Residual image reduction apparatus for a linear image sensor

Abstract: A residual image reduction apparatus for a linear image sensor comprises a charge storage unit for photo-electrically converting an input light to charges, a charge transfer unit for sequentially transferring the supplied charges, a shift electrode for shifting the charges stored in the charge storage unit to the charge transfer unit, and a shift electrode control circuit for periodically controlling the shift operation of the shift electrode. The shift electrode control circuit includes a residual image reduction circuit which carries out, in one image read period, an image read operation by turning on the shift electrode to shift the charges stored in the charge storage unit to the charge transfer unit and a residual image charge sweep-out operation by turning on the shift electrode to shift the charges stored in the charge storage unit to the charge transfer unit several times.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a residual image reduction apparatus for a linear image sensor used in an image scanner. Related Background Art A CCD (Charge Coupled Device) has been known as a linear image sensor used for an image scanner. The CCD has a residual image phenomenon in which previous line data remains in an output data of a line read after the previous line. This is because the charges generated by the photo-electric conversion in the charge storage unit of the CCD cannot be perfectly transferred to the charge transfer unit. The residual image phenomenon appears as a signal overlapped on one line of output data as shown in Fig. 3A and it lowers the resolution power. In Fig. 3A, assuming that the charges newly generated by the photo-electric conversion in the charge storage unit of the CCD in a period T are 100 and the charges remaining untransferred to the charge transfer unit (residual component) are 10 (10% of 100), the charges at the transfer storage unit a period T after a certain time are 100 plus 10, that is, 110. In the next period T, the charges are 100 plus 10 plus 1 (10% of 10), that is, 111. By the repetition of the above, it meas that the charges of 111.11 are outputted as the image data at the start of the fifth period. This extremely lowers the resolution power. As a method for reducing the effect of the residual image, it has been proposed to shut the light for several lines after the exposure of one line to sweep out the residual image component. However, in the above method of shutting the light for several lines after the exposure of one line to sweep out the charges of the residual image, no output data is produced for the time of several lines during which the charges of the residual image are swept out and a data read rate is lowered. EP-A-0456451 describes an image reading apparatus in which a solid state image sensor is controlled so as to transfer out a first light information signal integrated during a first interval, and to transfer out a second light information signal integrated during a second interval, and including a residual image signal from the first light information signal. Thus, the dark portion of the image signal can be read as well as the bright image portion by the processing circuitry. US-A-5,105,277 describes a linear image sensor having a two stage transfer circuit in which split clock transistor actuating passes are used to cancel cell-to-cell variations and neutralise undesired couplings of the rising edge of the charges into the transistors. Only one transfer from a storage circuit is performed per video output period. SUMMARY OF THE INVENTION It would be beneficial to improve a data read rate by sweeping out the charges of residual image in a short time. In accordance with the present invention there is provided a residual image reduction apparatus for a linear image sensor, comprising: a charge storage unit for photo-electrically converting light to charges and storing the charges; a charge transfer unit for sequentially transferring the charges during charge transfer operations to produce a valid image signal and an invalid image signal; a shift electrode for shifting the charges stored in said charge storage unit to said charge transfer unit by charge shift operations; and a shift electrode and charge transfer control circuit for controlling the operations of said shift electrode and said charge transfer unit, wherein said shift electrode and charge transfer control circuit is for carrying out, in an image read period, an image read operation by performing a first charge shift operation to shift charges stored during a valid charge storage period in said charge storage unit to said charge transfer unit and by performing a first charge transfer operation to produce said valid image signal, and wherein said shift electrode and charge transfer control circuit is for carrying out, in a residual image charge sweep-out period, a residual image charge sweep-out operation by performing a plurality of further charge shift operations to shift charges stored in said charge storage unit to said charge transfer unit to produce said invalid image signal, said plurality of further charge shift operations being performed before a subsequent valid charge storage operation during which charges associated with said valid image signal are stored in said charge storage unit, characterised in that said plurality of further charge shift operations are separated by a period which is shorter than said valid charge storage period. The present invention also provides a residual image reduction method for a linear image sensor, comprising the steps of: photo-electrically converting