JPH0444727B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical filter that is applied to a television camera that multiplexes color signals using a color stripe filter, and that acts in a direction perpendicular to the scanning direction of an image pickup tube. .

Conventional technology In television cameras that multiplex color signals using color stripe filters, the subject image and the color stripe filter are superimposed and projected, so if the subject image has a specific spatial frequency component, a false signal is generated. However, color beats appear on the image capture screen. Optical filters are used to prevent this spurious signal from occurring.

Conventional optical filters are constructed by combining quartz plates, and optical filters that act in a direction perpendicular to the scanning direction of the image pickup tube are also generally constructed from quartz plates.

Problems to be Solved by the Invention When an incident light ray passes through a crystal plate, which is an optical filter, it is birefringent and separated into two light rays, and one object point of a subject becomes two image points. Here, the two rays separated by birefringence have the same intensity,
That is, the incident light beam is separated into 50% parts, and the brightness of the two separated image points is always equal. Therefore, the MTF curve of an optical filter is a curve in which the MTF value always drops to zero at a certain spatial frequency. Note that the spatial frequency at which the MTF value becomes zero is determined by the thickness of the crystal plate.

On the other hand, when we imaged the zone plate and observed the occurrence of color beats, we found that in the direction perpendicular to the scanning line (hereinafter referred to as the scanning vertical direction), the third
In the figure, a color beat occurs near a spatial frequency f _v1 ^* where 1/2 cycle is equal to the distance between adjacent scanning lines. This color beat can be removed by using an optical filter whose MTF value is low near the above spatial frequency f _v1 ^* .

The optical filter using the above crystal plate is
As shown by graph line I in the figure, if the MTF value is set to zero at the spatial frequency f _v1 ^* , color beats are completely removed, but the spatial frequency
Images around f _v1 ^* will be blurry and have poor resolution. Therefore, in the past, as shown by the graph line, the color beat tolerance limit Q was set at which the color beats were removed to an acceptable extent and the resolution was satisfied to some extent.
An optical filter with MTF characteristics such as a curve passing through (MTF value approximately 0.4) was used. In this case, the occurrence of color beats is suppressed, but the spatial frequency
The problem remains that the resolution of images near the spatial frequency f _v2 ^* higher than f _v1 ^* deteriorates.

An object of the present invention is to provide an optical filter that solves the above problems.

Means for Solving the Problems The present invention consists of a half mirror surface and a mirror surface arranged close to each other in parallel, and the incoming light ray is separated from the light ray reflected by the half mirror surface and the half mirror surface. It passes through the mirror surface and is reflected by the mirror surface,
An optical filter configured to separate the light beam reflected by the half mirror surface and the parallel light beam, the distance between the half mirror surfaces and the reflectance of the half mirror surface being The configuration is such that the MTF value of the MTF curve of the optical filter is set to be minimum at a given spatial frequency.

Embodiment Next, an embodiment of the optical filter according to the present invention will be described.

As shown in FIGS. 1 and 2, the optical filter 1 has a structure in which a half mirror film 3 is provided on one side of an extremely thin glass plate 2, and a mirror film 4 is provided on the opposite surface. It is fixed on top. Geometrically, the optical filter 1 consists of a mirror surface _S1 constituted by a mirror film 4, and a half mirror constituted by a half mirror film 3 arranged parallel to the mirror surface _S1 in front of the mirror surface _S1 . It consists of a mirror surface _S2 .

This optical filter 1 has a half mirror film 3 and a mirror film 4 at an angle α (45 degrees) with respect to an incident light beam 6.
It is installed in the imaging optical system of the television camera in an inclined orientation, in other words, in such a posture that the angle β between the incident light ray 6 and the reflected light ray is 90 degrees.
Further, the X axis is the scanning direction of the image pickup tube, and the Y axis is the scanning direction perpendicular to this, and the incident side to the optical filter 1 and the output side from the optical filter 1 are shown separately.

The incident light ray 6 is first reflected at the point A at the half mirror film 3 and becomes a light ray 7a and heads upward, and the rest passes through the half mirror film 3, travels inside the glass plate 2, and passes through the mirror film. It is reflected at point B on 4, becomes a ray 7b, and heads upward. Ray 7a,
7b is parallel. Therefore, by passing through the optical filter 1, the incident light ray 6 has its optical path changed and is separated into two parallel light rays 7a and 7b. That is, one object point P of the subject is separated into two image points P ₁ and P ₂ separated by a dimension Py in the orthogonal scanning direction on the image captured by the image pickup tube. This produces a filter effect in the direction orthogonal to scanning.

For example, when the reflectance of the half mirror is 50%, the above-mentioned optical filter 1 has an MTF characteristic expressed by M=|cosπf ^* Py| in the direction perpendicular to scanning. Here, f ^* =
(1/2Py), M indicates the minimum value 0. The spatial frequency at which the MTF value is minimum is expressed by the same formula when the reflectance of the half mirror is 50%, and is expressed by the above separation dimension.
Determined in Py. This dimension Py is determined by the thickness t of the glass plate 2 (Py=(1/cos45°)t). The above-mentioned optical filter 1 is determined so that the MTF value is the minimum at the above-mentioned spatial frequency f _v1 ^* . The thickness t of the glass plate 2 varies depending on the screen size of the image pickup tube, but in one calculation example for a 2/3-inch image pickup tube, it was several μm.

