JPS6152411B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a light splitting device for photometry in single-lens reflex cameras and the like. 2. Description of the Related Art Conventionally, there has been known a photometry method in a single-lens reflex camera in which a part or all of the reflective surface of a quick return mirror is made semitransparent, and the light beam transmitted therethrough is guided to a photoreceptor for photometry. For example, in Utility Model Application Publication No. 52-14445, a reflective film is formed in one direction on a transparent flat plate provided with directional grooves in a certain direction on the back surface of a semi-transparent quick return mirror. discloses a photometric device that guides light transmitted through the back surface of a quick return mirror in a fixed direction.

However, in general, in order to maintain the brightness of the finder, it is not possible to reduce the luminous flux reflected toward the finder system, and for other reasons, it is desired to improve the light utilization efficiency in such photometry systems.

SUMMARY OF THE INVENTION An object of the present invention is to provide a light splitting device for photometry which improves the conventional technology, increases light utilization efficiency, and improves photometric characteristics.

The structure and operation of the present invention will be explained below with reference to the drawings.

FIG. 1 is a sectional view of a camera having a light splitting device according to a first embodiment of the present invention. The light splitting device of this embodiment is a reflecting mirror that splits a luminous flux in terms of amplitude (for example, a semi-transparent mirror) or a reflecting mirror that splits a luminous flux in area with a mixture of transparent and reflective parts (for example, a mirror that splits a luminous flux in terms of area). The mirror consists of a main mirror surface, at least a part of which has a semi-transparent reflecting surface, and a large number of minute Fresnel-shaped sub-mirrors 2 provided on the back surface of the main mirror surface, and the entire mirror is a conventional quick return mirror. It is placed in the position of In the figure, 3 is a photographing lens, 5 is a light receiving element, 12 is a focus plate, and 13 is a pentaprism for a finder.

Figure 2 shows an enlarged cross-sectional view of the light splitting device;
The figure shows a plan view seen from the back side. Further, FIG. 4 is a perspective view showing how the light beam splitting device of this embodiment divides the light beam and how the divided light beam is guided to the light receiving element. As is clear from the plan view of FIG. 3, each submirror 8 of the Fresnel-shaped submirror group 2 provided on the back surface of the reflecting mirror 1 of the light splitting device of this embodiment has an arcuate shape within its back surface. They are arranged concentrically to form sub mirror group 2.
is formed. Reference numeral 7 designates irregularities formed on the back surface of the reflecting mirror in order to provide the sub-mirror 8.

In the light splitting device of this embodiment, as shown in FIGS. 2 and 4, the light beam 4 emitted from the photographing lens system 3 in parallel to the optical axis is first reflected by the semi-transparent and semi-reflective surface of the reflecting mirror 1, and most of it is is directed toward the finder system, and the light beam 6 transmitted through the semi-transparent and semi-reflective surface is reflected by a group of minute sub-mirrors 2 on the back surface of the main mirror and guided to the light receiving element 5.

At this time, since the circular zone shape of each sub-mirror is set concentrically with a curvature that allows the reflected light beam to converge on the light receiving element, the light beam reflected by this Fresnel-shaped sub-mirror group 2 has an efficiency as shown in Fig. 4. It is well guided to the light receiving element.

Furthermore, as is clear from FIG. 2, in the photometry device of this embodiment, in order to efficiently guide the light flux that has passed through the main mirror to the light receiving element, the direction of the surface of the microscopic submirror changes depending on its position on the back surface of the main mirror. are doing. Here, each sub-mirror 8 can be obtained by depositing a reflective material such as a metal on one surface of the unevenness 7, but in order to avoid the manufacturing difficulty of having to deposit only the sub-mirror surface, each It is also possible to use total reflection on one surface of the unevenness to form a sub-mirror. Assuming that the refractive index of the material forming the unevenness 7 is 1.5, the angle of incidence must be approximately 42 for total reflection to occur on each submirror surface.
Although it is necessary that the angle is greater than 100 degrees, this condition can be achieved by placing the light receiving element 5 at an appropriate position. Furthermore, in FIG. 2, the cross-sectional shape of each sub-mirror 8 is represented by a straight line, but as shown in FIG. 5, by making the cross-sectional shape of each sub-mirror curved, only the light beam parallel to the optical axis is Instead, the light 18 that enters at a small angle with the optical axis can also be guided to the light receiving element, and it is possible to further improve the light collection efficiency.

Note that in FIGS. 2 and 5, the path of the light ray is illustrated in a simplified manner, and the actually existing refraction upon incidence on the reflecting mirror 1 and upon exit from the sub-mirror is omitted. FIG. 6 shows an example of the cross-sectional shape and optical path diagram of the present light splitting device when this refraction is taken into consideration. Due to refraction when incident on the reflecting mirror 1, the inclination of the sub-mirror 8 needs to increase as the refractive index of the material forming the reflecting mirror increases. Further, by forming the cross-sectional shape as indicated by the solid line or broken line in FIG. 6, it is possible to prevent the light beam from divergent due to refraction at the time of exit. Although FIG. 6 shows the case where the sub-mirror surface has a straight cross section, the same applies to the case where the sub-mirror surface has a curved cross section.

On the other hand, if it is necessary to reduce the amount of light near the optical axis in order to maintain F-number proportionality in photometry in a camera, this Fresnel-shaped sub-mirror group should not be provided near the optical axis on the back surface of the reflector, or Furthermore, this purpose is achieved by making the portion a diffusion surface.

Further, by changing the area of the sub-mirror surfaces and the positions where they are provided, it is possible to obtain a light splitting element having preferable photometric characteristics.

As shown in the description of the embodiments above, by using the light splitting device according to the present invention, it is possible to efficiently guide light from the photographing lens to the light receiving element. Furthermore, the size of the reflecting surface of the Fresnel-shaped sub-mirror group used in the light splitting device of the present invention,
By changing the shape and the location where the submirror group is provided, it is possible to form a light splitting device having various photometric characteristics.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a camera having a light splitting device of the present invention, FIG. 2 is an enlarged sectional view of the light splitting device of the first embodiment, and FIG. 3 is a plan view of a reflecting mirror of the device of the present embodiment. 4 is a perspective view showing how the light beam is divided in the device of this embodiment, FIG. 5 is a sectional view of the light splitting device of the embodiment having a sub-mirror whose cross section is a curve, and FIG. FIG. 3 is a cross-sectional view showing an optical path in the light splitting device of the present invention when refraction is taken into account. In the figure, 1 is a semi-transparent mirror, 2 is a sub-mirror group, 3 is a photographic lens system, 4 is light emitted from the photographic lens, 5 is a light receiving element, 6 is a light beam transmitted through the semi-transparent mirror, and 7 is provided on the back surface of the reflective mirror. 18 is a ray of light that enters at a small angle with the optical axis.

Claims (1)

[Scope of Claims] 1. A Fresnel-shaped sub-mirror group formed by arranging arc band-shaped sub-mirrors within the back surface is provided on the back surface of a reflecting mirror that partially has a semi-transparent surface. 1. A light splitting device for photometry, characterized in that a light beam transmitted through a transparent portion is guided to a light receiving element by the sub-mirror group. 2. The light splitting device for photometry according to claim 1, wherein in a cross section along the arrangement direction of the Fresnel-shaped submirror group, the reflecting surface of each of the individual submirrors is linear. 3. The light splitting device for photometry according to claim 1, wherein in a cross section of the Fresnel-shaped sub-mirror group taken along the arrangement direction thereof, the reflecting surface of each of the individual sub-mirrors has a curved shape for condensing light. .