The Design, Construction, and Testing of a Low Magnification Camera to Photograph Polished Ore Specimens

Citation

Bieler, Barrie Hill (1952) The Design, Construction, and Testing of a Low Magnification Camera to Photograph Polished Ore Specimens. Master's thesis, California Institute of Technology. doi:10.7907/WQ7Y-YE14. https://resolver.caltech.edu/CaltechTHESIS:02262010-144642155

Abstract

The color photography on "Bantam" film of polished ore specimens is the subject of this research. Preliminary tests showed that a diffused light source of large area (6 inch diameter) is necessary for even illumination of the subject which is up to 1 3/4 inches in diameter. An enlarging lens, designed to function best at low image magnifications, is used on the camera. Focusing and framing is done on a ground glass. A special camera was designed which has a magnification range of 1 to 4 3/4 times. Further testing with type A Kodachrome film showed that acceptable results can be achieved using for illumination either a #1 Photoflood bulb (color temp. 3400°K) giving bluish colors or a PS-25 lamp (color temp. 3200°K) giving reddish colors. If pictures are taken using nearly crossed polarized light: (1) the intensity of reflected light drops from 20 to 100 times de- pending on the minerals, and the intensity will vary with the orientation of the minerals and the degree of extinction of the polarized light; (2) the extinction color of the polaroid filters is not gray but blue-violet, and for some pictures an orange correction filter may be necessary. A study of the sharpness of the pictures indicates that the resolution of the lens, limited by aberrations and diffraction, is below that of the film. Kodachrome film is better than Ektachrome for miniature transparencies.

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