Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2002-06-27
2004-05-18
Lee, John R. (Department: 2881)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370080, C348S245000
Reexamination Certificate
active
06737654
ABSTRACT:
The present invention relates to a method of temperature compensation of an image detector, which makes it virtually insensitive to temperature fluctuations and which in particular guarantees that it has an approximately constant image dynamic range whatever the temperature.
In this type of image detector, the acquisition of an image takes place with the aid of one or more 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 spots, 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 by said spots are then generally 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 known type. It has only nine photosensitive spots in order not to 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 element having a switching function. The photodiode Dp and the switching diode Dc are connected together in a back-to-back 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 a signal 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.
The read device CL will now be explained in greater detail. It consists of as many read circuits as there are column conductors X
1
to X
3
and these read circuits are of the charge-integrating circuit type
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 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 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-to-digital converter
7
, the digitized signals output by the analog-to-digital converter
7
translating the content of image to be detected. These digitized signals are sent to a management system
8
which can store, process and display them.
Defects affect the quality of the useful images from such photosensitive devices.
The semiconductor components of the photosensitive spots exhibit remanence which is associated especially with their imperfect crystalline structure. Charges corresponding to an image record phase are not read during the associated read phase and are reproduced during the read phase for a subsequent image. To try to overcome remanence problems, it has been proposed, especially in patent application EP-A-364 314, to add to the charge due to the signal to be picked up a drive charge and to apply, between two read pulses, a bias pulse whose amplitude is generally less than that of the read pulse.
The semiconductor components of the photosensitive spots are not all exactly identical and the matrix of photosensitive spots has locally impaired regions. The components of the read device CL also contribute inhomogeneities.
It is common practice to correct the useful image with what is called an offset image, also known as a “black image”. This black image is made at the start of the operating cycle by exposing the image detector, during the image record phase, to a signal of zero intensity, and then carrying out the read phase.
The offset image is produced in the absence of any illumination and the charges read, at the photosensitive spot, during the corresponding read phase, fall within the following three categories. The first represents the drive charges, the value of which is given by:
Q
=(
VP
2−
VP
1)
Cp
with:
VP2, the amplitude of the read pulse;
VP1, the amplitude of the bias pulse (these read and bias pulses are delivered by the addressing circuit
3
); and
Cp, the capacitance of the photosensitive spot P
1
to P
9
.
The second category of charges corresponds to the charges arising from the leakage current of the photodiode Dp of the photosensitive spot read, this current being established between the application of two successive read pulses or bias pulses.
The third category of charges corresponds to the charges arising from the leakage current emanating from all the photosensitive spots connected to the same column conductor as that which is read, but only during the read phase.
However, it has turned out that although the first category of charges is relatively temperature-stable, the same does not apply in the case of the other two categories of charges. The offset image varies with temperature. This variation may be very significant, for example at 25° C., the electric charge, accumulated at a photosensitive spot of the offset image and converted into a voltage by the charge-integrating circuit
5
, may be 0.5 volts, while it may reach up to 2 volts at 50° C.
This phenomenon is annoying; it may be overcome, but in a restricting manner, by recording offset images often and by correcting the useful image with these offset images, at a sufficiently high frequency compared with the t
Gill Erin-Michael
Trixell
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