The first two techniques described in this chapter, namely slit-scanning and multi-angle light scatter, are very specialized and not yet available to the majority of users. Nevertheless, they are potentially important and beginners in flow cytometry should at least be aware of their existance and consequently I've included a brief summary of each. Rare-event and fluorescence emission spectrum analyses are also included in this section. The former presents particular statistical problems, although anyone could have a shot at it with a standard instrument and the latter can be performed on most instruments as long as they are equipped with the correct filter combination.

Slit-scanning

Slit-scanning is a term used to describe interrogation techniques where different parts of the object are sequentially illuminated and/or observed during their passage through the exciting beam. This can be achieved either in the object or the image plane and the recordings enable the shape of objects to be reconstructed. Progress in this area of flow cytometry has, to a large extent, been pioneered by Wheeless and colleagues in development of automated methods for cancer prescreening (Wheeless and Patten, 1973a,b; Wheeless, Hardy and Balasubramanian, 1975; Cambier and Wheeless, 1975; Cambier et al., 1976).

Object plane slit-scanning

Object plane slit-scanning applies where the vertical dimension of the excitation beam is physically smaller than the object being illuminated. It was first encountered in section 3.9.6 where the differences in fluorescence intensity from crossed cylindrical lens pair focussed excitation was compared with that from spherical lens focussing.