JPS5848020A - Optical system for image pickup - Google Patents

Abstract

PURPOSE:To remove a spurious signal generated from color filters effectively by providing at least one double refractive plates and diffraction grating space frequency filters in an optical path for image pickup in such a way that the directions of filter characteristics differ. CONSTITUTION:In an optical system for image pickup having dot-like color separating filters, it is necessary to select filters suited for suppressing the space frequencies which generate beats. In this case, a double refractive plate 1 having the trap characteristics shown in Fig. 3 is suited as the space filter in a perpendicular direction. In a horizontal direction, the diffraction grating space frequency filter which has the filter characteristics as shown in Fig. 4 and suppresses both 3.58MHz and 7.16MHz is suited. According to this invention, the high band components of the space frequencies in longitudinal and transverse directions are suppressed thoroughly; therefore the generation of spurious signals is eliminated effectively.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an imaging optical system, and more particularly to an optical system for a color video camera having a dot-like color separation film.

Conventionally, there is a single tube color camera as an imaging device that obtains a color signal from a single image pickup tube. In this single-tube color camera, a method for obtaining color signals is to use a striped color separation filter to separate the signals into each color. An example of this striped color separation filter is shown in FIG. This filter consists of a filter R that transmits red, a filter G that transmits green, and a filter B that transmits blue.

When an image is captured with an image pickup tube equipped with this filter, a beat between the spatial frequency of the filter and the pitch of the pixels occurs in the low-frequency components, and appears particularly in the color signal. This beat component is a false signal different from the original color and is a major cause of image quality deterioration.

As a conventional technique for suppressing this, a birefringent plate or a diffraction grating spatial frequency filter is used.

All of these suppress components of the spatial frequency that cause beats, and will be explained below.

First, the properties of birefringence will be explained using FIG. 2. Natural light 2 incident on the birefringent plate 1 is divided into two parts: normal light 3 having linearly polarized light vibrating perpendicular to the optical axis and extraordinary light 4 having linearly polarized light vibrating parallel to the optical axis. The refractive index of the birefringent plate is different for the normal ray 3 and the extraordinary ray 4, and the image becomes a double image. At this time, the response function R (force is expressed by the following formula %) Here, f is the spatial frequency and f is the trap frequency. This R (c) is shown in FIG.

Next, the diffraction grating spatial frequency filter will be explained.

This utilizes the diffraction phenomenon of light using a light-dark grating, a phase grating, etc., and suppresses the high-frequency components of the nocturnal frequency.

The response function of this '+1L spatial frequency filter is described in detail in Japanese Patent Publication No. 55-49723.

FIG. 4 shows this response function. In the figure, fQ indicates the cutoff frequency, and JP is the frequency at which the response function starts to rise again.

In striped color separation filters used in image pickup tubes,
The color is modulated only horizontally, creating spurious signals in beats with horizontal spatial frequencies.

Therefore, by suppressing the components that cause beats among the horizontal spatial frequency components using the above-mentioned spatial frequency filter, it is possible to suppress the generation of similar signals.

On the other hand, a dot-shaped color separation filter is used in a solid-state image sensor. This has a smaller number of pixels than a solid-state image pickup device or an image pickup tube, but it has the feature that the color separation filter's address is accurate, and arranging the color filters horizontally and vertically improves the resolution. This is because it is advantageous in terms of generating pseudo signals.

However, with the current number of pixels, a large amount of pseudo signals are generated, deteriorating the image quality. Moreover, in the dot-shaped color separation filter, the colors are varied horizontally and vertically.

FIG. 5 shows an example of a dot-like color separation filter, showing that the color signals are modulated both horizontally and vertically. In this figure, the filter is White W.

It consists of four colors: green G, cyan Cy, and yellow Ye. Red light R and green light B from these.

The demodulation calculation for is shown below.

