Abstract:
An optical scanner and method in which light beams from a plurality of sources are directed onto a medium to form a composite spot. The relative positions of the beams are detected and changed to change the diameter of the composite spot so that lines formed by such spots appear unbroken. The modulating times, intensities and number of sources of the beams can also be varied to provide a linear relation between the dither matrix and the picture element density.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical scanner having a plurality of light sources forming a composite spot and capable of changing the diameter of the composite spot. 
     As shown in FIG. 1, when a line inclined by 45° is printed in a laser printer, spot diameter D must be larger than pitch d of a scanning line or the inclined line will appear broken. When the intensity distribution of the spot is of an isotropic Gauss type with a central intensity I o  and the spot diameter is defined as a point having intensity I o  /exp(-2), the light intensity Ia at point a is provided by the following formula (1). ##EQU1## When a region having an intensity not less than I o  /exp(-2) is developed, the condition for preventing interruption of a line inclined by 45° is provided by the following inequality (2). 
     
         I.sub.a ≧I.sub.o /exp(-2) 
    
     From the formula (1) and the inequality (2), the following inequality (3) is formed. 
     
         D/d≧1.22 
    
     The dither method is one method for representing the gradation of an image. In the dither method picture elements are decomposed to a matrix and the density of the picture elements changed by changing the coating ratio of this dither matrix. When a dither pattern is printed under the condition of the inequality (3), as shown in FIG. 2, the spot diameter is larger than the area of one dither matrix constituting picture element 71. Therefore, as shown in FIG. 3, the relation between the dither matrix coating ratio and the density of the picture element is not a linear relation. Thus the gradation representation becomes inaccurate in the region in which the dither matrix coating ratio is high. In FIG. 3, the density of the picture element is 1 at its maximum. 
     SUMMARY OF THE INVENTION 
     To overcome the conventional problems mentioned above, an object of the present invention is to provide an optical scanner for providing a preferable gradation as represented by the dither method. 
     To attain the above object, in the present invention, a light spot on a scanning medium is constructed by a plurality of light beams. The diameter of a composite spot is changed by changing the relative positions of the light beams on the scanning medium which form the composite spot. The diameter of the composite spot is further changed by changing the ratio of the number of oscillated beams. The spot diameter is effectively changed by changing the intensity of the beams. The spot diameter is further effectively changed by changing the modulating time of the above beams to change the moving distance of the beams on the scanning medium. The size of the spot diameter is changed as above to provide a linear relation between the dither matrix coating ratio and the picture element density at the time of printing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more apparent from the following description of the preferred embodiments thereof in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic view showing the positions of a spot on a scanning face when a line inclined by 45° is printed; 
     FIGS. 2 and 3 are schematic views showing the relation between a picture element and the spot when a dither pattern is printed; 
     FIG. 4 is a schematic view showing an optical scanner in an embodiment of the present invention; 
     FIG. 5 is a schematic view showing the positions of two light spots on the scanning face; 
     FIG. 6 is a graph showing the relation between the distance between the two spots on the scanning face and the light intensity distribution thereof; 
     FIG. 7 is a graph showing the relation between the distance between the two spots on the scanning face and the diameter of a composite spot thereof; 
     FIG. 8 is a schematic view showing the modulating time of a light beam and the movement of the spot on the scanning face; and 
     FIGS. 9, 10 and 11 are graphs showing the relation between the dither matrix coating ratio and the picture element density. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiments of an optical scanner of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 4 shows an optical scanner in accordance with one embodiment of the present invention. Light beams 1, 2 emitted from two light sources 11, 12 are collimated by coupling lenses 13, 14 and are reflected by movable type reflectors 5, 6. The reflected light beams are incident into beam splitter 15 and separated onto the sides of rotary polygon mirror 16 and photodetector 3. The light beam separated on the side of rotary polygon mirror 16 is focused onto photosensitive drum face 18 by F theta lens 17, etc., and a scanning line is formed by the rotation of rotary polygon mirror 16. The light beam separated on the side of photodetector 3 is also focused onto photodetector 3 by throttle lens 19. Photosensitive drum face 18 and photodetector 3 are arranged on the focusing face of the lens system so that the photosensitive drum face 18 and the photodetector 3 and the light sources 11, 12 are arranged in an optically conjugate relation with respect to each other. When the relative positions of the two light spots produced on the photosensitive drum faces by light beams 1, 2 are changed, the positions of the two spots are detected by photodetector 3 and movable type reflectors 5, 6 are moved by a servo circuit 4 to change the positions of these reflectors, and hence the light spots. 
     FIG. 5 shows the relative positions of the two spots 21 having diameter D on the photosensitive drum face. The light intensity of the respective spots is assumed to have an isotropic Gauss distribution. When the central distance δ of the photosensitive drum in the rotary direction thereof (which is called the secondary direction in the following description) is changed, the intensity distribution I is provided by the following formula (4) and is shown by the solid line in FIG. 6. ##EQU2## 
     Further, when only one of the two light sources 11, 12 is oscillated the intensity distribution I is provided by the following formula (5) and is shown by the broken line in FIG. 6. 
     
