Beam splitting for camera using a multilayer film

Details Beam splitting for camera using a multilayer film Beam splitting for camera using a multilayer film

2001-01-31

2003-12-09

Mack, Ricky (Department: 2873)

Optical: systems and elements

Single channel simultaneously to or from plural channels

By partial reflection at beam splitting or combining surface

C359S638000, C348S341000

Reexamination Certificate

active

06661579

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a beam splitting element for a single reflex lens digital camera.
Recently, digital cameras have become widely used instead of cameras using silver-salt films. Among such digital cameras, an SLR (single lens reflex) type digital camera is advantageous since the image of the object formed by the photographing lens of the camera is observed through the finder, no parallax is generated between the image captured by an image capturing element such as a CCD (Charge Coupled Device), and the image observed through the finder.
An example of the SLR digital camera is provided with a beam splitter which splits light passed through the photographing lens into light directed to the image capturing element and light directed to the finder optical system. The beam splitter includes a beam splitting surface (i.e., a half mirror surface), which may be formed with a multi-layer film or coating made of dielectric material. The multi-layer film is generally designed to exhibit optimum reflecting/transmitting characteristics for the visible light, whose wavelength range is, for example, approximately from 400 nm to 700 nm.
FIG. 1
shows an example of a conventional photographing optical system
10
′ for an SLR digital camera employing a beam splitting element
2
′ and a CCD
3
. The beam splitter
2
′ is provided with a half mirror surface
2
&agr;′. The half mirror surface
2
&agr;′ is provided with a multi-layer film which is designed to have an optimum reflectivity (e.g., 30%) for the visible light incident thereon at a predetermined incident angle (e.g., 45°).
FIG. 2
shows a graph indicating a relationship between the reflectivity with respect to the wavelength of a beam incident on the half mirror
2
&agr;′ at the incident angles of 45° and (45±10)°.
As shown in
FIG. 2
, when the incident angle of the beam is 45°, the beam is reflected at the reflectivity of 30% substantially at any wavelength within the visible range (i.e., 400 nm through 700 nm). However, light from the object includes a beam which is inclined with respect to the optical axis of the photographing optical system. Generally, the incident angle of such a beam with respect to the half mirror surface
2
&agr;′ is within a range approximately from 35° to 55°. The characteristics of the beams incident on the half mirror surface
2
&agr;′ at the incident angles of 35° and 55° are also indicated in FIG.
2
. As shown in
FIG. 2
, the reflectivity characteristics vary depending on the incident angle. Specifically, when the incident angle is lowered with respect to a designed angle (i.e., 45°0), the characteristic shift in the longer wavelength side (i.e., right-hand side in FIG.
2
), while if the incident angle increases, the characteristic shift in the shorter wavelength side. If a wavelength range corresponding to the maximum reflectivity (i.e., 30% in
FIG. 2
) shifts as the incident angle changes, the color of an object cannot be captured accurately. For example, if the incident angle decreases, components having shorter wavelengths (e.g., 400 nm) are reflected by the half mirror surface
2
&agr;′ at a lower reflectivity. Accordingly, the reflected light includes less lower-wavelength components than the light from the object. This also causes the light transmitted through the half mirror surface
2
&agr;′ to include more lower-wavelength components. Similarly, if the incident angle increases, components having longer wavelengths (e.g., 700 nm) are reflected at a lower reflectivity. In such a case, the reflected light includes less higher-wavelength components than the light from the object, and the light transmitted through the half mirror surface
2
&agr;′ includes more higher-wavelength components than the light from the object. If such a phenomenon occurs, a part of an image captured by the CCD and/or observed through the finder appears reddish or bluish, which is different from the color of the object viewed by the naked eyes.
Conventionally, for the image captured by the CCD, an improved image processing system is provided in the digital camera to compensate for the shift of the reflectivity characteristics so that the change of the color of the image captured by the CCD and displayed on an LCD is not conspicuous. However, such a high-performance image processing system increases the manufacturing cost. Further, since a longer image processing time duration is required, movement of the object may not be viewed in real time through the LCD.
Further, with respect to the image observed through the finder, since the optical image is directly viewed, the image cannot be compensated. Thus, the operator is required to observe the image including the above-described defects, which may cause the operator to feel uncomfortable.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an improved beam splitting element which retains a predetermined reflectivity characteristic for the visible light including components having predetermined incident angle range.
For the above object, according to the invention, there is provided a beam splitter for a digital camera for capturing an image of an object using an image capturing element, the beam splitter splitting light passed through a photographing lens of the camera into a beam directed to the image capturing element and a beam directed to a finder optical system of the camera, which is provided with at least one optical element provided with a beam splitting surface that is inclined with respect to an optical axis of the photographing lens, and a multi-layer film formed on the beam splitting surface, the multi-layer film including a plurality of layers of dielectric materials, the multi-layer film being formed such that the beam splitting surface of the at least one optical element exhibits a substantially constant reflectivity at least for visible light that is incident on the beam splitting surface at any incident angle within a predetermined range.
With this structure, all the beams incident on the beam splitting surface are split thereby at a substantially same ratio, and therefore the defects in the conventional beam splitter can be overcome.
Optionally, the multi-layer film may be formed such that a layer of a dielectric materials having a relatively high refractive index and a layer of a dielectric material having a relatively low refractive index are alternately layered. In particular, the low refractive index may be defined to fall within a range from 1.30 to 1.66, and the high refractive index may be defined to fall within a range from 1.90 to 2.50.
In a specific structure, the at least one optical element comprises first and second right-angle prisms arranged from a photographing lens side, inclined surfaces of the first and second prisms being adhered to each other with the multi-layer film sandwiched therebetween, the inclined surface of the second prism being the beam splitting surface.
In the above structure, the multi-layer film may be formed such that the beam splitting surface exhibits a substantially constant reflectivity for light whose wavelength is within a range broader than the wavelength range of the visible light in either a lower wavelength direction or a higher wavelength direction, and which light is incident on the beam splitting surface at a predetermined incident angle.
Optionally, the multi-layer film may include at least two types of dielectric layers respectively having a relatively low refractive index and a relatively high refractive index, the two types of dielectric layers being alternately layered.
Also in this case, the low refractive index may fall within a range from 1.30 to 1.66, and the high refractive index may fall within a range from 1.90 to 2.50.
In a specific example, the multi-layer film includes six layers L
1
-L
6
respectively having the low refractive indexes and seven layers H
1
-H
7
respectively having the high refractive indexes. The layers L
1
-L
6
and H
1
-H
7
are a

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