1. Field of the Invention
The present invention relates to a color exposing device for a recording apparatus, which is adapted to imagewise scan a photosensitive recording medium simultaneously or sequentially with two or more modulated light beams having different wavelength bands corresponding to respective colors.
2. Discussion of the Prior Art
A conventional color exposing device is shown generally at 130 in FIG. 18, wherein a photosensitive medium is imagewise exposed to three wavelength components of light corresponding to three primary colors, i.e., blue, green and red. The color exposing device 130 has three beam generators 131, 132 and 133 for generating three modulated light beams corresponding to the three primary colors of light. Each of these beam generators 131, 132, 133 includes a light source 135 such as a halogen lamp which produces an incoherent radiation in the visible spectrum, a collimator lens 136 for converging the visible radiation produced by the light source 135, and an optical modulator 134R, 134G, 134B such as an electro-optic modulator using a PLZT crystal. The optical modulators 134R, 134G, 134B receive respective color image signals representative of red, green and blue color images, and modulate the intensities of the radiations according to the received color image signals, respectively. The color exposing device 130 uses a dichroic mirror 137 which reflects only the blue component of the modulated radiation generated by the blue beam generator 131, whereby a signal-modulated light beam corresponding to the blue color is produced. The device 130 also uses a dichroic mirror 138 which reflects only the red component of the modulated radiation generated by the red beam generator 133, to thereby produce a signal-modulated light beam corresponding to the red color. These blue and red modulated light beams are incident upon a converging lens 139, with the blue light beam passing through the dichroic mirror 138. The blue and red components of the modulated radiation generated by the green beam generator 132 are reflected by the dichroic mirrors 137 and 138, respectively. That is, only the green component of the radiation from the generator 132 is transmitted through the dichroic mirrors 137, 138, as a signal-modulated light beam corresponding to the green color is received by the converging lens 139. Thus, the signal-modulated light beams corresponding to the three primary colors of light are combined into a composite exposing radiation at the converging lens 139, and is converged at an aperture 140 of a stop. The diameter of the composite exposing radiation is controlled by the aperture 140 and is focused by lens 141 on the surface of a photosensitive recording medium on a platen roll 142. Consequently, a composite scanning spot to imagewise expose a local portion of the surface of the recording medium is formed. The scanning spot is moved along a line on the recording medium parallel to the rotation axis of the platen roll 142 by a suitable beam deflecting device, as the radiations produced by the light sources 135 of the generators 131, 132, 133 are modulated according to the respective color image signals. At the end of scanning of each line on the recording medium, the platen roll 142 is rotated to scan the next line.
The photosensitive recording medium has three different photosensitive materials which are sensitive to respective bands of wavelength of the blue, green and red light beams. Accordingly, a latent image corresponding to an original image represented by the color image signals applied to the modulators 134B, 134G and 134R is formed on the photosensitive recording medium.
In the color exposing device 130 constructed as described above, the three beam generators 131, 132, 133 use the respective light sources 135. However, only the appropriate wavelength band of the radiation generated by each light source 135 is utilized. Therefore, the operating efficiency of each light source 135 is considerably low, and the exposing device 130 tends to be large-sized and expensive.
Further, it is difficult to align the optical axes of the blue, green and red modulated light beams accurately with each other when the light beams are combined into a composite exposing radiation, i.e., white light having color image information to be reproduced. This difficulty in establishing the optical axis alignment lowers the manufacturing efficiency of the device 130, and results in misalignment of the blue, green and red spots formed on the photosensitive recording medium, for example, in the scanning direction parallel to the axis of rotation of the platen roll 142.
Where a white laser light source is used in each beam generator 131, 132, 133, the output of the laser source for each of the three primary colors of light is as low as about several mW. Namely, the intensity of the used wavelength component of the white laser radiation is relatively low, whereby a relatively long time is required to expose the recording medium to the modulated light beams corresponding to the three primary colors.
The color exposing device 130 described above is adapted such that the photosensitive recording medium is simultaneously exposed to the three modulated light beams having respective wavelength bands, for each scanning line. However, each scanning line may be exposed to the three light beams sequentially or at different times while the recording medium is fed at a constant speed in the feeding direction which intersects the scanning line, normally at right angles. In this arrangement, the recording medium is irradiated along each scanning line three times, first with a light beam modulated according to the blue image signal for the line, then with a light beam modulated according to the green image signal, and finally with a light beam modulated according to the red image signal.
In the color exposing device adapted for sequential exposure of each scanning with the three modulated light beams as indicated above, however, the lines of beam spots formed with the three light beams are more or less deviated from each other in the feeding direction of the recording medium, since the recording medium is continuously fed in that direction while the position of the nominal scanning line on the recording medium is held constant, for all the three light beams. Consequently, the three scanning spots formed by the three light beams along each scanning line are not precisely matched or aligned with each other, whereby the color reproduction accuracy is deteriorated.