Auto-ranging film densitometer

Abstract

A film densitometer for generating digital density values as a function of analog transmittance signals received at an input. A comparator coupled to the input compares the transmittance signals to one or more range references characterizing a plurality of ranges of transmittance signal magnitudes, and provides digital transmittance range signals as a function of the comparison. An amplifier amplifies the transmittance signals to provide amplified transmittance base signals. A gain control circuit coupled to the amplifier and the comparator controls the gain factor of the amplifier as a function of the magnitudes of the transmittance signals. The amplified transmittance base signals are converted to digital transmittance base values by an analog-to-digital converter. A lookup table of data characterizing the logarithmic relationship between transmittance base values and density base values is stored in base memory. A lookup table of data characterizing the relationship between the transmittance range signals and range gain values is stored in range memory. A digital processor accesses the memory as a function of the transmittance base values and transmittance range signals, and generates the digital density values as a function of a sum of the density base values and range gain values.