Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-04-30
2001-03-06
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C348S297000
Reexamination Certificate
active
06198086
ABSTRACT:
DESCRIPTION
The present invention relates to a method for detecting an image signal by means of an array of photosensitive devices with each of which the charge of a capacitance can be altered.
In monolithic integrated image sensor systems, arrays of sensor elements are usually fabricated together with the electronics for signal readout on an integrated circuit (IC). In the field of integrated optical sensors, photodiodes, bipolar phototransistors, charge-coupled devices (CCDS) and light-sensitive MOS transistors can be used as light-sensitive devices. The light-dependent signals, i.e. the charge, the voltage or the current, of the arrays of the above-named devices are normally read out and subsequently converted to a digital signal and processed further. Alternatively, these light-dependent signals can be displayed directly without an analogue/digital conversion.
The process of reading out image signals by means of an integrating method is known. For example, a signal current of a light-sensitive device is here integrated onto a capacitance over a certain period of time. As a result of this integration, a signal voltage proportional to the illumination intensity and to the time period is generated, which can then be read out easily from a peripheral circuit on the integrated circuit. In one example of a light-sensitive device according to the prior art, a capacitance which was initially charged with a predetermined potential is discharged by a photocurrent generated by the light-sensitive device.
In some applications, for example in the automobile branch, where there are frequent changes in brightness conditions and strong differences in brightness within an image, the demands made on the dynamics of the image signals are very great. Image signal dynamics of this order, a consequence of too strong a variation in the illumination intensity within an image, cannot be achieved by the method according to the prior art described above. If, for example, the illumination intensity is too high for certain image sensor elements, the voltage of the capacitance initially charged with a predetermined voltage drops very quickly, so that, after the time period during which the capacitance is discharged by the signal current of the light-sensitive device has elapsed, it is no longer possible to assess how high the incident illumination intensity was initially if the capacitance has discharged completely on termination of the time period. The known method thus exhibits a limited dynamic range.
U.S. Pat. No. 4,479,062 describes an apparatus for photoelectrical conversion in which an array of light-receiving elements is provided in order to accumulate information relating to incident light. The known apparatus has a saturation detection device for detecting a saturation of an output signal of the light-receiving element array. If the output signal of the light-receiving element array is saturated, the accumulation time is reduced. According to U.S. Pat. No. 4,479,062 the detection time is reduced progressively until none of the image-receiving elements of the image-receiving element array is oversaturated, i.e. until the output signal of the light-receiving element array no longer exceeds a saturation level. When the output signal of the light-receiving element array no longer exceeds the saturation level, all the image sensor elements provide a valid signal, whereupon the image acquired with the integration time which was ascertained last is then evaluated.
The method known from U.S. Pat. No. 4,479,062 is disadvantageous in that, despite the effort involved in multiple image acquisition, the resulting image may have image sensor elements having a low signal
oise ratio, e.g. image sensor elements with low brightness values. For an observer, however, it is advantageous if the signal
oise ratio for an acquired real scene is as large as possible. For the observer a large signal
oise ratio means that objects which are static and not subject to fluctuations in illumination really do have a static appearance in the image, and that it is possible to resolve low contrasts within a scene. A further disadvantage of the known method is that the dynamic range for the totality of all the image sensor elements of the acquired image is restricted to the physically limited dynamic range, i.e. the ratio of the maximum signal to the equivalent noise signal, of a single image sensor element for an image acquisition. For an observer, a high dynamic range means that both very bright and also very dark regions of a real scene can be represented without appearing equally bright or equally dark from a certain threshold on.
It is the object of the present invention to provide a method for detecting an optical signal that exhibits an increased dynamic range and increased precision.
This object is achieved by a method according to claim
1
.
The present invention provides a method for detecting an image signal by means of an array of photosensitive devices, with each of which the charge of a capacitance can be altered, wherein at first an optical signal of each photosensitive device is detected by means of the following substeps: creating a charge condition of the capacitance with a predetermined voltage, changing the charge of the capacitance either with a photocurrent generated in the photosensitive device by the optical signal or with a quantity derived from the same and detecting the voltage across the capacitance after a predetermined time period, deciding whether the detected voltage lies within a valid range and, if this is so, determining on the basis of the detected voltage a valid signal which characterizes the detected optical signal, and, if this is not so, repeating the cited steps with a time period which differs from the predetermined time period either a predetermined number of times or until it is determined that the detected voltage lies in a valid range. Subsequently the optical signal detected for each photosensitive device is stored together with the time period for which a valid signal has been detected. Finally the image signal is obtained from the stored optical signals for the individual photosensitive devices and the respective assigned time periods.
In an embodiment of the method according to the present invention it is decided in the decision step that the detected voltage does not lie in the valid range if the detected voltage corresponds to a complete discharge of the capacitance. In this case the time period after which the voltage is detected, which differs from the predetermined time period, is reduced for each repetition.
The method according to the present invention provides an extension of the dynamic range in the detection of optical signals, e.g. when imaging by means of an image sensor array consisting of image sensor elements. The method according to the present invention prevents departure from the operating range, i.e. an overdriving, of individual image sensor elements and also an overdriving of the complete image sensor array.
According to the present invention it is thus decided, after each image acquisition, whether a valid signal exists for each image sensor element. If this is so, the value of the respective image sensor element, together with the information on the integration time, is stored e.g. in an intermediate memory. This is also carried out even if there have already been invalid signals of image sensor elements in this partial image. The control of the brightness values and information on the integration time can e.g. be realized by means of a digital processor.
In preferred embodiments of the present invention, acquisition of a partial image commences with the greatest sensitivity, i.e. with the longest integration time. This ensures that all the image sensor elements exhibit a maximum signal
oise ratio in the prevailing circumstances. If subsequently, for a curtailed integration time, remaining image sensor elements are still overdriven, i.e. do not exhibit valid signals, the image sensor elements in the intermediate memory can e.g. adopt the maximum repre
Hosticka Bedrich
Kisakürek Hakan
Schanz Michael
Wertheimer Reiner
Duft, Graziano & Forest, P.C.
Fraunhofer-Gesellschaft zur Forderung der angewandten
Le Que T.
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