Abstract: An optical sensor has a collecting lens, filters and detectors. The colleng lens is fabricated from a single piece of optical material and has a truncated conical surface, an input surface adjacent the wide end of the truncated conical surface and an output surface adjacent the narrow end of the truncated conical surface. A plurality of detectors is positioned proximate the output surface and a filter, which allows the passage only of radiation of a predetermined band of wave lengths is positioned between each of the detectors and the output surface. When radiation is incident on the input surface, it is passed through the filters to the detectors. The resultant detected optical signals are translated into electrical signals which are compared to discriminate between a radiating source or sources having the desired spectral characteristics and a source or sources having undesired spectral characteristics. The result of the comparison is an output signal indicative of the presence of the desired radiation.