light to charges and storing the charges in a charge storage unit, sequentially transferring the charges in a charge transfer unit during charge transfer operations to produce a valid image signal and an invalid image signal; shifting the stored charges stored in said charge storage unit to said charge transfer unit by charge shift operations of a shift electrode; and controlling the operations of said shift electrode and said charge transfer unit, wherein said shift electrode and charge transfer control step comprises carrying out, in an image read period, an image read operation by performing a first charge shift operation to shift charges stored in said charge storage unit to said charge transfer unit and by performing a first charge transfer operation to produce a valid image signal, and wherein said shift electrode and charge transfer control step further comprises carrying out, in a residual image charge sweep-out period, a residual image charge sweep-out operation by performing a plurality of further charge shift operations to shift charges stored during a valid charge storage period in said charge storage unit to said charge transfer unit and by transferring said charges in said transfer unit to produce said invalid image signal, said plurality of further charge shift operations are performed before a subsequent valid storage period during which charges associated with said valid image signal are stored in said charge storage unit, characterised in that said plurality of further charge shift operations are separated by a period which is shorter than said valid charge storage period. In the residual image reduction apparatus and method of the linear image sensor of the present invention, the charges of the residual image can be swept out in a short time by the residual image charge sweep-out operation in which the shift electrode is turned on and the charges stored in the charge storage unit are shifted to the charge transfer unit several times. As a result, the time required to sweep out the charges of the residual image is shortened and the remaining time can be used to read the image data. Consequently, the data read rate is improved. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a block diagram of one embodiment of a residual image reduction apparatus for a linear image sensor of the present invention. Fig. 2 shows waveforms for explaining an operation of the residual image reduction apparatus for the linear image sensor of the present invention; and Figs. 3A and 3B show conceptual views of the operations of the residual image reduction apparatus of the present invention and a prior art apparatus, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention is now explained with reference to the drawings. Fig. 1 shows a block diagram of one embodiment of the residual image reduction apparatus for the linear image sensor of the present invention. In Fig. 1, numeral 1 denotes a CCD linear sensor which comprises a charge storage unit 1a, a shift electrode 1b and a charge transfer unit 1c. The CCD linear sensor 1 is driven by a pulse signal f and a clock signal c supplied from a drive pulse generator 2. Each cell of the charge storage unit 1a is principally equivalent to a parallel circuit of a photo-diode and a capacitor, and a plurality of such parallel circuits are arranged in line to form the charge storage unit 1a. In the charge storage unit 1a, an input light is converted to charges by the photo-electric conversion action of the photo-diode and the converted charges are stored in the capacitor. The charges stored in the capacitor are shifted to a charge transfer unit 1c through a shift electrode 1b at a predetermined timing. The shift electrode is equivalent to a switching circuit, and when the switching circuit is turned on, the charges stored in the charge storage unit 1a are shifted to the charge transfer unit 1c. A timing to turn on the switching circuit of the shift electrode 1b is determined by the pulse signal f supplied from the drive pulse generator 2 through an OR circuit 5 as will be described later. The charge transfer unit 1c comprises an analog shift register which sequentially transfers the charges in accordance with a two-phase clock signal to produce a signal g. A two-phase clock generator 1d generates the two-phase clock signal to be supplied to the charge transfer unit 1c based on the clock signal c supplied from the drive pulse generator 2. In Fig. 1, the charge storage unit 1a, the shift electrode 1b and the charge transfer unit 1c are shown as if they comprise four cells but in actual they have 5000 cells, for example, arranged in line. The pulse signal f and the clock signal c supplied to the CCD linear sensor are generated in the manner described below. A clock pulse signal of a fixed frequency is applied to the drive pulse generator 2 from a clock pulse generator, not shown. The drive pulse generator generates a pulse signal required for driving the CCD linear sensor 1 based on the clock pulse signal supplied thereto and supplies it to the OR circuit 5, a frequency divider 4, a counter 3 and the two-phase clock generator 1d. A pulse signal a shown in Fig. 2 is supplied to the OR circuit 5 and the frequency divider 4, and the clock pulse c shown in Fig. 2 is supplied to the counter 3 and the two-phase clock generator 1d. The frequency divider 4 frequency-divides the pulse signal a to produce a pulse signal b which is supplied to an AND circuit 6. The counter 3 counts the clock signal c to produce a pulse signal d which is supplied to the AND circuit 6. The AND circuit 6 ANDs the pulse signal b and the pulse signal d to produce a pulse signal