Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1998-09-24
2000-03-07
Lee, John R.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
2502141, H01J 4014, G01T 122
Patent
active
060343672
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to detectors and, more particularly, to the biasing of a linear array of photoconductive detectors.
2. Background Art
The use of photoconductive detectors for measuring radiation is well-known. Because of their high sensitivity, photoconductive detectors such as PbS and PbSe are particularly effective in measuring infrared radiation. Detection of infrared radiation is used by the military for tracking warm vehicles and in night vision devices, is used by medical instrument manufacturers for measuring glucose and other body constituents in a noninvasive manner and is used by scientific instrument manufacturers for measuring chemical composition and structure.
In general, the resistance of the photoconductive detector changes when the radiation falls on its surface. Resistance changes can be measured as an electrical signal change and the intensity of the detected radiation can be estimated by the magnitude of resistance change.
Photoconductive detectors typically require a bias current or voltage to operate, such as a direct current bias. The sensitivity of the detector is proportional to the magnitude of the applied bias. It is preferred to supply a high bias to such a detector to increase its sensitivity and to overcome the noise of the electronics associated with the detector in an overall detection system.
The bias can be delivered to the detector in various manners, including a voltage divider, a constant current or a constant voltage. FIG. 1A shows a known voltage divider arrangement in which a detector 2 is attached at one end to a bias electrode 4 and is connected at its other end to a load resistor 6 which is grounded. The voltage across the load resistor 6 is supplied through capacitor 8 to amplifier 10 which generates an output signal related to the intensity of the radiation 12 impinging upon the detector 2. In the voltage divider arrangement, the incident radiation 12 modulates the resistance of the detector 2 and, consequently, changes the current flowing through the detector 2 and the load resistor 6. These current changes are converted to voltage changes across the load resistor 6. The AC components of the voltage changes are passed through capacitor 8 to the amplifier 10 for further processing.
FIG. 1B shows a constant current mode of operation in which the detector 2 is connected to the bias electrode 4 and to a constant current source 14. As discussed above, the voltage drop across the detector 2 changes with incident radiation 12 and these voltage changes are coupled to amplifier 10 through capacitor 8.
A constant voltage mode of operation is shown in FIG. 1C. In this mode of operation, radiation induced resistance changes in the detector 2 are quantified by measuring the current flowing through the detector 2. This is typically accomplished by direct coupling of the detector 2 to the input of a current-to-voltage converter, often identified as a "transimpedance amplifier". The transimpedance amplifier shown in FIG. 1C includes operational amplifier 16 having one input terminal grounded and the other input terminal connected to the detector 2. Feedback resistor 18 extends between the output terminal of operational amplifier 16 and its input terminal receiving the output of the detector 2. The detector 2 is also connected to the bias electrode 4.
The bias voltage applied to a photoconductive detector also causes current to flow in the absence of incident radiation. This current, referred to as the "dark current", is usually large when compared to the current changes resulting from incident radiation. The detection of the small, radiation related signal, which is added to the large, dark signal, is often difficult. A typical arrangement for detecting the radiation related signal is shown in FIG. 2. Radiation from a radiation source 20, which could be a source of infrared radiation, passes through and is modulated by a modulator 22, such as a rotating slotted chopper disk, and then impinges upon a photoconductive
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patent: 4551623 (1985-11-01), Elliott et al.
patent: 4565928 (1986-01-01), Yamamoto et al.
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Grinch Dean S.
Mazurowski John S.
Waczynski Augustyn
Diasense, Inc.
Lee John R.
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