can isolate the Cherenkov photons that each particle emits, to form a single "ring image" from which an accurate can be determined. A polar plot of the Cherenkov angles of photons associated with a 22 GeV/c particle in a radiator with =1.0005 is shown below; both pion and kaon are illustrated; protons are below Cherenkov threshold, , producing no radiation in this case (which would also be a very clear signal of particle type = proton, since fluctuations in the number of photons follow Poisson statistics about the expected mean, so that the probability of e.g. a 22 GeV/c kaon producing zero photons when ~12 were expected is very small; e−12 or 1 in 162755) The number of detected photons shown for each particle type is, for illustration purposes, the average for that type in a RICH having ~ 25 (see below). The distribution in azimuth is random between 0 and 360 degrees; the distribution in is spread with RMS angular resolution ~ 0.6 milliradians. Note that, because the points of emission of the photons can be at any place on the (normally straight line) trajectory of the particle through the radiator, the emerging photons fill a light-cone in space. In a RICH detector the photons within this light-cone pass through an optical system and impinge upon a position sensitive photon detector. With a suitably focusing optical system this allows reconstruction of a ring, similar to that above, the radius of which gives a measure of the Cherenkov emission angle . The resolving power of this method is illustrated by comparing the Cherenkov angle per photon, see the first plot above, with the mean Cherenkov angle per particle (averaged over all photons emitted by that particle) obtained by ring-imaging, shown below; the greatly enhanced separation between particle types is very clear: This ability of a RICH system to successfully resolve different hypotheses for the particle type depends on two principal factors, which in turn depend upon the listed sub-factors; The effective angular resolution per photon, Chromatic dispersion in the radiator ( varies with photon frequency) Aberrations in the optical system Position resolution of the photon detector The maximum number of detected photons in the ring-image, The length of radiator through which the particle travels Photon transmission through the radiator material Photon transmission through the optical system Quantum efficiency of the photon detectors is a measure of the intrinsic optical precision