Noise power spectrum analysis of a scanning microdensitometer.

We have analyzed the principal sources of noise in a commercially available 2-D scanning microdensitometer which we use to estimate the noise power spectra of radiographic films. Two kinds of noise have been observed. One source, associated with the glass platen of the instrument, is correlated from scan to scan. This source of noise limits our ability to measure the NPS of film samples at low sample optical densities. The other major noise source is uncorrelated from scan to scan and increases exponentially with sample optical density. We have measured both of these component noise sources as well as the total instrument noise as a function of instrument density and spatial frequency. A method for minimizing the effects of instrument noise on estimates of the noise power of film samples is described and demonstrated.