Abstract: A sensor based on the technique of surface plasmon resonance (SPR) comprises an SPR device, a source of electromagnetic radiation from which radiation can be directed onto the device, and a detector to measure the intensity of radiation reflected from the SPR device. The electromagnetic radiation directed onto the SPR device contains both Transverse Electric-polarized and Transverse Electric-polarized components. A polarization analyzer is interposed between the device and the detector such that, at angles away from resonance, little or no light reaches the detector. The sensor is particularly useful in the qualitative and/or quantitative determination of biological, biochemical or chemical analytes.