The laser film digitizer: density, contrast, and resolution

This work evaluates differences in density linearity, contrast, and resolution of digital images acquired from a laser film scanner. A calibrated photographic step tablet tested linearity of diffuse optical density (OD). It was digitized within center openings of a dark and light film. The 14- x 17-inch films were scanned into 1024 x 1280 picture elements of 8 bits. A photographic tapered resolution guide evaluated the scanner's detail resolution. A single characteristic curve adequately represents the rate of change of OD per pixel value, but two piecewise linear functions are better. One ranges in light OD and a second in dark OD. Contrast shows a steeper slope in a light film rather than a dark film, yet a greater difference exists between two identical digitizers. The resolution test diminishes at 1.3 line pairs per mm (LP/mm), which compares to a Nyquist limit of 1.44 LP/mm. Such density, contrast, and resolution tests yield a benchmark to assure quality operations.