In the optical filter 1 having the above configuration, the reflectance of the half mirror film 3 is included in addition to the thickness t of the glass plate 2 as parameters that can be changed. By changing the reflectance of this half mirror film 3, the ratio of brightness between image points _P1 and _P2 changes. For example, when the reflectance is 50%, the ratio is 1:1, when the reflectance is 20%, the ratio is 1:4, and when the reflectance is 60%, the ratio is 3:2. In this way, the image point
When the ratio of brightness between P ₁ and P ₂ changes, the filter effect changes. For example, when the ratio is 1:1, the entire brightness of image points P ₁ and P ₂ is equal to that of the incident ray 6
The entire amount of light exhibits a filter effect. The ratio is 4:
1, of the brightness 4 of image point P ₂ , the image point
The brightness corresponding to the brightness ₁ of P1 exhibits a filter effect between it and the image point _P1 , and the remaining brightness does not exhibit a filter effect. That is, the incident ray 6
2/5 of the amount of light has a filter effect, and the remaining 3/5 has no filter effect. If the ratio is 3:2, 4/4 of the amount of incident ray 6
5 will have a filter effect and 1/5 will have no filter effect. As described above, by appropriately changing the reflectance of the half mirror film 3, the ratio of the amount of light that acts as a filter to the amount of light of the incident light beam 6 can be varied, and as a result, the minimum value of the MTF value can be varied.

The optical filter 1 described above has a reflectance of the half mirror film 3 of about 30%, and the minimum MTF value is
It is set to be 0.4.

As a result, the optical filter 1 has a valley at the spatial frequency f _v1 ^* , as shown by the graph line in FIG.
It has ideal MTF characteristics with an MTF value of approximately 0.4. Therefore, by using this optical filter 1, a television camera can be realized in which color beats in the direction perpendicular to the scan direction are removed and which has good resolution over the entire spatial frequency range in the direction perpendicular to the scan direction.

Note that even if the reflectance of the half mirror film 3 is set to be approximately 70%, the lowest MTF value will be 0.4 as described above. Further, instead of providing the mirror surface on the surface of the glass plate 2, it may be provided on the glass surface on the half mirror side.

In addition, in FIGS. 1 and 2, strictly speaking, the incident light ray 6 is refracted when it enters the glass plate 2,
The light is reflected by the mirror film 4 and refracted when exiting from the glass plate 2 to become a reflected light beam 7b. Now, passing through the intersection of the incident light ray 6 and the reflected light ray 7b, the mirror film 4
As shown in FIGS. 1 and 2, if we consider a mirror film parallel to It is equivalent to the light ray emitted from 2. Therefore, even in actual cases, results similar to those described above are obtained.

FIG. 4 shows an example in which the optical filter 1 described above is incorporated. The above-mentioned optical filter 1 also functions to reflect the incident light beam 6 at right angles, and the television camera 10 shown in FIG. 4 skillfully utilizes the reflection effect of the optical filter 1 to have a small and easy-to-use shape. That is, the television camera 10 shown in FIG. 4 has a configuration in which an imaging optical system 11 and a video tape recorder 12 are integrated. The imaging optical system 11 includes an imaging tube 13
are installed vertically to form an L-shape. 14 is an imaging lens, and 15 is a crystal plate that is an optical filter that acts in the scanning direction. The optical filter 1 of the present invention is installed on the lower side of the image pickup tube 13 at an angle of 45 degrees. The number of crystal plates incorporated in the imaging optical system 11 is one, and considering that optical crystal plates are expensive and have a long delivery time, the above-mentioned imaging optical system 1
1 is an imaging optical system (3
This method is advantageous in terms of cost and mass production compared to conventional methods (which require multiple crystal plates). In addition, the above-mentioned television camera 10 has a considerably shorter length L than in the case where the image pickup tube is arranged horizontally, making it compact and
This makes it easy to balance and convenient to use when carrying the camera on your shoulder to take an image.

FIG. 5 shows a modification of the optical filter of the present invention. This optical filter 20 is formed on one surface of the prism 21 and is composed of a half mirror film 22, an extremely thin glass plate 23, and a mirror film 24. The incident light ray 25 is divided into two light rays 26 as shown in the figure.
It is separated into a and 26b.

Effects of the Invention As described above, the optical filter according to the present invention has the following features.

(1) It can have both the function of changing the optical axis and the function of a low-pass filter.

(2) Even if there is an installation error in the rotational direction during installation, the separation direction will not change and installation is easy.

(3) Complete two-point separation is possible.

[Brief explanation of drawings]

1 and 2 are respectively a perspective view and a side view schematically showing one embodiment of an optical filter according to the present invention, and FIG. FIG. 4 is a diagram showing an example of a television camera to which the optical filter of the present invention is applied; FIG. 5 is a diagram showing a modification of the optical filter of the present invention. . 1, 20... Optical filter, 2, 23... Glass plate, 3, 22... Half mirror film (S ₂ ), 4, 24
... Mirror film (S ₁ ), 5... Glass substrate, 6... Incident light beam, 7a, 7b... Reflected light beam, 10... Television camera, 11... Imaging optical system, 12... Video tape recorder, 13... Image pickup tube, 14... Imaging lens, 1
5...Crystal plate, 21...Prism.

Claims (1)

[Claims] 1 Consisting of a half mirror surface and a mirror surface arranged close to each other in parallel, the incoming light ray is reflected by the half mirror surface, and the ray that passes through the half mirror surface and is reflected by the mirror surface. an optical filter configured to separate the light ray reflected by the half mirror surface and the parallel light ray, the separation dimension between the half mirror surface and the mirror surface and the reflectance of the half mirror surface being , of the optical filter
An optical filter characterized in that the MTF value of the MTF curve is set to be minimum at a given spatial frequency.