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The conditions for linearity in microdensitometry are discussed. It is shown that under proper conditions a microdensitometer may behave linearly regardless of the coherence properties of the source. A linear microdensitometer was constructed and design considerations are presented as well as test results.

This study has empirically shown that the maximum slope of an ideal edge scan in transmittance versus position is an excellent indicator of the cut-off frequency at which a microdensitometer system's modulation transfer function goes to zero. It was found that numerical aperture of collection objective, scanning slit width and alignment, system focus, and sampling interval are parameters which must be carefully chosen and set-up when implementing or using this slope technique to check system performance.

Research into the signal and noise characteristics that ultimately determine the performance quality of an imaging system has done little to isolate the contribution of the final image carrying substrate. A study is presented that experimentally analyzes some optical characteristics of six paper substrates representative of those used in lithography, xerography and photography. The optical characteristics studied can have a direct impact on image quality and are a result of the paper structure and its light scattering properties. Reflection microdensitometer techniques are used to provide representations of optical spread functions and an analysis of paper noise which is quantified as a single rms parameter and as a function of frequency in calculated Wiener or Noise Power spectra. The measured spread functions are found to vary in width from lOOiim to 250pm depending upon paper type. An exponential model is used to represent the spread functions and the corresponding modulation transfer functions are calculated. MTF values for the six papers range from 0.99 to 0.73 at 2 cycles per mm. Measured Wiener spectra are presented for the six papers under a variety of measurement conditions. The spectra are found to decrease rapidly with increasing frequency. While this spectrum shape is relatively independent of paper type or measurement condition, the noise power amplitude varies significantly. Data are presented to determine the effect of illumination geometry, backing substrate, paper side and grain direction. Paper noise, or the rms fluctuation in reflected density as a percentage of the reflectance signal, is found to be a 2% effect regardless of paper type under conditions of annular illumination; increasing the illumination directionality, the most sensitive variable tested, increases paper noise to a 4-6% effect.