Figure 7. The detected features being 'flagged' and counted. Figu. re 8. Operation of the size control, 'sizes' the small particles out first. Facing page 680

THE PARTICLE SIZE ANALYSIS OF PIGMENTS WITH THE QU•dNTIMET 681 final limit of resolution is set by the computer detection circuit, which is in turn worse than the television camera. The next stage in evaluating the image is to detect the features, and to measure their size. However, these processes are complicated by the uncertainty of particle diameter due to its variation with both threshold setting and image brightness. We may expect the light intensity to vary across the image of a per- fectly black feature due to diffraction at its boundary. This is shown in Fig. 9 and implicates not only the possibility of resolving the feature, but the ease with which it may be detected (due to unevenness of density) and its apparent size (due to variation in size with detected density and thresh- old setting). This difference between true size and apparent size with varying detected density is more apparent for small feature areas than for large ones (Fig. 10). If we consider any population of features with constant area, as the average size of the features decreases (and, their number in- creases) the errors produced by a small change in threshold setting becomes more significant. It follows that percentage inaccuracies in size measure- ment assume a greater significance for smaller features. LARGE FEATURES / AREA % I i I ,,, THRESHOLD AREA % SMALL FEATURES / I _ I THRESHOLD Figure 10. Variations in feature area with detected density. (a) Large changes in threshold give small changes in area. (b) Small changes in threshold give large changes in area. T he significance of these errors can be reduced by limiting the size of the smallest feature examined for a particular magnification making any evaluation of particle size distribution in relation to another one standard- ising illumination and detection conditions by using the same camera