Method for controlling a photosensitive device including a read phase and a calibration phase

A method of driving a photosensitive device including a matrix of photosensitive pixels of the type produced in particular by techniques for depositing semiconductor materials. The method may be particularly directed to the driving of such devices used for radiological image detection. The method subjects the photosensitive pixel during a read phase to a cycle of successive images including a useful image preceded by at least one offset image taken at an initial instant. A correction is applied to the useful image, the correction being based on the offset image taken at the initial instant and based on the time separating the useful image from the offset image taken at the initial instant.

The present invention relates to a method of driving a photosensitive device comprising a matrix of photosensitive pixels of the type produced in particular by techniques for depositing semiconductor materials. The invention relates more particularly (but not exclusively) to the driving of such devices used for radiological image detection.

Techniques for the thin-film deposition of semiconductor materials, such as hydrogenated amorphous silicon (a-SiH), on insulating, for example glass, supports are used to produce matrices of photosensitive pixels that can produce an image from visible or near-visible radiation. To use these matrices for the detection of radiological images, all that is required is to interpose, between the X-radiation and the matrix, a scintillator screen that converts the X-radiation into light radiation within the band of wavelength at which the photosensitive pixels are sensitive.

The photosensitive pixels that form these matrices generally comprise a photosensitive element associated with an element providing a switch function. The photosensitive area is mounted between a row conductor and a column conductor. Depending on the requirements, the photosensitive device then comprises a plurality of photosensitive pixels arranged as a matrix or as a linear array.

The photosensitive element usually consists of a diode connected in series with the switch element. The switch element may, for example, be a switching diode whose “closed” or “on” state corresponds to the bias that drives it into forward conduction and whose “open” or “off” state corresponds to its reverse bias. The two diodes are connected with opposed conduction directions in a configuration called “back-to-back”. Such an arrangement is well known, for example from French patent application 86/14058 (publication No. 2 605 166) which describes a matrix of photosensitive pixels of the type with two diodes in a “back-to-back” configuration, a method of reading the photosensitive pixels and a way of producing such a photosensitive device. The amorphous semiconductor material produces persistence. This is due to its amorphous structure, which includes a large number of traps, much more than in crystalline materials. These traps are structure defects which extend over the entire band gap. They retain charges generated during image acquisition. The material stores an image corresponding to a given irradiation and restores the charges relating to this image while the following image is being read, or even while several following images are being read. The quality of the images is thereby degraded.

A defect affects image quality. The semiconductor components used in such photosensitive devices are not all identical and the photosensitive device inherently possesses inhomogeneities which are manifested by impaired regions and which vary over time.

To attempt to obtain a useful image of optimum quality, a correction is applied to the useful image on the basis of what is called an “offset image”, also known as “dark image” that is generally acquired and stored at the start of an operating cycle. This offset image is the image obtained when the photosensitive device is exposed to a signal of zero intensity and corresponds to a kind of background image. The offset image varies according to the electrical state of the components of the photosensitive pixels and to the dispersion in their electrical characteristics. The useful image is that read whenever the photosensitive device has been exposed to a useful signal that corresponds to exposure to X-radiation. It includes the offset image. The correction consists in subtracting the offset image from the useful image. This correction is reliable only if the offset image has not varied between the moment when it was taken and the moment when the useful image was taken at the moment when the useful image is taken. The photosensitive pixels must be in the same electrical state just before acquisition of the offset image and before that of the useful image. When not being driven, semiconductor components are permanently seeking an equilibrium state, which can be achieved in a few hours since the time taken to drain the traps of the stored charges extends over time ranges of between a few microseconds and a few minutes, or even a few hours. After this lapse of time, their state may still vary depending on the temperature or on infinitesimal variations in residual irradiation.

Since the offset image is generally taken at the start of an operating cycle of the photosensitive device and the useful image, initiated at the discretion of the radiographer, is taken randomly when the need arises, there is no reason for the semiconductor components to be all in the same state at these two instants, which are separated by a variable time interval.

FIGS. 1aand1bshow symbolically the state of filling of the traps of the components of a photosensitive pixel of a photosensitive device, to which the invention applies, over the course of time. The arrows represent image cycles. The term “image cycle” is understood to mean the sequence formed by an image acquisition phase followed by a read phase and then by an erase and reset phase, as explained in patent application FR-A-2 760 585. During the image acquisition phase, the photosensitive pixels are exposed to a signal to be picked up, whether this signal is a maximum illumination signal or a dark signal; during the read phase, a read pulse is applied to the row conductors addressed for reading the amount of charge stored during the image acquisition. During the erase and reset phase, the photosensitive pixels are erased, generally optically, and reset to a state in which they are receptive to a new image acquisition.

Between two successive image cycles, the photosensitive pixels are left at rest, but their electrical state will change. It will be assumed that the first image cycle shown provides the offset image and the other cycles deliver useful images to be corrected by the offset image.

It may be clearly seen that if the image cycles occur randomly, as inFIG. 1a, since the electrical states of the photosensitive pixel are different at the start of a cycle, the useful images corrected by the offset image cannot be reliable.

By contrast, inFIG. 1bthe image cycles occur regularly, for example every five seconds, and at the start of each cycle the electrical state of the photosensitive pixel is substantially the same.

The offset image has not fluctuated and the correction of a useful image taken during a cycle by the offset image taken during another, previous cycle is reliable. The major drawback with this method of operation is that it introduces many constraints since the various cycles must follow one another in a periodic manner in order to obtain the expected result.

This use is very restrictive and is not compatible with the expectations of radiologists who wish to be able to take useful images as and when they require. The aim of the present invention is to avoid this major drawback, while still guaranteeing an image of optimum quality.

For this purpose, the subject of the invention is a method of driving a photosensitive device comprising at least one photosensitive pixel with a photodiode connected to a switching element, consisting in subjecting the photosensitive pixel during a read phase to a cycle of successive images comprising a useful image preceded by at least one offset image produced at an initial instant, characterized in that a correction is applied to the useful image, said correction being based on the offset image taken at the initial instant and based directly on the time separating the useful image from the offset image taken at the initial instant.

FIG. 5shows, for a non-limiting embodiment of the present invention, a method for driving a photosensitive device. In step502, the calibration phase is performed. In step504, an initial offset image is taken. In step,506, the read phase is performed. In step508, a current offset image is taken. In step510, the temperature at which the useful image was taken is determined. In step512, a correction is applied.

As a variant, it is possible to dispense with an actual calibration phase and produce the offset ti image after the useful image. This variant may be employed by carrying out, after the useful image, a cycle comprising an offset0image40followed by an offsettiimage41. The images40and41are then separated by a time of approximately ti. A simpler and more rapid procedure may be to be content with taking an offsettiimage separated from the useful image by the time ti.