A beam splitter divides incident light into reflected and transmitted beams at a specified R/T ratio. For a lossless beam splitter, R + T = 1. R = reflectance, T = transmittance, A = absorptance (ideally zero) When comparing beam splitters, always check whether the specified R/T ratio is for. Key Words: Optical attenuation, optical beam splitter. If a narrow pencil beam is approximately at normal incidence to one face, it pentrates the prism and undergoes multiple reflections. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding widespread application in fibre optic telecommunications.