1. Field of the Invention
The present invention relates generally to an image sensor, and more particularly to an image sensor that performs a Time Delay Integration (TDI) scanning at high speed.
2. Description of Related Art
A charge coupled device (CCD) line sensor is a one-dimensional sensor in which photo-sensors receiving image light are arranged in line. To image a two-dimensional image, the CCD line sensor or the object is moved so that the object can be imaged on a line-by-line basis. A scanner having the CCD line sensor is used in a copy machine and a product inspection machine, for example.
When the CCD line sensor captures the object that is moving at high speed, or when the CCD line sensor is moved at high speed to capture the object, the signal charge must be repeatedly accumulated and transferred at high speed on a line-by-line basis. Consequently, the signal charge can be accumulated for only a short period of time per line. For this reason, there is a problem in that a sufficient quantity of light is not obtained for imaging.
To solve this problem, a TDI sensor is used to scan the object at high speed. The TDI sensor is constructed in such a way that a plurality of CCD line sensors are arranged in a scan direction. The TDI sensor transfers signal charge accumulated in the CCD of each line to the CCD of the next line in synchronism with the movement of the image. Consequently, the signal charge is successively accumulated in the plurality of CCD line sensors, and the high-speed scanning can obtain a sufficient quantity of light for imaging.
The scan speed of the TDI sensor is limited. That is because there is a limit to a speed at which the signal charge accumulated in the CCD is shifted to the outside. Specifically, the signal charge accumulated in the CCD line sensors is finally discharged to the outside as serial analog signals, and an A/D converter converts the analog signals into digital signals. Owing to the limited processing capacity of the A/D converter, the scan speed is limited.
To raise the scan speed, one CCD line sensor is divided into a plurality of patches, and the signal charge accumulated in each patch is discharged as serial analog signals, which are converted into digital signals by each of a plurality of A/D converters provided for each patch. In this case, however, there is a limit to the width that can be divided into patches, and therefore, the scan speed cannot be raised sufficiently.
The present invention has been developed in view of the above-described circumstances, and has as its object the provision of an image sensor that realizes a high-speed scanning.
To achieve the above-mentioned object, the present invention is directed to an image sensor comprising: a light receiving part having photo-sensors, the photo-sensors receiving object light and converting the object light into signal charge, the photo-sensors being arranged in a plurality of lines in a line direction and being arranged in a plurality of lines in a scan direction perpendicular to the line direction; a plurality of scan direction signal charge transfer parts for transferring the signal charge, read from the photo-sensors, in the scan direction in accordance with a scan speed and accumulating the signal charge representing the same image read from the photo-sensors in the same line, each of the scan direction signal charge transfer parts being provided for each line of the photo-sensors in the scan direction; a plurality of line direction signal charge transfer parts for transferring the signal charge, transferred through the scan direction signal charge transfer parts, in the line direction, each of the line direction signal charge transfer parts connecting with every a predetermined number the scan direction signal charge transfer parts; a plurality of digitizing parts for converting a plurality of signal charge lines, transferred through the line direction signal charge transfer parts, to digital signals; and an image signal producing part for producing image signals according to the digital signals output from the digitizing parts.
According to the present invention, the signal charges are read from the photo-sensors and are transferred in the scan direction in accordance with the scan speed, and the signal charges read from the photo-sensors on the same line, which represent the same image, are accumulated. The signal charges of every the predetermined number lines in the line direction are sequenced in a signal charge line in the line direction. The signal charge lines are transferred in the line direction, and are separately converted to the digital signals, from which the image signals are generated.
Thus, the signal charge line representing the image of one line can be converted into the digital signal by a plurality of processing circuits. For this reason, one processing circuit processes only a small amount of data, and the object can be scanned at high speed.
The photo-sensors are arranged at intervals of more than predetermined distance at the light receiving part, and the light receiving surface of each photo-sensor can be enlarged by shifting the peripheral circuits. As a result, the light receiving surface can receive a large quantity of light.