Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2000-07-10
2002-04-16
Sugarman, Scott J. (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370120, C250S370130
Reexamination Certificate
active
06373063
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for recording image information as an electrostatic latent image on an electrostatic recording material and for reading the electrostatic latent image having been recorded.
2. Description of the Related Art
In the field of radiography in medicine for example, radiation image information reading systems have been widely known. In radiography, in order to reduce a radiation dose on a patient and to improve diagnostic performance, a photoconductive material such as a selenium plate comprising a-Se (amorphous selenium) sensitive to X rays is used as an electrostatic recording material (a photosensitive material, a solid-state radiation detector). Radiation for recording, such as X rays, representing radiation image information is irradiated on the electrostatic recording material and a latent image charge representing the radiation image information is stored in a capacitor of the electrostatic recording material. Thereafter, reading light (an electromagnetic wave for reading) such as a laser beam scans the electrostatic recording material and a latent image represented by the latent image charge, that is, the radiation image information, is read by detecting a currrent generated by the scan within the electrostatic recording material by using plate electrodes or comb electrodes on both sides of the electrostatic recording material.
In such a system, the electrostatic recording material comprising the electrodes at both ends thereof and at least one photoconductive layer located inside of the electrodes are used. Radiation for recording is irradiated when a voltage for recording is supplied to the electrodes at both ends, and the latent image is formed in the capacitor of the electrostatic recording material. Thereafter, the electrodes at both ends are made to have the same potential (generally short-circuited), and the photoconductive layer of the electrostatic recording material is scanned with the light for reading, via an electrode having transmissivity to the reading light (hereinafter called an electrode on the reading light side). The latent image is electrically read by photoinductive discharge caused by pairs of electrons and holes (changed couples) generated at an interface between the electrode on the reading light side and the photoconductive layer. In this system, when the latent image is read, no electric current flows in a dark portion of the image while a larger current flows in a lighter portion of the image. This type of system in which the electrodes at both ends of the electrostatic recording material are short-circuited after recording and a larger current flows in a lighter portion of the image is called a positive system, and the electrostatic recording material used in the positive system is called a positive electrostatic recording material.
As a specific layer configuration of such a positive electrostatic recording material, several types can be used. For example, a configuration comprising a first conductive layer (electrode layer on the recording light side; hereinafter this layer is called a first conductive layer), a photoconductive layer for recording, a trap layer as a capacitor, a photoconductive layer for reading, and a second conductive layer (the electrode layer on the reading light side; hereinafter this layer is called a second conductive layer) can be used (U.S. Pat. No. 4,535,468, for example). Another configuration comprising a first conductive layer, a photoconductive layer for recording and reading, and a second conductive layer with a capacitor formed at an interface between the photoconductive layer and the second conductive layer can also be used (see Medical Physics, Vol. 16, No. 1, Jan/Feb 1989; P105-109). Alternatively, a layer configuration comprising a first conductive layer, an insulating layer, a photoconductive layer for recording and reading, and a second conductive layer, with a capacitor formed at an interface between the insulating layer and the photoconductive layer can be used.
The applicant of the present invention has also proposed a positive electrostatic recording material comprising a first conductive layer having transmissivity to radiation for recording, a photoconductive layer for recording exhibiting photoconductivity when receiving the radiation for recording, an electric charge transport layer acting approximately as an insulator to an electric charge having the same polarity as an electric charge in the first conductive layer while acting approximately as a conductor to an electric charge having the reverse polarity of the electric charge in the first conductive layer, a photoconductive layer for reading exhibiting photoconductivity when receiving reading light (an electromagnetic wave for reading), and a second conductive layer having transmissivity to the reading light, with these layers disposed in this order and a capacitor formed at the interface between the photoconductive layer for recording and the electric charge transport layer (Japanese Patent Applications Nos. 10(1998)-232824, 11(1999)-87922, and 11(1999)-89553).
However, in any of the positive electrostatic recording materials described above, a barrier electric field is created at the interface between the second conductive layer having transmissivity to the reading light and the photoconductive layer comprising a-Se or the like. As a result, a so-called photoelectromotive force noise problem, which is a problem caused by a current generated by the reading light even in an area exposed to 0mR recording radiation, occurs.
Furthermore, the photoelectromotive force noise will have local position dependency if the electrostatic recording material is continuously used. As a result, an artifact will be created.
In the photoconductive layer of the electrostatic recording material, a high-resistance amorphous material (having traps) such as a-Se is generally used. During the time from voltage supply (generally a high voltage) between the electrodes at both ends of the electrostatic recording material to the short circuit, an electric charge pours directly from the electrodes to the photoconductive layer. The charge is trapped as a space charge within the photoconductive layer or at the interfaces between the photoconductive layer and the electrodes, and leaked as a dark current in the photoconductive layer instead of being trapped as the space charge. This dark current is stored in the capacitor as a dark latent image upon reading and appears as a dark latent image noise in a reproduced image. This dark current is large in the beginning of the voltage supply, and reduced with time. Thereafter, the dark current reaches a certain leakage current value. In other words, the level of the dark current immediately after the voltage supply is larger than the level of the dark current in a stable state (the state of stable leakage current). This phenomenon is more conspicuous when the voltage is higher, and 10 minutes or more is necessary in some cases to reach the stable leakage current level. Furthermore, even if the stable state is temporarily established, the dark current level tends to reach the previous value when the voltage supply is resumed after a temporary voltage supply cut due to the short circuit between the electrodes at both ends. Therefore, the dark latent image due to the high-level dark current immediately after the voltage supply contributes largely to the reading noise. Moreover, the amount of the dark latent image changes with time from the voltage supply to irradiation of the recording light and with usage history. Therefore, correction of image data so as not to cause the dark latent image noise to appear in a reproduced image is difficult.
Moreover, as has been described above, the electric field generated by the space charge due to the supply of the recording voltage is created at the interfaces between the electrodes and the photoconductive layer. As a result of the short circuit prior to reading, a new electric field due t
Fuji Photo Film Co. , Ltd.
Hasan Mohammed
Sugarman Scott J.
Sughrue & Mion, PLLC
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