Signal attenuation refers to the reduction in the intensity of a light beam as it passes through a medium or a device. In the context of beam splitters, attenuation can occur due to several factors, including absorption, reflection, and scattering. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding widespread application in fibre optic telecommunications. For a lossless beam splitter, R + T = 1.