Optical: systems and elements – Holographic system or element – Copying by holographic means
Reexamination Certificate
2000-12-07
2003-02-04
Chang, Audrey (Department: 2872)
Optical: systems and elements
Holographic system or element
Copying by holographic means
C359S001000, C359S015000, C348S097000
Reexamination Certificate
active
06515775
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a diffusive optical element, an illumination optical system, and an image-shooting apparatus such as a film-reading apparatus.
2. Description of the Prior Art
A film-reading apparatus (film scanner) for reading images recorded on photographic film is provided with an illumination optical system for illuminating the film, a taking lens for imaging the illumination light transmitted through the film, and an image-sensing device for shooting images by receiving the light exiting from the taking lens. The film is disposed on the illumination target plane of the illumination optical system, and the image-sensing device is disposed on the imaging plane of the taking lens that lies conjugate with the illumination target plane.
In a high-performance film-reading apparatus, uniform illumination is essential, and therefore, to make the uneven intensity distribution of a light beam emitted from a light source uniform on the illumination target plane, a Köhler illumination optical system provided with an integrator is used. As the integrator, a lens array or rod is used, either of which functions basically in the same manner. Specifically, the light beam from the light source is split into a plurality of light beams, which are then imaged as secondary light-source images in the vicinity of an aperture stop of the taking lens so that the thus split light beams are superimposed on one another on the illumination target plane. This permits every portion on the film to receive all of the split light beams, which makes it possible to realize highly uniform illumination.
From the perspective of efficient use of light, it is preferable to illuminate the film with illumination light that has made into a reasonably parallel beam. However, in a film-reading apparatus, if the film is illuminated with a parallel beam, a scratch or foreign particle, if any, present on the film appears distinctly in the image shot, i.e. read, by the image-sensing device. This can be avoided by disposing a diffusive optical element in the vicinity of the film so that the film is illuminated with illumination light that has a uniform intensity distribution but is nevertheless diffusive.
The more the illumination light is diffused, the more effectively it is possible to reduce the effect on the image shot of a scratch or the like present on the film. For this reason, the diffusive optical element is generally designed to diffuse light in such a way that part of the diffused light reaches outside the area determined by the entrance-side numerical aperture of the taking lens.
A film-reading apparatus employs a light source that emits white light. In general, the intensity of the light emitted from a light source varies greatly from one wavelength range to another. For example, with a halogen lamp, the ratio of the intensity of light it emits in red, green, and blue wavelength ranges is about 10:4:1. Therefore, if the light from such a light source is used intact as illumination light, it is impossible to reproduce correctly the colors of images recorded on film.
This can be overcome by disposing, somewhere in the optical path from the light source to the image-sensing device, a filter that transmits only part of light in a wavelength range in which light intensity is comparatively high so that, in each wavelength range, an amount of light equal to that in the wavelength range in which light intensity is lowest reaches the image-sensing device. For example, with light having an intensity ratio as mentioned above, it is possible to make the amounts of light in individual wavelength ranges uniform by discarding 90% of red light and 75% of green light.
While diffusing illumination light wide is effective in reducing the effect of a scratch or the like present on the film, it simultaneously reduces light use efficiency. In particular when a common diffusion plate formed of frosted glass or of microparticles is used as the diffusive optical element, since such a diffusion plate diffuses light nearly on a perfect-diffusion basis, i.e. in such a way that the intensity of the diffused light is proportional to the cosine of the angle thereof relative to the direction normal to the diffusion plate, while the effect of a scratch or the like present on the film is reduced further, light use efficiency lowers further.
Moreover, a conventional diffusive optical element does not exhibit wavelength dependence, and thus diffuses light of different wavelengths to approximately identical degrees. Therefore, the proportion of light lost through diffusion is constant in all wavelength ranges irrespective of whether light intensity is high or low therein.
FIG. 9
shows the diffusion characteristic of a perfect-diffusion surface as plotted in a polar coordinate system, and
FIG. 10
shows the efficiency with which the light from a light source is used in a conventional film-reading apparatus employing a perfect-diffusion surface. As shown in
FIG. 10
, within a range in which the numerical aperture NA of the taking lens has practical values of 0.2 or less, light use efficiency is about 2% or less.
Low light use efficiency imposes severe restrictions on enhancement of the image-reading performance of a film-reading apparatus. In particular in a scanning-type apparatus that is provided with a line sensor as an image-sensing device and that reads an image line by line, low light use efficiency makes high-speed scanning impossible, and thus imposes a rigid limit on image-reading speed. It is of course possible to enhance image-reading performance to a certain degree by increasing the output of the light source, but doing so leads to higher electric power consumption, and in addition shortens the operating life of the light source.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an image-shooting apparatus that excels in light use efficiency, and to provide a diffusive optical element and an illumination optical system suitable for use in such an optical apparatus.
To achieve the above object, according to one aspect of the present invention, a diffusive optical element is designed to diffuse light in such a way that light incident thereon from a predetermined direction is diffused so as to exhibit the maximum intensity in an identical direction irrespective of wavelength and that, the lower the intensity of incident light in a wavelength range, the higher the rate at which the intensity of diffused light in that wavelength range is reduced with increasing angle relative to the maximum intensity direction.
This diffusive optical element diffuses light in such a way that light incident thereon from a predetermined direction is diffused so as to exhibit, after diffusion, the maximum intensity in an identical direction irrespective of wavelength. In addition, light of different wavelength ranges is diffused in such a way that, the lower the intensity of incident light in a wavelength range, the higher the rate at which the intensity of diffused light in that wavelength is reduced with increasing angle relative to the maximum intensity direction. That is, the lower the intensity of incident light in a wavelength range, the narrower the range of angles at which light is diffused in that wavelength range. Accordingly, by using diffused light of different wavelength ranges within an identical range of angles relative to the maximum intensity direction, it is possible to use light of a wavelength range in which light intensity is low (hereinafter called a low-intensity wavelength range) with high light use efficiency and use light of a wavelength range in which light intensity is high (hereinafter called a high-intensity wavelength range) with low light use efficiency.
For example, in cases where, as in a film-reading apparatus, part of light of a high-intensity wavelength range needs to be discarded to make the amount thereof equal to the amount of light of a low-intensity wavelength range, enhancing light use efficiency in a low-intensity wavelen
Chang Audrey
Curtis Craig
Minolta Co. , Ltd.
Sidley Austin Brown & Wood LLP
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