e which is supplied to the OR circuit 5. The OR circuit 5 ORs the pulse signal a supplied from the drive pulse generator 2 and the pulse signal e supplied from the AND circuit 6 to produce a pulse signal f to control the turn-on and turn-off of the shift electrode 1b. By controlling the turn-on and turn-off of the shift electrode 1b by the pulse signal f, the operation to shift the charges stored in the charge storage unit 1a to the charge transfer unit is controlled. The charges stored in the charge storage unit 1a at a period of T1 of the pulse signal f shown in Fig. 2 are shifted to the charge transfer unit 1c at a time G1. Similarly, the charges stored in the charge storage unit 1a at a period of T2 are shifted to the charge transfer unit 1c at a time G2, the charges stored in the charge storage unit 1a at a period of T3 are shifted to the charge transfer unit 1c at a time G3, and the charges stored in the charge storage unit 1a at a period of T4 are transferred to the charge transfer unit at a time G4. The charge transfer unit 1c sequentially transfers the charges shifted through the charge transfer unit 1c by the two-phase clock signal. Since the two-phase clock signal is continuously supplied to the charge transfer unit 1c from the two-phase clock generator 1d, the charge transfer unit 1c eventually produces a signal g shown in Fig. 2. Of the signal g shown in Fig. 2, a signal A corresponds to the charges stored in the charge storage unit 1a at the period T1 and shifted to the charge transfer unit 1c at the time G1. Namely, the signal A is one line of output image data. Of the signal g, a signal B corresponds to the charges stored in the charge transfer unit 1a in the periods T2 - T4 and shifted to the charge transfer unit 1c over three times of the timers G2 - G4. Namely, the signal B is the superposition of the signal corresponding to the image data and the signal corresponding to the residual image. The signal A and the signal B are separated by time and the only the signal A is externally outputted as the image data. As described above, the signal B includes the signal corresponding to the residual image after one line of image data has been read. Since the charges are shifted to the charge transfer unit 1c over three times of short period G2 - G4, the signal B is the superposition of the three signals including the residual image signal as shown by a broken line in Fig. 2. By shifting the charges from the charge storage unit 1a to the charge transfer unit 1c a plurality of times in a short time, the charges of the residual image can be swept out in a short time. This is explained in conjunction with Figs. 3A and 3B. Fig. 3B differs from Fig. 2 in that the signal including the residual image component is swept out over two times. In Fig. 3A, it is assumed that the charges newly generated by the photo-electric conversion in the charge storage unit 1a in the period T are 100, and the charges remained without being shifted to the charge transfer unit 1c (residual image component) are 10 (10% of 100). In Fig. 3B, it is assumed that the charges of 100 are swept out at a certain period (first period). A period from the end of the first period to a second period is assigned to the sweep-out of the charges of the residual image. Theoretical amount of charges to be swept out a 4T/5 period after the first period as a first sweep-out is 80. The charges of 10 remains as the residual image. Theoretical amount of charges to be swept out a T period after the first period as a second sweep-out is 20 but the charges of 8 (10% of 80) which are the residual image component of the previous sweep-out and the charge of 1 (10% of 10) are also subject of the sweep-out. In a third period, theoretical amount of charges to be swept out is 102.9 (100 + 2 + 0.8 + 0.1). The charges of 2 (10% of 20), 0.8 (10% of 8) and 0.1 (10% of 1) are residual image component of the previous sweep-out (second sweep-out). The amount of charges swept out in the third period as the image data is 102.9 and the effect by the residual image is very small. Comparing it with the example of Fig. 3A, the amount of charges swept out in the third period as the image data in Fig. 3A is 110, which includes much more effect of the residual image than the amount of charges of 102.9 in Fig. 3B. Similarly, the amount of charges swept out in the third period as the image data in Fig. 3B is 102.9029 which includes a very small effect of the residual image. Comparing it with the example of Fig. 3A, the amount of charges swept out in the third period as the image data in Fig. 3A is 111, which includes much more effect by the residual image. As described above, in a certain period, the charges are shifted from the charge storage unit 1a to the charge transfer unit 1c at the time G1 to read as the image data, and in other period, the charges are shifted from the charge storage unit 1a to the charge transfer unit 1c over three times of G2 - G4 to sweep out the residual image components. Accordingly, the charges of the residual image can be swept out in a short period of one period. In accordance with the residual image reduction apparatus for the linear image sensor of the present invention, the residual image sweep-out operation is carried out by turning on the shift electrode to shift the charges stored in the charge storage unit to the charge transfer unit over several times so that the charges of the residual image can be swept out in a short period. Thus, the time required to sweep out the charges of the residual image is shortened and the remaining time can be used to read the image data. As a result, the data read rate is improved.

