Image sensing apparatus.

The present invention relates to image sensing (or reading) apparatus for converting optical image infor­mation into electrical signals, and more specifically, relates to image sensing apparatus of the integrated type having a plurality of electro-optical conversion elements aligned linearly in a row.

Recently, image sensing apparatus has been widely used in facsimiles and word processors for reading and inputting characters and figures as image data. Conventionally, such apparatus is comprised of a plurality of electro-optical conversion elements for converting image information into electrical signals corresponding switching elements and a scanning circuit for sequentially reading the electrical signals obtained by the electro-optical conversion elements, and a control circuit for controlling the switching elements and the scanning circuit.

Figure 3 is a timing chart showing the operation of a conventional image sensing apparatus of the line scanning type. A scanning circuit receives an initiating signal SI effective to initiate each line scanning of <u style="single">n</u> linearly arranged electro-optical conversion elements through corresponding switching elements and outputs an ending signal SO indicative of ending of each line scanning when the n-th conversion element is accessed to read out a corresponding n-th electrical signal SIG. The scanning circuit is responsive to clock pulses CK to scan sequentially the conversion elements. The scanning circuit is comprised of <u style="single">n</u> flip-flops, and the last flip-flop produces an output pulse, which is used directly as the ending signal SO so that it coincides to the n-th electrical signal SIG.

Accordingly, in the above described conventional image reading apparatus, the n-th electrical signal SIG produced by the n-th conversion element includes switching noise due to the ending signal SO. There­fore, the signal-to-noise ration (S/N ratio) is reduced and this is a drawback.

The present invention seeks to provide image sensing apparatus which avoids the reduction of S/N ratio of the electrical signal of the last electro-­optical conversion element in spite of the occurrence of an ending signal for a scanning operation.

According to one aspect of the present invention there is provided an image sensing apparatus characterised by comprising: a plurality of electro-­optical conversion elements arranged integrally in a row to constitute a line image sensor for effecting electro-­optical conversion to produce respective bit image signals; a plurality of switching elements connected to the corresponding conversion elements and switchable between ON and OFF states to read out the corresponding respective bit image signals; a scanning circuit for scanning sequentially the switching elements sequentially to turn them ON thereby to effect the sequential reading of the bit image signals; an initiating signal generating circuit connected to the scanning circuit for generating an initiating signal effective to initiate the scanning circuit; an ending signal generating circuit connected to the scanning circuit and operative after the last switching element is turned OFF for generating an ending signal indicative of the ending of scanning operation; a control circuit receptive of the initiating and ending signals to produce a control signal; and a switching circuit switched according to the control signal to output the bit image signals.

Preferably, each of the switching elements comprises a MOS transistor.

The control circuit may include means for producing a control signal after the receipt of the initiating signal and before the receipt of the following ending signal.

The switching circuit may comprise a gate circuit connected commonly to the switching elements and made open according to the control signal to output the series of image bit signals during the scanning operation for one line.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- <ul id="ul0001" list-style="none"><li>Figure 1 is a schematic block diagram of one embodiment of an image sensing apparatus according to the present invention;</li><li>Figure 2 is a timing chart to illustrate the operation of the image sensing apparatus of Figure 1; and</li><li>Figure 3 is a timing chart to illustrate the operation of a conventional image sensing apparatus.</li></ul>

As shown in Figure 1, the inventive image reading or sensing apparatus includes a plurality of opto-­electro conversion elements 11, 12, ..., 1n arranged integrally in a row to constitute a line image sensor. Each of the conversion elements operates to convert bit image information into a corresponding bit electrical signal. A plurality of switching elements 21, 22, ..., 2n are connected between the corresponding respective conversion elements 11, 12, ..., 1n and a common output signal line 32, and are operable to switch between ON and OFF states selectively to feed the corresponding bit electrical signals to the common output signal line 32. A scanning circuit 3 is composed of, for example, a plurality of multi-stage flip-flops and is connected to the switching elements 21, 22, ..., 2n for sequentially scanning them and to selectively switch them. The scanning circuit 3 is responsive to clock pulses CK fed from a clock input terminal 30 sequentially to produce scanning pulses effective sequentially to turn ON the switching elements 21, 22, ..., 2n in order. An initiating signal generating circuit 4 is connected between a pulse input terminal 9 and the scanning circuit 3 for generating an initiating signal SI effective to initiate the scanning circuit 3 to start the scanning operation. An ending signal generating circuit 5 is connected to the last stage flip-flop of the scanning circuit 3 and operative after the last switching element 2n is turned OFF for generating an ending signal SO to an output terminal 31 indicating that the scanning operation of one line is completed. In an image sensing apparatus of multi-chip line sensor type, this ending signal SO is utilised as the next initiating signal SI for initating the next line scanning operation. A control circuit 6 receives the initiating signal SI and the ending signal SO and produces a controlling or selecting signal SEL. An additional switching or gate circuit 7 is connected to the common image signal line 32 and is driven according to the controlling signal SEL to feed sequentially an electric bit image signal SIG to an image signal output terminal 8.

Figure 2 is a timing chart illustrating the reading or sensing operation of the image sensing apparatus of Figure 1. The scanning circuit 3 is initiated in response to the initiating signal SI to start the sequential scanning of the switching elements 21, 22, ..., 2n sequentially to turn them on thereby to transfer the bit image signals SIG produced by the corresponding conversion elements 11, 12, ..., 1n to the common image signal line 32. In order to turn on the i-th switching element 2i (i = 1, 2, ..., n), the scanning circuit 3 is driven in response to the trailing edge of the i-th clock pulse CK to produce a scanning pulse effective to turn on the corresponding i-th switching element 2i for the duration during which the i-th clock pulse is held low level. Consequently, the i-th bit image signal SIG generated by the i-th conversion element 1i (i = 1, 2, ..., n) is transferred through the corresponding conductive i-th switching element to the common image signal line 32. The trans­ferred i-th bit image signal is outputted through the switching circuit 7 to the image signal output terminal 8.

The ending signal generating circuit 5 is operated immediately after the last n-th bit image signal is outputted from the n-th conversion element 1n for generating the ending signal SO. As opposed to the conventional operation shown in Figure 3, the ending signal SO does not coincide to the last bit image signal, but is shifted therefrom so that the last bit image signal does not have an added noise component due to the ending signal Therefore, even when reading or sensing a dark image, each bit image signal SIG has uniform signal level substantially free from noise as shown in Figure 2.

Lastly, the controlling circuit 6 produces the control signal SEL during one line scanning operation immediately after the occurrence of initiating signal SI and immediately before the occurrence of the ending signal SO. The control signal SEL is effective to turn on the switching circuit 7 only during each line scanning operation thereby sequentially to output a series of bit image signals SIG from the common image signal line 32 to the image signal output terminal 8.

Though each switching element is composed of a single MOS transistor for simplification in the embodiment shown in Figure 1, each switching element may be composed of CMOS transistors. Further, the scanning circuit 3 is driven in response to the trailing edges of the clock pulses CK, but it may be driven in response to the leading edges of the clock pulses CK. In such a case, the other circuits may need to be modified accordingly.

Thus, in the image sensing apparatus of Figure 1, the ending signal indicative of the ending of one line scanning is produced after the last bit image signal is read out from the last conversion element to prevent noise due to the ending signal from being superposed on the bit image signal thereby to achieve the advantage that the S/N ratio of the image sensing apparatus is improved.