.
Particles pass through a radiator, the radiated photons may be directly collected by (or are focused by a mirror onto) a position-sensitive photon detector.
Respectively, these are called direct focusing
or mirror-focused RICH detectors. For direct focusing, radiators have to be kept thin (e.g. a liquid radiator), to avoid broadening the ring or filling it; however, [Fabjan95b] report a use of a similar setup as a threshold counter.
The Cherenkov radiation emitted at angle 
is focused onto a ring of radius r at the detector surface, and can be determined by a measurement of r.
For photon detection one uses thin photosensitive (an admixture of e.g. triethylamine to the detector gas)
proportional or drift chambers, see [Barrelet91].
A detailed treatment of errors in Cherenkov detectors can be found in [Ypsilantis94]. An outlook for the future use is given in [Treille96].
For the various currently successful ways of building practical RICH detectors, see [Ekelof96] or [Ypsilantis94], and literature given there. An example is the combined RICH with liquid radiator (unfocused) and gas radiator (mirror-focused) of the DELPHI experiment at LEP (see [Abreu96], [Aarnio91]):
A combined tracking-cum-RICH project, including even identification of particles by energy loss, has been described in [O'Brien91]. For using a RICH as a significant trigger device, see [Baur94].