Claims: A residual image reduction apparatus for a linear image sensor, comprising: a charge storage unit (1a) for photo-electrically converting light to charges and storing the charges; a charge transfer unit (1b) for sequentially transferring the charges during charge transfer operations to produce a valid image signal and an invalid image signal; a shift electrode (1c) for shifting the charges stored in said charge storage unit to said charge transfer unit by charge shift operations; and a shift electrode and charge transfer control circuit (2,3,4,5) for controlling the operations of said shift electrode and said charge transfer unit, wherein said shift electrode and charge transfer control circuit (2,3,4,5) is for carrying out, in an image read period, an image read operation by performing a first charge shift operation to shift charges stored during a valid charge storage period in said charge storage unit (1a) to said charge transfer unit (1b) and by performing a first charge transfer operation to produce said valid image signal, and wherein said shift electrode and charge transfer control circuit (2,3,4,5) is for carrying out, in a residual image charge sweep-out period, a residual image charge sweep-out operation by performing a plurality of further charge shift operations to shift charges stored in said charge storage unit (1a) to said charge transfer unit (1b) to produce said invalid image signal, said plurality of further charge shift operations being performed before a subsequent valid charge storage operation during which charges associated with said valid image signal are stored in said charge storage unit (1a), characterised in that said plurality of further charge shift operations are separated by a period which is shorter than said valid charge storage period. Apparatus according to claim 1, wherein the residual charges stored in said charge storage unit (1a) after each of said further charge shift operations, respectively, decreases during said residual image charge sweep-out operation. Apparatus according to claim 1 or 2, wherein said plurality of further charge shift operations are begun immediately after said image read period. A residual image reduction method for a linear image sensor, comprising the steps of: photo-electrically converting light to charges and storing the charges in a charge storage unit (1a), sequentially transferring the charges in a charge transfer unit (1b) during charge transfer operations to produce a valid image signal and an invalid image signal; shifting the stored charges stored in said charge storage unit (1a) to said charge transfer unit (1b) by charge shift operations of a shift electrode (1c) ; and controlling the operations of said shift electrode (1c) and said charge transfer unit (1b), wherein said shift electrode and charge transfer control step comprises carrying out, in an image read period, an image read operation by performing a first charge shift operation to shift charges stored in said charge storage unit (1a) to said charge transfer unit (1b) and by performing a first charge transfer operation to produce a valid image signal, and wherein said shift electrode and charge transfer control step further comprises carrying out, in a residual image charge sweep-out period, a residual image charge sweep-out operation by performing a plurality of further charge shift operations to shift charges stored during a valid charge storage period in said charge storage unit (1a) to said charge transfer unit and by transferring said charges in said transfer unit (1b) to produce said invalid image signal, said plurality of further charge shift operations are performed before a subsequent valid storage period during which charges associated with said valid image signal are stored in said charge storage unit (1a), characterised in that said plurality of further charge shift operations are separated by a period which is shorter than said valid charge storage period. A method according to claim 4, wherein the residual charges stored in said charge storage unit (1a) after each of said further charge shift operations, respectively, decreases during said residual image charge sweep-out operation. A method according to claim 4 or 5, wherein said plurality of further charge shift operations are begun immediately after said image read period.

Inventors: TAKAHASHI MASAMI, TAKAHASHI, MASAMI
Applicants: NIPPON KOGAKU KK, NIKON CORPORATION
Application year: 1993
Main classification: H04N3
IPC codes: H04N5, H04N1, H04N3