R=W×Ye −(Cy×G)
(1) B = W×Cy −(Ye×G)
(2) For example, the demodulation calculation formula (1
) is considered corresponding to the filter arrangement shown in FIG. Focusing on the four pixels on the left side of the figure, we can see that Cy is subtracted from W and G is subtracted from Ye.
Two horizontal operations: subtraction of G from W and Y
It consists of two vertical operations: subtraction of Cy from e. Thus, in the dot-like color separation filter, the color signal is demodulated from both horizontal and vertical components, resulting in a beat with the horizontal and vertical spatial frequencies. Therefore, suppressing only the spatial frequency in one horizontal or vertical direction using the above-mentioned spatial filter has a small effect of suppressing pseudo signals, and conventionally, no comprehensive method for suppressing such pseudo signals has been known.

It is therefore an object of the present invention to provide a spatial filter that effectively removes spurious signals arising from color filters that are color multiplexed both horizontally and vertically.

In order to achieve the above object, an imaging optical system according to the present invention is provided with at least one birefringent plate and one diffraction grating spatial frequency filter in the imaging optical path, with the former and the latter having different directions of filter characteristics with respect to the spatial frequency. The main point is that

The basic idea is that the direction in which double images are generated due to birefringence and the direction in which multiple images are generated due to the diffraction grating are 9
This is to shift 0 degrees and form multiple images in the vertical and horizontal directions, suppressing high spatial frequencies.

Hereinafter, the present invention will be explained in detail using Examples.

First, a description will be given of the pseudo signals generated by the dot-shaped color separation filter used in this embodiment. .

An example of a dot-shaped color separation filter is shown in FIG. 5, and in this example, the horizontal pitch is 23 μm.

The vertical direction is 13.5 μm. If the frequency of the clock that reads out the signal from this pixel is 7.16 MHz, the horizontal pitch of 23 μm corresponds to the frequency of 7.1 MHz in the electrical signal.
It becomes 6MH2. Let's focus on one more pixel. For example, if we look at W in Figure 5, this repetition is 46 times in the horizontal direction.
μm, which corresponds to the frequency of the electrical signal of 3.58 MHz.

Also, the vertical pitch of 135 μm is 5 μm on the monitor screen.
25 This is the wavelength of the spatial frequency corresponding to TV mains. Therefore, among the spatial frequencies that cause the pixel pitch and beat of the dot-shaped color separation filter, the components with high energy are 3.58 MHz and 7.16 MHz in the horizontal direction.
In the vertical direction, the frequency corresponds to 52 STV lines.

Next, an example of generation of a false signal when an image is actually taken with the camera having the dot-shaped color separation filter shown in FIG. 5 will be shown. The concentric pattern shown in FIG. 6A was selected as the object having spatial frequencies of various values in each direction. In this concentric circle pattern, the intervals between the outer circles are closer, and the two
Dimensionally, the outer side contains high-frequency spatial frequency components in all directions, and a beat is generated in the vicinity of matching the pitch between pixels. When this concentric pattern 5 is photographed and observed on a monitor, two signals are observed: an original signal and a pseudo signal caused by the beat. The two signals on the monitor are plotted on the horizontal axis 6 (X-X
') direction, as shown in Figure 6B, the original signal 8
and pseudo signals 9 and 10 are observed. The pseudo signal 9 is a component in which a spatial frequency around 3.58 MHz in the horizontal direction is folded back to a lower frequency band due to the beat, and the pseudo signal 10 is a component in which a spatial frequency around 7.16 MHz in the horizontal direction is folded back.

Furthermore, when viewed in the direction of the vertical axis 7 (Y-Y'), as shown in FIG. 6C, an original signal 11 and a pseudo signal 12 are observed. The pseudo signal 12 is a folded component to the low frequency band due to the beat of the spatial frequency corresponding to 52 STV lines in the vertical direction.

