The Sampling Aperture For Linear Microdensitometry

Abstract

In the development of modern linear microdensitometry, the trend of optical system design has been towards the condition of underfilling the efflux optics, with total collection of light after it passes through the sample. The system transfer function is therefore attributable to the influx optics, and the sampling aperture is the light distribution impinging on the sample, reduced from an illuminated slit or circular aperture through the influx optical system. The maximum frequency response of the system is obtained when the sample is illuminated with the impulse response of the influx optics. However, the theoretical impulse response can only be realized by imaging a delta-function and this is photometrically impossible. Similarly, because the system images an illuminated aperture onto the sample, scanning with a pure, geometrically-characterized slit or spot is not possible due to lens response and diffraction. These two problems are investigated, for both coherent and incoherent illumination. For both impulse response and slit image, the MTF is investigated, and its deviation from the ideal is calculated. The results are characterized in terms of RMS-MTF differences over the spectrum for 10, 5, 2 and 1% levels. The controlling parameter is the ratio N/R, where R is the reduction factor employed for the influx optics, and N is the number of resolution elements contained within the nominal object slit width. The study shows that there are significant differences in these values for the same RMS difference level, with coherent and incoherent illumination, and that there are compromises to be made with both kinds of illumination. The results of this study facilitate calculation of system response for any configuration of object slit and influx optics (within the linear microdensitometer system), and defines limits on slit sizes for operation with impulse response and geometrically characterized slit images for the sampling aperture. The effects expected with the linear polarization associated with laser illuminat