Abstract: The combination of a very high gain image intensifier, a CCD camera and an image processor which eliminates read out noise of the CCD camera, enables one to detect single photon events by photon counting imaging. The detected photons are then counted in the image processor to generate an image. The image intensifier is also designed to minimize dark noise that determines the sensitivity limit. The photon counting imaging system can be used as a standard intensified CCD camera by lowering the gain of the intensifier. This is useful for focusing and getting an image of the entire object in brightfield, which is used as the reference for the luminescent image. This is a very flexible imagin system used to monitor luminescence; such as gene expression, genetically modified luminescent microorganisms, ATP from microorganisms and luminescence from aequorin stimulated by calcium ions. This system also makes it possible to see luminescence generated inside of small animals through their skin without harming them and it is becoming an important tool for In-Vivo monitoring of small animals. In this paper, we discuss recent technique of photon counting imaging. !6
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