The basic optical configuration of an SPR biosensor consists of a prism coated with a thin metal film (usually silver or gold) (figure). In this configuration, which was originally proposed by Kretschmann & Raether, the total internal reflection of light is used to excite nonradiative surface plasmons in the metal film. Surface plasmons are waves of oscillating surface charge density traveling along the metal surface. The electromagnetic field amplitudes of nonradiative surface plasmons decay exponentially with increasing distance perpendicular to the sensor surface, with a decay length in the biosensor of a few hundred nanometers. These plasmons can be resonantly excited by light only at a well-defined angle of incidence, which occurs when the wave vector of the light in the plane of the sensor surface matches that of the surface plasmon. The resonance causes an energy loss in the reflected light, which is visible as a sharp minimum in the angle-dependent reflectance, which is, experimentally, the primary recorded quantity. The resonance angle strongly depends on the refractive index (or dielectric constant) profile of the sample within the evanescent field above the sensor surface. Adsorption or desorption of macromolecules at the sensor surface change the local refractive index and produce a shift in the resonance angle, which, to a good approximation, has been shown to be proportional to the surface concentration of macromolecules up to a concentration of 50 ng mm-2. For a given refractive index increment of the macromolecules, the signal is approximately proportional to the mass that is bound to the sensor surface. The time-dependent change of the refractive index in the vicinity of the surface upon binding is commonly measured in RU (resonance units), with 1 RU corresponding to ~1pg protein mm-2.