Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1995-06-26
1997-03-04
Le, Que T.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
2502141, 257448, H01J 4014
Patent
active
056082045
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The present invention relates to an image cell, in particular, for an image-recorder chip having a multiplicity of image cells disposed in the form of a two-dimensional array and having a readout logic, for the projection of high input signal dynamics onto reduced output signal dynamics Natural scenes have irradiation intensity dynamics of, in some instances, more than 1,000,000:1. In order to be able to image scenes of this type simultaneously with an image-recording system, each element of the signal processing chain, i.e. the sensor element, the readout logic, as well as, if need be, A/D converters connected thereafter for reading the image into an image-processing system, would have to have a dynamic range of 120 dB. A dynamic range of this type can be achieved with individual components, such as special semiconductor diodes or discrete A/D converters, however, elements which have a dynamic range of 120 dB are not suited for integration into a digital CMOS process. On the other hand, almost any dynamics can be achieved on the digital "side", i.e. in a subsequently connected image-processing unit with the respective hardware complexity.
2. State of the Art
An image-recorder chip whose light-sensitive elements are photo diodes or MOS transistors whose generated signals are all converted logarithmically has been proposed in U.S. Pat. No. 4,973,833. A charge coupled device (CCD) is utilized for storage and transfer of the signals from the light-sensitive elements.
The image-recorder chip known from U.S. Pat. No. 4,973,833, however, has the drawback that the input signal dynamics cannot be controlled, but rather in the case of the image recorder described in U.S. Pat. No. 4,973,833, the input signal dynamics are defined "logarithmically". Furthermore, direct and nondestructive readout of the image data is not possible.
EP 0 390 205 A2 describes an electrical circuit that is distinguished by high amplification during the conversion of the electrical charge into voltage. This circuit is predominantly employed for charge coupled devices (CCDs). In this circuit described in EP 0 390 205 A2, there is also no possibility of controlling the compression of the input signal. The amplifier circuit according to EP 0 390 205 A2 permits high amplification for small charges, however, cannot control the amplification factor and quickly reaches its limits with high charges.
U.S. Pat. No. 4,473,836 proposes an image-recorder chip in which a MOS field-effect transistor is the light-sensitive element. The gate electrode of the MOS field-effect transistor is electrically connected to a floating photo-sensitive diffusion area, yielding an image cell that projects high input signal dynamics onto reduced output signal dynamics. This cell permits further signal processing with conventional circuits produced in the same MOS process, because greatly reduced dynamic demands are made on these components.
The image-recorder chip known from U.S. Pat. No. 4,473,836 has, however, a number of drawbacks:
For one thing, the projection of the input signal dynamics onto the output signal dynamics occurs only approximately logarithmically, with the exact course of the characteristic line being greatly dependent on the fabrication parameters. The initially logarithmic characteristic line changes into a "root characteristic line". Moreover, supply voltages are needed that are imcompatible with the supply voltages of digital CMOS-VLSI circuits.
Furthermore, the known image recorders with initially logarithmic signal compression are not manufactured for array arrangement or have to be made using special processes due to the intolerance to fluctuations of the process parameters.
DESCRIPTION OF THE INVENTION
The object of the present invention is to provide an image cell, in particular, for an image sensor, which projects high input signal dynamics with a characteristic line which can be selected within a specific range and, in particular, logarithmically onto output signal dynamics.
It should be possbile to fabrica
REFERENCES:
patent: 5146074 (1992-09-01), Kawahara et al.
patent: 5153420 (1992-10-01), Hack et al.
Hofflinger Bernd
Landgraf Marc
Seger Ulrich
Institut fur Mikroelektronik Stuttgart
Le Que T.
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