Single Photon Counting

Recently the demand for the measurement of very low-light levels has been increasing in the fields of biology, chemistry, medicine, and many others; for instance, analytical chemistry needs to employ fluorescence, luminescence and Raman scattering light for the quantitative operation of substance. Biochemistry calls for the measurement of very low-light level to observe gene expression (1 ,2 ) and the activity of living cell (3 ,4 ), etc. It also calls for radioimmunoassay and fluorescent immunoassay. Medicine asks for a positron computerized tomography (CT) to research a brain physiologically and diagnose cancers. In any case, the absolute levels of light to be detected are very low in every field, so that light that changes with time at a very fast speed can be captured.