Recording apparatus with beam scan information reading
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a recording apparatus in which the information on a document is read by beam scanning with a semiconductor laser generating plural beams, and more particularly to such apparatus in which the shading phenomenon is prevented at the beam scanning of the information on the document. 
     2. Description of the Prior Art 
     In a laser beam printer, the overlaying of a determined format on the output image can be achieved either electrically or optically. In the electrical method a frame memory corresponding to one page of the output image is provided for storing the format information to be overlaid, and the stored information is released simultaneously with the image signals. In the optical method, a format to be overlaid is prepared as a transparency for optical exposure, which is overlapped with a beam irradiation modulated with image signals. These methods are however defective in that the electrical method requires the expensive frame memory and cumbersome software for entering the necessary information into said frame memory, while the optical method requires a negative transparency for overlay in case the laser beam printer performs so-called negative exposure in which the exposed area turns out black in the output image. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a recording apparatus with beam scan information reading capable of overlaid recording regardless whether the laser beam printer functions on the basis of a positive or negative exposure method. 
     Another object of the present invention is to provide a recording apparatus with beam scan information reading capable of overlaid recording by means of extremely simple structure and using an optical system. 
     Still another object of the present invention is to provide a recording apparatus with beam scan information reading featured in an extremely compact dimension. 
     Still another object of the present invention is to provide a recording apparatus with beam scan information reading capable of preventing the effect of the shading phenomenon at the original scanning. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a recording apparatus embodying the present invention; 
     FIG. 2 is a perspective view of a semiconductor array laser; 
     FIG. 3 is a circuit diagram for overlaid recording; 
     FIG. 4 is a chart showing the intensity change in the original scanning beam; 
     FIG. 5 is a view showing the relation between the beam scanning position and the detector position; 
     FIG. 6 is a circuit diagram for shading prevention; 
     FIG. 7 is a circuit diagram showing another example for shading prevention; and 
     FIG. 8 is a chart showing the intensity change in the original scanning beam. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, plural laser beams La, Lb generated by a semiconductor laser 1 are collimated by a single collimating lens 2, then deflected together by a rotary polygonal mirror 3, and converged by an imaging lens 4. 
     The semiconductor laser 1 is provided with two elements a and b which are able to be independently driven on a wafer as shown in FIG. 2 in such a manner that the light-emitting points of said elements are arranged perpendicularly to the scanning plane or parallel to the direction of the rotary or deflecting axis of the deflector. Because of the collimation by a single lens, the laser beams La, Lb emerging from said two laser elements proceed in different directions and fall on spatially different positions on a mirror 5. Because of such different proceeding directions, the distance between the beams La and Lb can be arbitrarily regulated by the suitable selection of the optical path length even if the elements a, b are positioned close together. Said mirror 5 is provided with a slit SL parallel to the scanning direction. A beam Lb from the semiconductor laser passes through said slit SL and is focused on a photosensitive drum 7 (spot 1). The other beam La, falling on the mirror 5 on a position other than said slit SL, is reflected by said mirror 5 and is focused (spot 2) on the original 6 on which a form is written. When the original is moved in the direction of arrow F while the beam spot 2 of a constant intensity moves over the original 6 in a scanning motion, the reflected or transmitted light modulated according to the density of the original and is detected by a photodetector 8 to generate the original signal. The position of said photodetector 8 varies according to whether the original is light transmissive or reflective. 
     FIG. 3 shows the signal processing circuit of the present embodiment in which the original signal from the photodetector 8 is amplified by an amplifier 9 and converted into a binary signal by slicing at a suitable level in an analog-digital converter 10. The original signal thus obtained is synthesized in a logic circuit 12 consisting of an OR gate with the image signal read from an unrepresented magnetic tape or the like and stored in a semiconductor memory 11 to form a synthesized image output signal for activating a drive circuit for the element b of the semiconductor laser 1. Consequently said element b is simultaneously modulated both by the original signal and the image signal, thus achieving an overlaid recording. A positive or negative original 6 may be employed by simply inserting an inverter circuit behind the analog-digital converter 10. The time delay caused by the photodetector and the digitalization may become an obstacle in case of a high-speed image output. However if such time delay is always constant, it is possible to overcome by displacing the original in the direction of arrow F corresponding to said constant delay time. 
     It is further possible, as shown by the broken line in FIG. 3, to provide a delay circuit 14 for delaying the supply of the image signal to the logic circuit 12, and furthermore the overlapping of two forms of information can be regulated by using a delay circuit with a variable delay time. 
     The photodetector 8 in the embodiment shown in FIG. 1 is extended to cover the entire scanning width of the original 6. The use of a pin detector is desirable for a high-speed operation. However the use of such high-speed detector is often associated with the shading phenomenon or unevenness in the amount of light received from the original because of the limited light receiving area. 
     In case a single photodetector 8-1 is positioned as shown by the broken line in FIG. 1 at the center of the original scanning line in place of the photodetector 8, the shading phenomenon will result due to the lowered amount of light received from the peripheral area of the original. Such phenomenon can be prevented by causing a change in the amount of light emerging from the semiconductor laser element a. For this purpose the light intensity should be so modulated as to be proportional to 1/cos 4  θ, wherein θ is the angle between a line connecting the photodetector and the spot position and the perpendicular line to the original plane as shown in FIG. 5. Such function may be generated by a function generator. As shown in FIG. 6, a counter CA for counting clock signals supplies the count signal to digital-analog converter CO for providing a corresponding voltage signal which is supplied to a function generator FG to drive the semiconductor laser L through a driver DR, thereby modulating the intensity of the beam La as shown in FIG. 4. 
     Such function generator can be composed of an approximating function generator known in the art, and in this manner it is rendered possible to eliminate the effect of the shading phenomenon from the detection output signal from the photodetector 8-1. However, in consideration of the positional error of the photodetector 8-1, it is preferable to achieve such correction in an empirical way as explained in the following with reference to FIG. 7. This empirical method consists of: 
     (1) using a white original giving uniform reflection or transmission to the laser beam; 
     (2) regulating the laser element at a constant beam intensity; 
     (3) performing beam scanning of one line; 
     (4) converting the output signal of the photodetector into digital signal by means of an analog-digital converter 8-2 and storing the thus obtained signal in a memory ME for each spot position; and also storing the detection output of the photodetector 8-1 corresponding to the central position of the beam in a center value memory CM; and 
     (5) upon completion of the scanning of one line, supplying the data stored in said memory ME, consisting of a shift register, in succession to an arithmetic unit ALU for dividing the center value stored in said center value memory CM with said data and again storing the results of said divisions in said memory ME. 
     After the original data stored in the memory ME are completely replaced by said results of divisions obtained from the arithmetic unit, the content of the memory ME is read in succession to drive said driver DR for obtaining laser output corresponding thereto. This procedure is repeated during the scanning operation to eliminate the effect of shading from the photodetector output. 
     Although the above-explained method requires a line buffer memory for storing the information of one line, the memory capacity need not be very high since a relatively low spatial sampling frequency is enough for practical purpose. 
     FIG. 8 shows the intensity behavior of the illuminating laser in case there are provided two photodetectors. Such characteristic can also be obtained by an apparatus as shown in FIG. 7.