Proposal of the Detectivity D** for Detectors Limited by Radiation Noise†

A new kind of detectivity, called D**, is proposed for cells that have a detectivity that is limited by radiation noise. The detectivity D** takes specific account of the solid angle Ω, from which external radiation can reach the responsive element. Usually, the external radiation is produced by objects at room temperature. D** is also appropriate for use with detectors that are non-Lambertian for reasons other than cooled radiation shields: for example, detectors that are immersed in a high-index medium, or detectors that are closely associated with a lens.