Page:Journal of the Optical Society of America, volume 30, number 12.pdf/84



8. A Quarter Century of Optics Reviewed. , Bell Telephone Laboratories.

9. Quality Control in the Manufacture of Optical Instruments—Twenty-five Years’ Progress. , Bausch & Lomb Optical Company.

10. Recent Developments in Photography. , Eastman Kodak Company.

11. Producing Low Reflecting Glass. , Corning Glass Works.

12. Changes in Lens Characteristics with Temperature. , Bausch & Lomb Optical Co.

The calculation of the change of correction of a lens with temperature requires, besides data on the coefficient of expansion of the metal of the mount, also a knowledge of the coefficient of expansion of the glass, together with its temperature variation of index and dispersion. Measured data on glass for low temperatures are lacking. Using published results obtained above room temperature one calculates, for instance, a decrease in focal length in a simple imaging system of a few hundredths percent per 50°C decrease in temperature. Although such temperature effects are small, they cannot always be ignored in the design of optical instruments, as for use in airplanes where -40°C may be encountered. A need is felt by the industry for more low temperature data on optical glasses.

13. Chemical Methods for Increasing the Transparency of Glass Surfaces. , Mellon Institute.

The amount of light transmitted by a lens or prism can be increased by suitable chemical treatment of the polished surface. The effect was described by Taylor in 1892, Kollmorgen in 1916, and Wright in 1921. The general use of treated lenses was delayed because of a lack of knowledge of the physical and chemical principles involved in the process. The chemical treatment involves the formation of a transparent surface film of silica by the removal of the higher refractive index oxides to a depth approximately equal to one-fourth the wave-length of the light for which maximum transmission is desired. Such removal is possible without damage to the surface polish if the solvent does not dissolve silica. The surface film is not noticeably different from the base glass in hardness. The gain in light transmission and the decrease in surface reflection is slightly less than that produced by evaporated films of materials of lower refractive index than silica. Solvents that will remove higher refractive index materials from a glass surface include water, fused salts, alkaline phosphate solutions, salt solutions and acid solutions. A dilute acid solution such as 1 percent nitric acid is suitable for most optical glasses with the exception of crown glass. The treatment time is short for glasses containing large amounts of lead or barium and long for borosilicate glasses. The solution speed for a given glass approximately doubles for each 10°C rise in temperature. Freshly prepared glass surfaces react with the solution at a uniform rate that depends on the glass composition, the solution composition, and the temperature. When a silica surface film has been formed, it may be processed in various ways that will render the surface unreactive so that a second treatment will not appreciably change the thickness of the film. Glass surfaces not freshly prepared are usually unevenly reactive due to accidental stabilizing of the surface by' handling or processing.

14. An Automatic Telescope Control. , Massachusetts Institute of Technology.

During the progress of some tests on the flatness of window glass, it became desirable to determine the deviation experienced by a small parallel bundle of rays traversing the glass normally to its surface. Measuring this deviation by means of a collimator and telescope is unsatisfactory because modern drawn glass is of such high quality that the maximum deviation is usually less than a few minutes of arc. It would, therefore, be necessary to use a telescope having an entrance pupil of such size as to mask the local variations in the deviation over the glass surface. This problem was satisfactorily solved by using a photoelectric cell to control the telescope. Light from a tungsten lamp enters a collimator whose slit is replaced by a circular aperture. At the image of this aperture, where cross-hairs would normally be located, there is a sort of photometric field consisting of two pieces of Polaroid placed side by side with their axes at right angles. Behind this photometric field is a single piece of Polaroid which is rotated in its own plane at a rate of thirty revolutions per second. When the image of the circular diaphragm is exactly centered on the dividing line of the photometric field, the rotation of the second Polaroid produces no variation in the light flux transmitted by the system. If the image is decentered, a sixty-cycle current is produced in the output circuit of an amplifier actuated by a photoelectric cell, the phase of the current depending upon which half of the photometric field receives the greater amount of light. This current is used to operate a motor in a manner previously described. The motor drives a tangent screw which controls the telescope and another screw which controls a recording pen.