As mentioned above, the pseudo signal has a spatial frequency of 3.5 in the horizontal direction.
It occurred at Nojito from around 8MHz and 7.15MHz, and it also occurred at a beat at a spatial frequency corresponding to 525 TV lines in the vertical direction. Therefore, it is necessary to select a filter suitable for suppressing the spatial frequencies that generate these beats. As shown in FIG. 6C, the pseudo signal due to the beat with the spatial frequency in the vertical direction has the largest spatial frequency component around 525 TV lines. Therefore, a birefringent plate having the trap characteristics shown in FIG. 3 is suitable as a vertical spatial filter. On the other hand, in the horizontal direction, as shown in Figure 6B, 3.58MHz and 7.12MHz
Generates pseudo signals at two locations near MHz. Therefore, a diffraction grating spatial frequency filter that has filter characteristics as shown in FIG. 4 and can suppress both 3.58 MHz and 7.16 MHz by selecting the cutoff frequency is suitable.

Numerical examples used in this example are shown below. In order to set the trap frequency by the birefringent plate to a spatial frequency corresponding to 525 TV lines, the shift distance of the double image should be set to 13.5 μm in the vertical direction at a pixel pitch of 7. When a quartz plate is used as a birefringent plate, its thickness is approximately 2β.

Next, the characteristics of the diffraction grating spatial frequency filter are as follows:
MHz and 7.12 MHz t:r) is also good. This allows both frequencies to be suppressed. For example, for green light with a wavelength of 5465 m, fQ is 5.
The R of the filter that is 4 MHz is as shown in Figure 7, and as you can see in this figure, it is 3.58 MHz, 7.12
The filter has a sufficient suppression effect on two MHz frequencies. Above, we have discussed green light with a wavelength of 546 nm, but the characteristics of this diffraction grating spatial frequency filter depend on the wavelength, and at a wavelength of 436 nm, fQ: 6.8
MHz, and at a wavelength of 700 nm, the IQ is 4.2
MHz. Therefore, it is possible to change the characteristics of the spatial frequency filter depending on the wavelength, that is, the color. For example, the filter shown in FIG. 5 (W, Cy.

Ye, G) all pass green light. This green light occupies a large component of the luminance signal. The cutoff frequency for this green light is set to fQ 5.4 MHz, while the red light, which stands out as a pseudo signal, is set to fQ 4.2 MHz.
Can be cut off at Hz. Therefore, it is possible to design a filter that can sufficiently suppress pseudo signals without reducing the resolution of the luminance signal.

In this example, a birefringent plate was used as the spatial filter in the vertical direction, and a diffraction grating spatial frequency filter was used in the horizontal direction. This does not change the spirit of the invention.

According to the present invention, high-frequency components of vertical and horizontal spatial frequencies can be sufficiently suppressed, so generation of pseudo signals can be suppressed. Furthermore, filter characteristics for vertical and horizontal spatial frequencies can be changed independently, depending on filter type and pitch.

This method has the advantage that it is possible to design an optimal pseudo signal suppression effect against changes in factors such as chi and the like.

[Brief explanation of drawings]

Fig. 1 is a conceptual plan view of a striped color separation filter, Fig. 2 is a conceptual perspective view showing the birefringence phenomenon caused by a birefringent plate, Fig. 3 is a diagram showing the characteristics of a spatial frequency filter using a birefringent plate, and Fig. 4 is a conceptual plan view of a striped color separation filter. The figure shows the characteristics of the diffraction grating spatial frequency filter, Figure 5 is a conceptual plan view of the dot-shaped color separation filter, and Figure 6
The figure shows a concentric pattern and a video signal, and FIG. 7 is a diagram showing the characteristics of an actual diffraction grating spatial frequency filter. 1... Birefringent plate 2... Natural light 3... Normal light 4... Extraordinary light 5... Concentric circle pattern 6... Horizontal axis 7... Vertical axis 8,1
1...Original signal 9, 10.12...Pseudo signal representative patent attorney Junnosuke Nakamura・11 ・1. 7 Zufuku Horei

Claims (1)

[Claims] In an imaging optical system having a dot-shaped color separation filter,
1-parallel imaging optics characterized in that a small number of birefringent plates and diffraction grating spatial frequency filters are provided in the imaging optical path so that the directions of the filter characteristics with respect to the spatial frequency of the former and the latter are different. system.