Abstract: Light guides (1) capable of encoding the transverse and longitudinal coordinates of light emission induced by the interaction of photons in an array of a plurality of the light guides. Each light guide has at least two discrete crystal segments (4) adjacently disposed along a common longitudinal axis of the light guide (1). Between adjacent segments is a boundary layer (7) having less light transmission than the light transmission of the crystal segments (4). A light absorbing mask (8) increases light adsorption in a segment (4). Photons enter the light guide (1) and cause the emission of scintillation light which is delivered in different and resolvable quantities to light sensing devices. The differences in quantity of delivered light is caused by successive decreases in light in part by the boundary layers (7). The differences in quantity of light establish the segment from which the light emission took place.