1. Field of the Invention
The present invention relates to an optical system for use in a copier utilizing a combination of a halogen lamp and a CdS photosensitive member, said optical system comprising a multi-layered coating capable of matching the spectral characteristics of the light from an original with the spectral sensitivity of the photosensitive member.
2. Description of the Prior Art
In a conventional copier utilizing a reflective projection lens system, the mirror contained in said lens system is usually composed of a metal coated mirror for example of aluminum, which shows a uniformly high reflectivity from the near-ultraviolet to infrared wavelength region. Thus, in case of using a halogen lamp light source, having a light-emission energy distribution highest in the infrared region of 800 to 900 m.mu. and gradually decreasing toward the shorter wavelength region at an ordinary filament temperature of approximately 3000.degree. C., in combination with a CdS photosensitive member having highest spectral sensitivity in the infrared to near-infrared wavelength regions, there will frequently result an excessive exposure in the red wavelength region in comparison with the green or blue wavelength region due to the additive effect of the above-explained characteristics of the light source and photosensitive member, leading to the defect that a red image, such as a red stamp, is reproduced unsatisfactorily or not at all on the copy.
In order to avoid such unmatching between the spectral characteristics of the incident light and the spectral sensitivity of the photosensitive member there is already known, as disclosed in the specification of German OLS No. 2350281, FIG. 6, a method of intercepting the light of unnecessary wavelength region by means of a filter placed in the imaging lens or in the vicinity thereof where the diameter of the light beam is smaller, but such method is unsatisfactory, as pointed out in said specification, in that a reflective lens system, different from a transmission lens system, will result in a significant loss of quantity of light as the imaging light bean passes the filter twice by entering into and emerging from the lens system and in that the imaging performance becomes inevitably deteriorated due to the reflections on the filter surfaces. Also in the IBM Technical Disclosure Bulletin, vol. 15, No. 2, July 1972, P. 522, there is disclosed a dichroic mirror for suppressing the light in the infrared and red wavelength regions. It is therefore expected to maintain desirable transmittance and imaging performance if the light of required wavelength region is reflected while that of unnecessary wavelength region is removed from transmission by providing the reflecting surface of the reflective lens system with a multi-layered interference coating of a property similar to that of the color separation dichroic mirror used in a color television camera, but the desired performance cannot be obtained when there is employed a multi-layered interference coating prepared by merely displacing the central wavelength of a conventional dichroic mirror. In order to obtain a reproduced image of an elevated quality, therefore, it is necessary to suitably suppress the exposure in the near-infrared wavelength region in response to the spectral characteristics and sensitivity of the combination of the halogen lamp and CdS photosensitive member while maintaining the exposures in the blue and green wavelength regions.