The SQM Series instruments are based on the principle known as Optically Stimulated Electron Emission (OSEE), also known as Photo Electron Emission (PEE). It is a well known fact that when metals or certain other surfaces are illuminated with ultraviolet light of the proper wavelength (energy), electrons are emitted from the surface. The process by which UV photons interact with the surface to produce electrons is known as OSEE. The emitted and subsequently scattered electrons can be collected across an air gap by a biased collector (located at the end of the the OSEE sensor), and measured as a current. By maintaining the surface to collector distance relatively constant, changes in the measured photocurrent (which is in the order of 10^-10 amps) can provide information about the surface, e.g., electronic structure, composition, and chemistry. Any contaminant on the surface, depending on its own photoemission characteristics, can either enhance or attenuate the inherent emission from the surface. In simple electronic terms, the surface is a current generator, and a non-photoemitting contaminant acts as a resistance, because the current is attenuated by interactions between the electrons and the contaminant. The thicker the contaminant, the higher the resistance, and consequently the greater the decrease in the measured signal.

The Sensor response to various materials generally depends on the magnitude of the photoelectron work function. Generally, materials with a work function of less than approximately seven EV (electron volts) will produce a photocurrent. Both conducting and non-conducting materials can be photoemissive. For example, epoxy primer, carbon phenolic, glass phenolic, graphite/epoxy, and fiberglass/epoxy will emit. Most plastics and ceramics are photoemissive. Some materials that have extremely low response include pure teflon, pure glass, and magnesium fluoride. In general measurements can be made on most materials of engineering importance, proving OSEE of great practical use.

The presence of thin contaminant films on surfaces can result from inadequate or incomplete cleaning methods, from oxide growth after cleaning, or from failure to properly protect cleaned surfaces from oils, greases, fingerprints, release agents, or deposition of facility airborne molecules generated by adjacent manufacturing or processing operations. Often these contaminants are invisible, making detection and quantitative measurement difficult and expensive. The SQM Series has the sensitivity and operational simplicity required to provide fast and cost effective surface evaluations.