Some Problems in Measurement of Fluctuation in Film Density

Abstract

Modulation transfer functions (MTFs) for the defocused objective lens system of the microdensitometer used for analysis of radiographic image qualities, are investigated for two objective lenses. Medical X-ray films have a unique structure characterized by the use of double-coated emu lsion. However, When MTFs are measured using an objective lens with a focus shallower than the thickness of the film in a screen-film system, large measurement errors do not occur. For an objective lens with a focal depth of 0.10mm, the measured MTFs errors are approximately 5% at 4 cycles/mm.
The Wiener spectrum values measured using lenses with focal depths of 0.1 0 m m and 0.24mm almost agree. However, when measured with the 0.24mm objective lens, the Wiener spectr a show significantly higher values in the low frequencies due to the effect of the stage glass.