Replacement of quarter-wave plates by photographic double exposure for controlled density evaluation

Abstract

Isoclinics are eliminated by double exposure of a film in the camera of a plane polariscope and by rotation of the coupled polaroids through 45° between the two exposures of equal duration. The resultant photographic isochromatic pattern can be evaluated by interpreting and calibrating as lines of constant fractional fringe orders the sharp boundaries between dark and bright regions seen in a developed high-contrast film negative or the narrow equidensity lines obtained in a developed Agfacontour film negative. Alternatively a developed low-contrast film negative may be copied by contact or enlargement on high-contrast or Agfacontour film.

The fractional fringe orders of the lines on any photograph depend on the exposure time, or copying exposure time, and on the coupling angle of the polaroids. Monochromatic light, or white light with blue-sensitive or orthochromatic film, may be used. An evaluated example and error analyses show that discrimination of fractional orders is about the same as in conventional compensation methods.