement method was within 0002, which can be further improved by enhancing the angle measurement accuracy of the angle rotating stage used in this method

Absolute refractive index measurement method over a broad wavelength region based on white-light interferometry

We describe a variation of the liquid-flat technique fordetermining the absolute flatness of a 240-mm-diameter optical surfaceto an accuracy better than 1/100lambda in both its horizontal(three-point support) and vertical orientations. Using theappropriate mathematics to calculate the surface deformation of a diskdue to gravity, we achieved verification of the method by comparingmeasurements carried out on a pair of optical flats and a liquidreference surface.

Absolute figure measurements with a liquid-flat reference.

Profile images of cylinders and spheres produced by conventional methods of optical inspection are frequently flanked by systems of interference fringes. This phenomenon is analysed theoretically and the results are confirmed by experiment. The conclusions are relevant not only to unfiltered inspection systems but also to those employing spatial filtering techniques. In discussing the latter, a distinction is drawn between thin specimens, for which the design of spatial filter is unrestricted, and those of finite radius, for which the filter design must be carefully optimized. It is shown that the central minimum of the spatially filtered pattern delineates the true edge only if the radius of the specimen is zero. In other circumstances the positional error is a function of the cube root of the radius of the specimen and is predictable. The applicability of spatial filtering to profile microscopy and profile projection is discussed. Photographic examples of screw-threads taken with two-dimensional filters...

The Application of Spatial Filtering Techniques to Profile Inspection, and an Associated Interference Phenomenon

The paper describes a recording refractometer which has been designed to measure the refractive indices of optical glasses throughout their spectral transmitting range with greater reliability than has hitherto been possible; uncertainties of ±1 × 10-5 are obtainable. After a short history of earlier instruments, the design and construction are discussed in some detail. A section on measuring and calculating procedures is followed by an analysis of uncertainties and a brief consideration of future trends.

An Improved Recording Refractometer for Optical Glasses in the Wavelength Range 300 to 2600 nm

Two potential causes of instability in optical flats are considered A number of experimental results are quoted in which significant changes of figure have occurred within periods of a few years Attempts to correlate these changes with birefringence measurements have met with limited success The relative merits of glass and fused silica flats are discussed

G.D. Dew

Papers

The Application of Spatial Filtering Techniques to Profile Inspection, and an Associated Interference Phenomenon

An Improved Recording Refractometer for Optical Glasses in the Wavelength Range 300 to 2600 nm

Some Observations on the Long-term Stability of Optical Flats