Photometer-digitizer system

DETAILED DESCRIPTION In a preferred embodiment of the present invention a two-dimensional vidicon photometer having desirable response characteristics is caused to operate in an integrating mode for exposure to weak astronomical or laboratory radiation.
The vidicon target stores a charge pattern in response to the radiation and the pattern is subsequently read from the target for digitizing and recording in a form which retains image accuracy and is suitable for further digital processing or display.
In the implementation of the present system, a silicon vidicon tube is selected for the properties of good linearity, dynamic range, quantum efficiency and spectral response.
The silicon target of the vidicon tube is provided with an electric charge, back biasing a P-N junction between two semiconducting regions thereof.
To induce an integrating mode of operation in the target and inhibit "dark current" the vidicon tube is placed in a dry ice environment to produce a greatly reduced target temperature.
Illumination is focussed onto the target surface and incident photons penetrate the silicon target to react with the molecular structure of the silicon and produce a charge carrier pair.
A charge carrier released by each incident photon drifts to a nearby point of the P-N junction where a quantum change in stored charge and consequently junction voltage is produced.
As a result of target temperature, the same quantum change is induced to occur with each received photon over the duration of long exposures to permit the target to accumulate a net charge change pattern linearly reflecting total incident radiation over a large dynamic range.
The increased sensitivity of the long exposure is augmented by the high quantum efficiency and large spectral range of silicon as the target material.
After exposure completion, the vidicon target is scanned by an electron beam to produce an electrical signal sequence representative of the cumulative charge change pattern.
The signal sequence is immediately digitized to retain amplitude and position accuracy and is then stored on tape for later data processing or plotting