The use ofEMCCDs as silicon-based photon counting devices

Photon Counting (PC) efficiency has increased since the first photon counting devices. Third generation image intensifiers with high Quantum Efficiency (QE) photocathodes (of the order of 30%) are used with CCDs in intensified photon counting systems. But, the efficiency of this kind of camera is limited by the capability of the tube to transform an incoming photon into an electron and amplifying it so that the output signal is largely over the CCD Read-out Noise (RON). Improvements in CCD QE, which now reaches 90% at some wavelengths, and RON, as low as $3\overline{\mathrm{e}}$, have no effect on the total counting efficiency of these cameras. RON of $3\overline{\mathrm{e}}$ is still too high for an application such as photon counting for extreme faint fluxes. It is now possible to amplify the pixel signal into the CCD before it reaches the output amplifier and before it is affected by its noise. These Electron Multiplying CCD make it is possible to get sub-electron effective RON, low enough for PC. PC capability combined with a very low dark current, very good pixel response uniformity, and high QE offered by the CCD technology have made EMCCD the detector of choice for high-performance applications such as time resolved spectroscopy and low light imaging.