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Conoscopy Measures with High Resolution

Photonics Spectra
Jun 1997
Lawrence Normie in Jerusalem

JERUSALEM -- Manufacturers of electronic components and data storage equipment are among those who can benefit from the high (up to 0.04 µm) resolution of gauging instruments based on conoscopic interferometry.
The technique exploits the phenomenon of conoscopy, the interference behavior of doubly refractive crystals under convergent, polarized light. In the devices, a ray from an incoherent monochromatic point light source strikes a uniaxial crystal, which splits it into two rays. These rays propagate along nearly identical geometrical paths within the crystal at different velocities: One is isotropic and the other anisotropic, according to the original ray's angle of incidence. The wavefronts emerge from the crystal with different relative phase and cross-polarization angles. Two circular polarization plates align the directions of the rays' electrical field components. Recombining the rays produces a fringe pattern (Gabor Zone Lens) at the output.
The technique eliminates stability and sensitivity problems associated with conventional laser-based interferometry. In addition, whereas fringe pattern scale limits the use of interferometry to optically flat surfaces, conoscopic techniques can be adapted to any scale of measurement.
Conoscopy also offers advantages over triangulation methods, which employ
separate light paths and so are subject to platform vibration and local temperature variations. Conoscopic configurations are inherently collinear, which provides immunity to mechanical disturbances, optical noise and temperature. And their collinearity is not dependent on a wide-angle aperture, thus enabling the development of instruments that can work with telescopes, microscopes and charge-coupled device cameras along a common optical axis and with little modification. A conoscopic system also can work with difficult geometries, such as steep slopes of up to 85°, where triangulation-based distance probes are not accurate.
Optical Metrology Ltd. (Optimet), a spinoff of Ophir Optronics Ltd., has developed prototype conoscopic demonstration devices including rangefinders, surface profilers and roughness gauges.
A point probe from the company determines the distance of more than 4000 measuring points over a 50-mm section at a mean working distance of 100 mm and a rate of 1000 points per second. A single lens can adjust working distance and depth of field. Versions with X-Y stages, scanning capabilities and 1-MHz measurement capabilities are under development.
A general-purpose probe provides 10-µm precision over a 40-mm working range. The company will soon offer a microscopic module with 0.1-µm precision over a 200-µm range through a 103 objective.

The science of measurement, particularly of lengths and angles.
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