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The detection offaint light sources, such as distant planets in astronomy; the fluorescence of biological trace elements in biosensors; and the spectral emissions from spectroscopy experiments; ideally requires instrumentation that can log the arrival of single photons in a temporally and spatially resolved fashion. Until recently, the most widely used sensor for such low light level optical measurements was the photomultiplier tube (PMT) but solid state solutions are now replacing them as the preferred detector. There are still a number of applications which include astronomical low light level imaging and photon counting spectroscopy which use conventional PMT and multichannel plate detectors because, until now, the technology has not been sufficiently flexible to replace them. Three solutions are currently available for low light level observation in the application areas mentioned above. Firstly, there are vacuum tube (PMT or multichannel plate) based systems. An example ofwhich is the PAPA Faint Object Camera [1] which indeed enables very low light level imaging to be performed but is extremely expensive and fragile: Secondly, Charge Coupled Device (CCD) based systems. These are very successful in many imaging applications, but have some limitations when low light level signals are to be addressed, and finally Avalanche Photodiode (APD) detectors. These detectors are not currently used for imaging but provide a robust replacement for the PMT in many applications. This work has involved the development of a novel, Avalanche Photodiode based optical integrated circuit, which enables the imaging of low light level signals and may potentially be a realistic alternative to PMTs in all of the above application areas. This paper describes the development of this novel and completely self contained integrated circuit for low light level imaging applications [2]. The technology developed to implement his integrated circuit has been called SPARCE.CMOS. (Single Photon ARray Control Electronics).
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