Image reading appratus

Abstract: An image reading apparatus, in which a solid state pick-­ up device receives light reflected from an image irradiated by a light source supplied with AC power, comprises: a zero cross pulse generating circuit to which AC supply voltage is applied; and received light amount detection starting time setting device which sets the time delayed by a specified period of time from the pulse generation by the zero cross pulse generating circuit as the time for the solid state pick-up device to start detecting the quantity of light received.

Description: Background of the Invention The present invention relates to an image reading apparatus such as a document size reading apparatus and an image density detecting apparatus for a copying machine in which a solid state pick-up device receives light reflected from the image on a document. A CCD line sensor is most widely used as the solid state pick-up device of an image reading apparatus. The light source of the image reading apparatus is normally a halogen lamp. For the CCD line sensor to read an image, it is necessary to control the quantity of light emitted from the light source according to the density of the image. For this reason, the conventional image reading apparatus is not capable of reading the image on a manuscript accurately. Summary of the Invention The object of the present invention is to provide an improved image reading apparatus which synchronizes the timing for a solid state pick-up device to detect the quantity of light received with the zero cross timing of AC power supply, to ensure that an image is irradiated with a constant quantity of light from a light source for all periods between adjacent detection start timings, thereby reading the image accurately. Briefly described, in accordance with the present invention, an image reading apparatus, in which a solid state pick-up device receives light reflected from an image irradiated by a light source supplied with AC power, is characterised in that the apparatus comprises a zero-cross pulse generating circuit to which the AC supply voltage for said light source is applied; and reflected light amount detection starting time setting means which sets the time delayed for a specified period of time after the pulse generation by the zero-cross pulse generating circuit as the time for the solid state pick-up device to start detecting the quantity of light received. Brief Description of the Drawings The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: Fig. 1 is a block diagram of the image reading apparatus of an embodiment of the present invention; Fig. 2 is a schematic construction drawing of the essential part of the image reading apparatus of the present invention; Fig. 3 is a flow chart for explaining the processing sequence by the controller of the image reading apparatus of the present invention; Fig. 4 is a timing chart of the image reading apparatus of the present invention; and Fig. 5 shows a model of a solid state pick-up device for use in the image reading apparatus of the present invention. Description of the Preferred Embodiment Fig. 2 is a schematic construction drawing of the essential part of the image reading apparatus of an embodiment of the present invention. A document rest 21 is made of hard transparent glass. A halogen lamp 22 is provided as a light source under the document rest 21. Light emitted from the halogen lamp 22 is distributed by a reflector 22a to the image-carrying surface of a document placed on the document rest 21. The halogen lamp 22 scans the image on the document by reciprocating in the directions "A" and "B" with respect to the document rest 21. Light emitted from the halogen lamp 22 is reflected by the image-carrying surface of the document and passes through mirrors 23, 24 and 25 and a lens 26 and enters a CCD line sensor 1 as indicated by a chain line in Fig. 2. Fig. 1 is a block diagram of the controller of the image reading apparatus of the present invention. The halogen lamp 22 connected with an AC power source is phase-controlled by a triac 5. Gate pulse is applied by a gate circuit 4 to the triac 5. The gate circuit 4 receives gate pulse application timing data from a phase control circuit 3. A zero cross pulse generating circuit 2 detects zero cross timing of the AC power supply to the halogen lamp 22. Receiving zero cross pulse from the zero cross pulse generating circuit 2, the phase control circuit 3 outputs to the gate circuit 4 the gate pulse application timing data based on the zero cross pulse. The above construction is typical for phase control by using a zero cross pulse generating circuit. The zero cross pulse generating circuit 2 supplies zero cross pulse to a delay circuit 6 as well. The delay circuit 6 outputs zero cross delay signals to an AND gate 7 at timings lagged for a specified period of time from the zero cross timing. A CPU 8 which constitutes a controller outputs shift pulse control signals to the AND gate 7. When both of the zero cross delay signal from the delay circuit 7 and the shift pulse control signal from the CPU 8 are "H", the AND gate 7 supplies a shift pulse "P1" to the CCD line sensor 1. On receiving the shift pulse "P1", the CCD line sensor 1 opens transfer gates 34 and 35 to send electric charge photoelectrically converted by a photo diode 31 to CCD shift registers 32 and 33. The CPU 8 supplies clock pulses 1 and 2 at specified intervals to the CCD line sensor 1. With the input clock pulses 1 and 2 as driving pulses, the CCD line sensor 1 reads the content of the CCD shift registers 32 and 33 sequentially. The read data is input through an A/D converter 13 to the CPU 8 as image data and stored in a RAM 11. Fig. 3 is a flow chart for explaining the processing sequence by the controller of the image reading apparatus. With a document set on the document rest 21, a reading switch (not shown) is ON (n1). Then, timing data is supplied to the phase control circuit 3 so that the halogen lamp 22 is turned ON (n2). The document rest 21 is moved horizontally (n3), allowing the halogen