An optical technique for the detection and quantification of binding which has been incorporated into some biosensors. The material to which binding will be detected is immobilised on the surface of a very thin metallic film. When brought into contact with a solution of a ligand, the properties of the film are sufficiently altered so that light reflected from the other side of it will be detectably altered. In coupled plasmon-waveguide resonance (CPWR) spectroscopy, the properties of reflected polarised monochromatic light yield data on anisotropic membranes.