Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2002-06-28
2004-12-07
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370010, C250S336100, C250S369000
Reexamination Certificate
active
06828563
ABSTRACT:
The present invention relates to solid-state image detectors and its purpose is to eliminate the variations in their sensitivity, especially those due to temperature variations.
In these image detectors, the acquisition of an image takes place with the aid of several photosensitive spots each formed from a photodiode and a switch. The photosensitive spots are produced with the aid of techniques for the thin-film deposition of semiconductor materials such as hydrogenated amorphous silicon (aSiH). These photosensitive points, arranged in the form of a matrix or linear array, make it possible to detect images contained within visible or near-visible radiation. The signals that are produced are then digitized so as to be able to be stored and processed easily.
These arrangements of photosensitive spots find one particularly advantageous application in the medical field or the field of industrial inspection, in which they detect radiological images. All that is required is to cover them with a scintillator and to expose the latter to X-radiation carrying a radiological image. The scintillator converts the incident X-radiation into radiation in the band of wavelengths to which the photosensitive spots are sensitive.
There are now large photosensitive matrices which may have several million photosensitive spots.
FIG. 1
shows a matrix image detector of the known type. It has only nine photosensitive spots in order not unnecessarily clutter up the figure. Each photosensitive spot P
1
to P
9
is formed from a photodiode Dp and an element having a switch function Dc represented in the form of a switching diode. It would have been possible to choose a transistor as the element having a switching function. The photodiode Dp and the switching diode Dc are connected together in a head-to-tail arrangement.
Each photosensitive spot P
1
to P
9
is connected between a row conductor Y
1
to Y
3
and a column conductor X
1
to X
3
. The row conductors Y
1
to Y
3
are connected to an addressing device
3
known as a driver. There may be several drivers
3
if the matrix is of large size. The addressing device
3
generally comprises shift registers, switching circuits and clock circuits. The addressing device
3
raises the row conductors Y
1
to Y
3
to voltages which either isolate the photosensitive spots P
1
to P
3
connected to the same row conductor Y
1
from the rest of the matrix or turn them on. The addressing device
3
allows the row conductors Y
1
to Y
3
to be addressed sequentially.
The column conductors X
1
to X
3
are connected to a read device CL.
During an image record phase, during which the photosensitive spots P
1
to P
9
are exposed to information to be picked up and are in a receiving state, that is to say their reverse-biased photosensitive diodes Dp and switching diodes Dc each constitute a capacitor, there is a build up of charges at the junction point A between the two diodes Dp, Dc. The amount of charge is approximately proportional to the intensity of the signal received, whether this is very intense illumination, provided that the photosensitive diodes remain in the linear detection range, or darkness. There then follows a read phase, during which a read pulse is sequentially applied to the row conductors Y
1
to Y
3
, which read pulse turns on the photodiodes Dp and makes it possible for the charges accumulated in the column conductors X
1
to X
3
to drain away to the read device CL and for them to be integrated.
A read device CL will now be explained in greater detail. It consists of as many read circuits
5
as there are column conductors X
1
to X
3
and these read circuits are of the charge-integrating circuit type. Each photosensitive spot is connected to a read circuit
5
. Each charge-integrating circuit is formed by an operational amplifier G
1
to G
3
mounted as an integrator by means of a read capacitor C
1
to C
3
. Each capacitor C
1
to C
3
is mounted between the negative input of the operational amplifier G
1
to G
3
and its output S
1
to S
3
. Each column conductor X
1
to X
3
is connected to the negative input of an operational amplifier G
1
to G
3
. The positive input of each of the operational amplifiers G
1
to G
3
is taken to a constant input reference voltage VR, which sets this reference voltage on each column conductor X
1
to X
3
. Each operational amplifier G
1
to G
3
comprises a resetting switch I
1
to I
3
mounted in parallel with the capacitor C
1
to C
3
.
The outputs S
1
to S
3
of the integrating circuits are connected to a multiplexing device
6
which delivers, as a series, signals corresponding to the charges which were integrated by the charge-integrating circuits. In the read phase, these signals correspond to the charges accumulated over an integration time by all the photosensitive spots of the same row. The signals delivered by the multiplexing device
6
are then digitized in at least one analog-digital converter
7
, the digitized signals output by the analog-digital converter
7
translating the content of the image to be detected. These digitized signals are sent to a management system
8
which can store, process and display them.
It has turned out that the sensitivity of such detectors varies, which results both in local and overall variations in the brightness of the image detected. There are several causes of the variations in sensitivity. Firstly, there is a spatial variation and secondly a thermal variation. This means, on the one hand, that two photosensitive spots of the detector cannot give the same response when they are exposed to precisely the same luminous flux and, on the other hand, that a photosensitive spot exposed to the same luminous flux does not give the same response at 25° C. as it does at 35° C. These discrepancies are partly due to the semiconductor components constituting the photosensitive spots, which components do not all come from the same manufacturing batch, and partly to the scintillator material used in radiology. This results in images with nonuniform areas which should not be there and which become increasingly pale as the temperature increases.
Although it is known how to overcome the spatial variation in sensitivity by making a correction to the image with a so-called gain image, it is not possible to use the gain image to overcome thermally induced variations in sensitivity.
The gain image is an image taken with a calibrated uniform illumination in the absence of a subject or object to be examined. This gain image allows the spatial variations in sensitivity to be properly corrected, since with a uniform illumination the image should be uniform. This gain image is produced with a very low frequency, of the order of one year. The signals delivered by the photosensitive spots when the gain image is being read are stored in the management device
8
and then serve to correct, for spatial inhomogeneity in the sensitivity, any useful image.
This method cannot be used to overcome thermally induced variations in sensitivity: this would require producing gain images in synchronism with the temperature variations, which would significantly increase the frequency at which gain images are recorded. This is not compatible with the manner in which operators use such image detectors.
The present invention proposes the use of a gain image or a quasi gain image matched to the ambient temperature in order to obviate variations in the sensitivity of the image detector, especially thermally induced variations, but this gain image is not simply recorded just before making the correction, in order to be matched to the ambient temperature, but it is generated from a calculation resulting in the determination of the ambient temperature.
To achieve this, the present invention provides a method for temperature compensation of the sensitivity of an image comprising photosensitive spots, each with a photodiode, these being connected to read circuits, characterized in that the photosensitive spots are divided into detecting photosensitive spots, capable of detecting an image when they are expo
Hannaher Constantine
Sung Christine
Thales
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