lamp 22 to scan the image on the document. Simultaneously, the CPU 8 outputs a shift pulse control signal to the AND gate 7 (n4) and clock pulses 1 and 2 to the CCD line sensor 1 (n5). Data detected by the CCD line sensor 1 is read and stored in the RAM 11 (n6). After the above process of the steps n2 through n6 is repeated until the entire document has been scanned by the halogen lamp 22 (n7), power supply to the halogen lamp 22 is shut off (n8) and the rotation of the scanning motor is inverted to return the document rest 21 to the initial position. Figs. 4 (A) through 4 (E) are timing charts of the image reading apparatus of the present invention. When supplied with AC power of sine wave form shown in Fig. 4 (A), the zero cross pulse generating circuit 2 generates pulses at the zero cross timings t₀, t₂, etc. of the AC power of sine wave form, as shown in Fig. 4 (B). Zero cross pulse is applied to the delay circuit 6, which then outputs to the AND gate 7 a zero cross delay signal lagged by (t₁-t₀) from the zero cross pulse as shown in Fig. 4 (C). Therefore, the zero cross delay signals are output from the delay circuit 6 at the same intervals as the zero cross pulses supplied from the zero cross pulse generating circuit 2. Meanwhile, the CPU 8 outputs a shift pulse control signal to the AND gate 7 for the period from t₃ to t₈. Everytime the AND gate 7 receives a zero cross delay signal from the delay circuit 6 during this period, the AND gate 7 outputs a shift pulse "P1" to the CCD line sensor 1. Thus, the intervals of the shift pulse 1 supplied to the CCD line sensor 1 coincide with those of the zero cross pulse generated by the zero cross pulse generating circuit 2. Since the zero cross pulse generating circuit 2 detects zero cross timing of the AC power supply to the halogen lamp 22, the intervals of ripples contained in the light emitted from the halogen lamp 22 also conform to those of the zero cross pulses output from the zero cross pulse generating circuit 2. Therefore, the intervals of the shift pulses supplied to the CCD line sensor 1 are the same as those of the ripples of the light from the halogen lamp 22. Accordingly, constant quantity of light is emitted from the halogen lamp 22 for all intervals between any adjacent shift pulses "P1". Here, considering that each shift pulse "P1" occurs at the same time as the CCD line sensor 1 begins detecting the quantity of light received, the quantity of light emitted from the halogen lamp 22 is constant for all intervals between any adjacent detection start timings. As shown in Figs. 4 (A) and 4 (C), when the timing of a zero cross delay signal output from the delay circuit 6 conforms to that of a gate pulse supplied to the gate circuit 4 from the phase control circuit 3, the CCD line sensor 1 can start detecting the quantity of received light when the quantity of light emitted from the halogen lamp 22 is almost at the maximum. According to the present invention, the zero cross timing of the AC power supply to the light source is detected by the zero cross pulse generating circuit, which generates a pulse when the AC power crosses the zero line. The reflected light amount detection starting time setting means sets the time delayed for a specified period of time after this pulse generation as the timing for the CCD line sensor to start detecting the quantity of light received, so that the solid state pick-up device starts detecting the quantity of received light at the set timing. Therefore, the detection of the quantity of received light by the solid state pick-up device is synchronous with the zero cross timing of the AC power supply. So, if supply voltage applied to the light source is maintained constant for all intervals between any adjacent zero cross pulses, the quantity of light projected on the image on a document should be constant for all intervals between any adjacent received light amount detection start timings. According to the present invention, as mentioned above, since the solid state pick-up device starts detecting the quantity of received light at a specified period of time after the zero cross timing of AC power supply to the light source, the intervals of received light amount detection start timings coincide with those of the zero cross timings of AC power supply. Consequently, the intervals of ripples contained in the light emitted from the light source conform to those of the timings for starting detection of the quantity of light received. As a result, the image reading apparatus of the present invention is capable of reading an image accurately with a constant quantity of light irradiated from the light source onto the image. While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the present invention as claimed. There are described above novel features which the skilled man will appreciate give rise to advantages. These are each independent aspects of the invention to be covered by the present application, irrespective of whether or not they are included within the scope of the following claims.

Claims: An image reading apparatus, in which a solid state pick-up device (1) receives light reflected from an image irradiated by a light source (22) supplied with AC power, characterised in that the apparatus comprises: a zero-cross pulse generating circuit (2) to which the AC supply voltage for said light source is applied; and received light amount detection starting time setting means (6, 7, 8) which sets the time delayed by a specified period of time from the pulse generation by said zero-cross pulse generating circuit as the time for said solid state pick-up device to start detecting the quantity of light received. The image reading apparatus of claim 1, wherein said time setting means (6, 7, 8) produces shift pulses supplied to said solid state pick-up device (1) separated by intervals which conform to those of the zero-cross pulses generated by said zero-cross pulse generating circuit (2).

Inventors: YAMADA YOSHIKADO, YAMADA, YOSHIKADO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: H04N1
IPC codes: G03G15, H04N1