Abstract: A method and apparatus for photon counting are disclosed. In an implementation, the present disclosure provides a dynamic-range photon-counting spectrometric sensor for low signal detection embedded in large parasitic background light signal. Rather than using a known forward illuminated imager approach of counting photons directly, embodiments of the present disclosure use a backward illuminated imager approach to generate a burst of electrons based on the photons, and perform photon detection by counting the generated burst of electrons using the plurality of CMOS chip pixels. An electronic circuit, in communication with the CMOS chip pixels, includes a fast clock enabling discrimination between electrons in time. The electronic circuit can be built on the face of a semiconductor wafer on which the CMOS chip pixels are provided.