Optical: systems and elements – Prism – With reflecting surface
Reexamination Certificate
2000-03-16
2002-01-29
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Prism
With reflecting surface
C359S638000, C359S583000, C348S338000
Reexamination Certificate
active
06342980
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a beam splitting prism system for splitting light from an objective lens into a plurality of light beams, and an image pickup apparatus using the beam splitting prism system.
2. Description of Related Art
First, an explanation will be made about a color separation optical system for separating a light beam exiting from an objective lens into a plurality of color components. Here, a color separation prism system for television cameras, which is composed of three prisms for separation into light beams of blue, green and red, is taken as an example.
FIG. 6
is a sectional view showing the essential parts of an image pickup apparatus including such a conventional color separation prism system and an objective lens.
Referring to
FIG. 6
, the image pickup apparatus (television camera) is provided with the prism system
1001
for color-separating light from the interchangeable objective lens Le, and a plurality of solid-state image sensors
1011
B,
1011
G and
1011
R. Light beams obtained by color separation at the prism system
1001
are respectively made to reach the solid-state image sensors
1011
B,
1011
G and
1011
R. The solid-state image sensors
1011
B,
1011
G and
1011
R pick up images formed with the respective color light beams and convert the picked-up images into electrical signals.
In a first prism of the prism system
1001
, only blue color light included in light from the objective lens Le, which enters an entrance surface
1002
, is reflected by, and the rest of the light is made to pass through, a blue-reflecting dichroic film applied to a surface
1003
. The reflected blue color light is totally reflected by the surface
1002
and is then made to exit from a surface
1004
, advancing to the solid-state image sensor
1011
B for blue color.
A red-reflecting dichroic film applied to a surface
1007
of a second prism reflects only red color light included in light having passed through the surface
1003
and an air separation
1005
, and transmits the rest of the green color light. The reflected red color light is totally reflected by an entrance surface
1006
adjacent to the air separation
1005
and is then made to exit from a surface
1008
, advancing to the solid-state image sensor
1011
R for red color.
The green color light having passed through the surface
1007
is made to exit from a surface
1010
, advancing to the solid-state image sensor
1011
G for green color. In the manner as described in the foregoing, the color separation prism system separates a light beam.
FIG. 7
is an optical path diagram illustrating a light beam which passes through the first and second prisms of the prism system
1001
from the objective lens Le and then reaches an effective portion of the solid-state image sensor
1011
R for red color. A ray of light D included in such a light beam is a peripheral ray of an off-axial light beam passing at the lowest position of an image pickup plane as viewed in FIG.
7
. While it is necessary to totally reflect the ray of light D at the second prism, the following condition has to be satisfied so as to reflect the ray of light D at the entrance surface
1006
:
&thgr;
2
>(&thgr;
1
+&dgr;+&thgr;max)/2 (1)
where
&thgr;
1
: an apical angle of the first prism,
&thgr;
2
: an apical angle of the second prism,
&dgr;=sin
−1
(1
),
n: a refractive index of each of the first prism and the second prism,
&thgr;max=sin
−1
{1/(2·n·Fno) },
Fno: an F-number of the objective lens.
In Japanese Laid-Open Patent Application No. Hei 7-281012, there is disclosed that, in order to attempt to reduce the size of the color separation prism system, it is necessary to limit the condition (1) to the following range:
−0.5°<&thgr;
2
−{(&thgr;
1
+&dgr;+&thgr;max)/2}<5.5°
In the past, while an image pickup tube was used as an image sensor, there was such a problem that the image pickup tube might be stuck, so that it was rare to photograph an intense light source such as the sun directly. Even when such an intense light source was photographed, the photography generally was performed using a light-reducing optical member such as an ND filter.
Meanwhile, in recent years, a solid-state image sensor such as a CCD has been becoming a main trend in place of the image pickup tube, In such a solid-state image sensor, there is no problem with respect to sticking, and smear or blooming is also improved, so that it has become possible to photograph an intense light source such as the sun directly.
However, a new problem has arisen with the use of a CCD. This problem is caused by the construction of the solid-state image sensor or CCD itself. The surface of a CCD is coated with metal, so that a reflection factor thereof is relatively high. Therefore, when an intense light source is directly photographed, strong reflection occurs at the surface of the CCD. Further, since the image pickup surface of the CCD has pixels regularly arranged thereon, diffraction is also caused. This point will be described with reference to
FIGS. 8A and 8B
.
It is found that such an adverse effect by reflection is caused by a light beam which passes through an optical path within the second prism as shown in
FIG. 8A and
, then, re-enters the solid-state image sensor
101
R, thereby becoming ghost.
FIG. 8B
illustrates one ghost optical path, as a diagram obtained by expanding the second prism along the ghost optical path. In particular, a ray of light, which is made incident vertically on the reflecting surface
1007
and is reflected thereby, is illustrated in FIG.
8
B. Referring to
FIG. 8B
, it is apparent that, with regard to a ray of light, among light beams reflected by the CCD, which is made incident on the surface
1006
at an angle &agr; immediately after entering the second prism, such an angle of incidence &agr; and an angle &bgr; at which the ray of light is made incident again on the surface
1006
after being reflected by the surface
1007
are equal to each other, being the angle &thgr;
2
.
In the past, since such a problem as ghost has not occurred due to the problem of the sticking of an image pickup tube, a range of the angle &thgr;
2
has been determined only taking into consideration the reduction in size of the color separation prism system. That is, it has been only necessary that the angle &thgr;
2
is set to such a small angle as to be enough to make light totally reflected as much as possible. For example, also in Japanese Laid-Open Patent Application No. Hei 7-281012, in order to reduce the size of the color separation prism system by setting the condition of “−0.5°<&thgr;
2
−{(&thgr;
1
+&dgr;+&thgr;max)/2}”, the apical angle (&thgr;
2
) of the second prism is set within such a range that light which is not totally-reflected has little influence, without satisfying the condition of total reflection, so that the reduction in size of the color separation prism system is given priority.
Here, supposing the angle &thgr;
2
is a little smaller than &dgr;, referring to
FIG. 8B
, total reflection would not take place at the point p
1
and the point P
2
. Therefore, at the point P
1
and the point P
2
, interference would occur between the two surfaces
1003
and
1006
which are opposed to each other across the air separation
1005
.
FIGS. 9A and 9B
are diagrams illustrating a ghost optical path in which the angle of light incident on the surface
1007
has slightly shifted from the vertical angle.
FIG. 9A
shows a ghost optical path of a ray of light which advances slightly upward, and
FIG. 9B
shows a ghost optical path of a ray of light which advances slightly downward.
At the point P
3
and the point P
6
, since the angle of incidence becomes large, total reflection is more apt to take place, so that there is no problem. However, at the point P
4
and the point P
5
, since, conversely, the angle of incidence becomes small, the condition of total reflection i
Spyrou Cassandra
Treas Jared
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