         I=I.sub.o /2 exp{-2(x-δ/2/D).sup.2 } 
    
     When the outputs of both light sources 11 and 12 are half that in the case of the formula (4), the intensity distribution I is provided by the following formula (6) and is shown by the broken line in FIG. 6 if δ=0, similar to the case of the formula (5). ##EQU3## 
     FIG. 7 shows the relation between the above central distance δ and diameter D&#39; of a composite spot of the two spots when the spot diameter is defined at a point providing the intensity I o  /exp(-2). In this figure, the solid line, the broken line and one dotted chain line respectively show the spot diameters corresponding to the formulas (4), (5) and (6). The spot diameter by the broken line is constant irrespective of the central distance δ. 
     As shown in FIG. 8, with respect to the scanning direction of the light beam by the rotary polygon mirror, the moving distance of the spot on the photosensitive drum face is changed by changing the modulating time I of the light beam so that the spot diameter can be effectively changed. 
     As shown in FIG. 9, a condition for providing the linear relation between the dither matrix coating ratio and the picture element density cannot be found by the relation of the inequality (3). FIG. 3 shows the position relation of the spot when the dither matrix coating ratio is 100% at the printing time of the dither pattern. The light intensity Ib at point b is provided by the following formula (7). ##EQU4## 
     When a region having a light intensity not less than the intensity I o  /exp(-2) is developed, the condition in which the point b is developed and the picture element density is maximum is provided by the following inequality (8). 
     
         I.sub.b ≧I.sub.o /exp(-2) 
    
     From the formula (7) and the inequality (8), the following inequality (9) is formed. 
     
         D&#39;/d≧1.09 
    
     FIG. 10 shows the relation between the dither matrix coating ratio and the picture element density. This relation is approximately a linear relation when D&#39;/d=1.12 and provides a preferable gradation representation. Accordingly, the line inclined by 45° is not interrupted and the print having a preferable gradation can be provided when D&#39;/d≧1.22 at the printing time of a line drawing image and D&#39;/d=1.12 at the printing time of the dither pattern. The spot diameter can be changed by changing the distance δ between the centers of the two spots on the scanning face and the modulating time t of the light beams as mentioned before. 
     By using this system, the gradation representation can be arbitrarily changed in accordance with what is necessary at the printing time of the dither pattern. For example, when the dither matrix coating ratio is R, D&#39;/d is changed in accordance with the following formula (10). 
     
         D&#39;/d=1+0.12R.sup.4 
    
     The picture element density is changed as shown in FIG. 11 and is gradually changed in a region of a small dither matrix coating ratio and is steeply changed in a region of a large dither matrix coating ratio. 
     Further, this system can be similarly applied to a method for representing other gradations such as a reticular point method, etc. This system can be similarly applied to the case in which it is preferable to change the spot diameter by switching the printing density, etc., in the line drawing image as well as an image providing the gradation representation. 
     As mentioned above, in accordance with the present invention, a plurality of light beams are employed so the spot diameter on a scanning medium can be effectively changed by changing the relative position of the light beams on the scanning medium, by changing the number of oscillated light beams, and by changing the modulating time of the light beams. Accordingly, the relation between the dither matrix coating ratio and the picture element density becomes linear at the printing time of the dither pattern, thereby providing a preferable gradation representation.