Even if judged clean on delivery, virtually all optical systems eventually become contaminated during use. Fortunately the low residual level of contamination always present has little effect on the appearance or function of most optical devices. Certain types of optical component, such as those employed in the manufacture of graticules or laser optics, are more sensitive to contamination than others used, for example, in photographic systems but thresholds of acceptance are needed for every application. This requirement presupposes the existence of acceptable methods of measurement. This paper discusses the effect of various forms of contamination on the appearance and function of a variety of optical systems and describes a new technique for inspecting and quantifying, in radiometric terms, the significance of contamination which can build up on all the air glass surfaces of an assembled optical system. The method quantifies the amount of radiation absorbed and scattered by a single particle as assessed by a given measuring instrument which can at the same time be used to measure artefacts of known characteristics on the surfaces of a similar design of reference optical system. Results of measurements undertaken on typical optical systems are presented to illustrate the capabilities of the technique.