1. Field of the Invention
The present invention relates to a reading method for reading a document or the like by using plural photodetectors.
2. Description of the Prior Art
A prior art relating to this invention is shown in an electric circuit diagram shown in FIG. 1. Photo detectors P1, P2, P3, . . . , Pn are respectively composed of parallel circuits which are composed of photodiodes D1, D2, D3, . . . , Dn and electric charge storage capacitors C1, C2, C3, . . . , Cn, wherein a specified quantity of electric charge is charged into these electric charge storage capacitors C1 to Cn from a bias power supply VB, and stored therein. When, for example, photodiode D1 receives light reflected from a document, the electric storage capacitor C1 discharges its stored charge corresponding to the exposed quantity, and when a individual reading switch AS1 composed of electronic switch such as analog switch is closed, the electric charge storage capacitor C1 is recharged with an electric charge corresponding to the discharged electric charge, and according to the movement of the electric charge due to this recharging, the voltage generated at output resistance RO is led out into output terminal OP through coupling capacitor CO, thereby, the document being read. Incidentally, capacitors CS1 to CSn shown in FIG. 1 are input capacities connecting respectively to individual reading switches AS1 to ASn, whereas capacitor CL is described below.
In the above electric circuit, the individual reading switches are opened and closed sequentially in time series by a switch driving circuit 10 for reading realized, for example, by shift register, and the output signals from photodetectors P1 to Pn are sequentially led out into the output terminal OP through line l51 which makes up a common electrode commonly connecting the terminals at one end of individual photodetectors P1 to Pn.
According to this prior art, the number n of photodetectors is extremely great, for example, 1,000 to 3,000 are needed even on the shorter side of A4 or B4 (Japanese Industrial Standard) format document, and since these elements are connected in parallel, the capacitance possessed by other (n-1) elements affects one element, and, as the whole, between line l51 and line l52 forming common electrodes as shown in FIG. 1 have a parallel element capacity indicated by CL. As a result, when one individual reading switch is turned on, the recharging current flows in also from the parallel element capacity CL, so that the stored charge in the electric charge storage capacitors C1 to Cn are decreased. Therefore, the output signal intensity led out into the output terminal OP is lowered, which produced a problem of relative deterioration of signal to noise ratio.
Besides, since output resistance RO is used for picking up output, a differential circuit is formed by the output resistance RO and said capacitor capacity, whereby the output signal is made up a narrow differenial waveform, making it necessary to install a shaping circuit to expand the pulse length in the later stage, which complicated the circuitry. Yet, the reading speed cannot be raised over the time constant determined by said capacitance and output resistance RO, which was a bottleneck for high speed reading.
To alleviate the abovementioned effects of such parallel element capacity CL, in the prior art, it was attempted to divide the common electrode into multiple sections (for example, 27 to 30 pieces in A4 size), and assemble the photodetectors into the groups corresponding to the number of divisions of the common electrode. However, to divide the common electrode into such a multiplicity further complicated the entire structure of the reading element and added to the manufacturing cost, while it also gave rise to the problem of deterioration of signal to noise ratio by interference of noise due to crossing and nearing of the common electrode lines and other signal lines.
Furthermore, an array of photoluminescence (such as LED array) disposed corresponding to the array of photodetectors is not uniform in the intensity of illumination in all elements, and this discrepancy of illumination resulted in uneven output voltages in the photodetector array.
In addition, when mass producing such reading devices, it is hard to attain a unifprm light receiving sensitivity characteristic in the individual photodetector arrays, and the receiving sensitivity differed among individual reading devices.