Testing and Performance of Two-Dimensional Optical Array Spectrometers with Greyscale

Two laboratory optical array spectrometers with greyscale were evaluated for their sizing, depth of field and timing performance; these three factors are necessary to calculate concentrations and liquid water contents. The probes were of 10 and 150 μm resolution with 40, 60, 80% and 25, 50, 75% threshold detection, respectively. Sizing was a function of the particle location in the sample area and may be in error by up to four channels, the effect is most pronounced for smaller particles. The greyscale feature is a quantitative measure of the sharpness of focus of the particle image. It permits the determination of the particle location in the sample area which, in turn, permits the rejection of extraneous or the missized particles. It also allows for a more accurate and user-definable sample volume. The timing check revealed substantial errors which were corrected via software. Comparison with several methods of estimating liquid water content showed consistent results. The sizing and depth of field results are applicable to the more popular nongreyscale probes since these effects depend on the optics and not on the greyscale feature. The greyscale probes provide definite advantages which would be useful in airborne applications.