Low-light level detection

PHOTOEMISSION AND PHOTOMULTIPLIERS. Photoemission is the ejection of electrons from a substance as a result of radiation filling on it Photomultipliers make use of the phenomena of photoemission and secondary-electron emission in order to detect very low light levels The electrons released from the photocathode by incident light are accelerated and focused onto a secondary-emission surface (called a dynode). Several electrons are emitted from the dynode for each incident primary electron. These secondary electrons are then directed onto a second dynode where more electrons are released. The whole process is repealed a number of times depending upon the number of dynodes used, In this manner, it is possible to amplify the initial photocurrent by a factor of 10s or more in practical photomultipliers. Thus, the photomultiplier is a very sensitive detector of light. 

Glow. Chemiluminescent products formed in surface-assisted reactions involving atomic oxygen are believed to be a source of space vehicle glow, which may seriously interfere with the function of sensitive optical instruments designed to detect low light levels. The most widely accepted... 

Measurements at low light levels are routinely performed with photon-counting techniques. The development of ultrasensitive optical detectors has made great progress in the last couple of years. Integrated photon-counting modules with cooled avalanche photodiodes (APD) have been available for some years [31]. These detectors can have quantum efficiencies of 50% with less than 10 dark counts per second. The light sensitive area of such a device has a diameter of about 200 (im and can serve directly as a pinhole in a confocal detection channel. 

The output of the single photon detection system electronics may be such as to form equal height, equal width pulses for each photon detected, whose output rate is high for high-intensity illumination and low at low-light levels. This form of signal is an ideal input for the digital autocorrelation process. 

Photomultipliers are used in circumstances where very low light levels need to be detected ultimately, with careful suppression of system noise, single photons can be detected. 

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