The ring-imaging Cherenkov, or RICH, detector is a device for identifying the type of an electrically charged subatomic particle of known momentum, that traverses a transparent refractive medium, by measurement of the presence and characteristics of the Cherenkov radiation emitted during that traversal. RICH detectors were first developed in the 1980s and are used in high energy elementary particle- , nuclear- and astro-physics experiments. This article outlines the origins and principles of the RICH detector, with brief examples of its different forms in modern physics experiments. Ring-imaging Cherenkov (RICH) detector Origins The ring-imaging detection technique was first proposed by Jacques Séguinot and Tom Ypsilantis, working at CERN in 1977. Their research and development, of high precision single-photon detectors and related optics, lay the foundations for the design development and construction of the first large-scale Particle Physics RICH detectors, at CERN's OMEGA facility and LEP (Large Electron–Positron Collider) DELPHI experiment. Principles A ring-imaging Cherenkov (RICH) detector allows the identification of electrically charged subatomic particle types through the detection of the Cherenkov radiation emitted (as photons) by the particle in traversing a medium with refractive index > 1. The identification is achieved by measurement of the angle of emission, , of the Cherenkov radiation, which is related to the charged particle's velocity by where is the speed of light. Knowledge of the particle's momentum and direction (normally available from an associated momentum-spectrometer) allows a predicted for each hypothesis of the particles type; using the known of the RICH radiator gives a corresponding prediction of that can be compared to the of the detected Cherenkov photons, thus indicating the particle's identity (usually as a probability per particle type). A typical (simulated) distribution of vs momentum of the source particle, for single Cherenkov photons, produced in a gaseous radiator (n~1.0005, angular resolution~0.6mrad) is shown in the following figure: The different particle types follow distinct contours of constant mass, smeared by the effective angular resolution of the RICH detector; at higher momenta each particle emits a number of Cherenkov photons which, taken together, give a more precise measure of the average than does a single photon, allowing effective particle separation to extend beyond 100 GeV in this example. This particle identification is essential for the detailed understanding of the intrinsic physics of the structure and interactions of elementary particles. The essence of the ring-imaging method is to devise an optical system with single-photon detectors, that