Image Enhancement and Film-grain Noise

Film-grain noise severely limits the possibilities for enhancing degraded photographic images. Because the grain noise increases monotonically with the photograph's mean or bias transmittance, poor signal-to-noise ratios result whenever the emulsion receives a low-contrast, high-level exposure. Image degradation—such as that resulting from inadvertent camera motion—preserves the level of the exposure, but lowers the image contrast inversely at the spatial frequency in question. Consequently, the observed signal-to-noise ratio and the theoretical enhanceability of the degraded image drop with increasing spatial frequency. Unfortunately, the tendency for low spatial frequencies to dominate the highs in the typical object scene, and the relative importance of the highs in obtaining quality image recordings, compound the enhancement problem still further. Nevertheless, Wiener-filter theory—reworked under the assumption of multiplicative film-grain noise—still provides some resolution improvement for imaging devices gathering large amounts of light from the object scene.