SESSION F: AUTOMATIZED COUNTING AND SIZING: THEORY: Some fundamental aspects of particle counting and sizing by line scans

Some of the simpler ways of using line scans are noted, particularly with reference to counting and sizing particles which approach the limits of optical resolution. A general treatment relating derivable information to observed data of frequency of interception of particles by line scans and combinations of line scans is given, and it is shown that to achieve a desirable accuracy a large aperture must be used and the presence of particles which do not fill the whole aperture must be recognized; this is best done by using an aperture composed of a line of independent probes so that the field is scanned as with a comb. Other methods, favoured at present, which depend on the use of apertures or probes whose size and position must be known accurately, cannot realize the potential and desirable accuracy of counting and sizing owing to limits set by optical resolution. Estimates are made of the increased counts required in statistical methods and also of the relative merits of criteria of association of signals in methods where particles are assessed individually.