JPS6217778Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical low-pass filter that optically suppresses the generation of false signals due to the spatial sampling in an imaging system that spatially samples a subject image to obtain an image output.

Generally, a CCD with a discrete pixel structure
Imaging that obtains imaging output by optically spatially sampling the subject image, such as single-tube color imaging devices that perform color coding of the subject image using solid-state images such as (Charge Coupled Device) or color filters. In this system, frequency components resulting from the above-mentioned spatial sampling are included in the imaging output as false signals, resulting in deterioration of image quality.
For example, in a color imaging device that optically spatially samples a subject image using a color filter and performs color coding to obtain each color signal, a subcarrier is modulated by a frequency component resulting from the spatial sampling, that is, a false signal. Folding distortion occurs, resulting in image quality deterioration due to so-called cross color.

In order to prevent the adverse effects of spurious signals due to spatial sampling as described above, it has been conventional to place a birefringent plate 1 made of a quartz plate or calcite plate having birefringence characteristics on the front side of an imaging section 2 of a discrete imaging system, as shown in Fig. 1. That is, the birefringent plate 1 has the property of separating incident light into an extraordinary light component 3 and an ordinary light component 4, so that the distance between the optical axes of the extraordinary light component 3 and the ordinary light component 4 is
Pr, the optical low-pass filter characteristic of the transfer function _H0 expressed by the following equation ₁ ^is given to the discrete imaging system.
By setting the distance Pr between the optical axes with respect to Px as Pr=1/2Px, the sideband components of the spatial sampling carrier are prevented from folding back into the baseband.

That is, the optical low-pass filter characteristic of the birefringent plate 1 is PxU/2 as shown in FIG.
When π = 2k, (Ho) = 1. However, with the above low-pass filter characteristics, the Nyquist limit (PrU/2π=1
/2) also exhibits a fairly high amplitude (√2/2) characteristic, which is insufficient to suppress the false signals caused by the spatial sampling. In other words, in the conventional optical low-pass filter, the first trap point (Ho = 1) is only given to the spatial sampling frequency s of the discrete imaging system, so the band compression effect in the Nyquist region is not sufficiently achieved. In addition, it has not been possible to sufficiently suppress aliasing distortion to the baseband due to the high-order spatial sampling carrier.

Therefore, in view of the above-mentioned problems, the present invention was developed by
a first birefringent plate providing trap points at least 2s/3 and 2s for a spatial sampling frequency s in the discrete imaging system; a quarter wave plate; and a second birefringent plate providing trap points at least s and 3s. An optical low-pass filter is provided in which two birefringent plates are arranged in series to enhance the band compression effect in the Nyquist region and to prevent aliasing distortion due to a high-order spatial sampling carrier.

Hereinafter, the present invention will be described in detail with reference to the drawings showing one embodiment.

FIGS. 3 and 4 show an embodiment of the present invention. To explain the configuration of the embodiment of the present invention with reference to FIG. 5 is placed. The imaging unit 11 has picture elements 11A arranged at a distance of pitch Px, and performs spatial sampling of a subject image at a frequency of s. On the other hand, the optical low-pass filter 5 includes a first birefringent plate 6, 1/
The configuration is such that a four-wavelength plate 7 and a second birefringent plate 8 are arranged in series. The first birefringence plate 6 separates the incident light into a first-order extraordinary light component 9 and a first-order normal light component 10, and the distance between the optical axes of the first-order extraordinary light component 9 and the first-order normal light component 10 is By adjusting the thickness of the first birefringent plate 6 so that becomes 3/4Px, (Ho ₁ )=1/2(1+e ^-3jPxU/4 ) ...transmission shown by the second equation, which is the second equation Gives the optical low-pass filter characteristics of the function Ho ₁ . Therefore, the first birefringent plate 6 provides a first trap point at 2/3s for the spatial sampling frequency s. On the other hand, since the first-order extraordinary light component 9 and the first-order normal light component 10 separated by the first birefringence plate 6 are in a polarized state, the above-mentioned extraordinary light component 9 and normal light component 10 are separated by the birefringence plate 6 again. When performing spectroscopy, it is necessary to cancel the polarization state and make it into normal light. In order to eliminate the above polarization state, a quarter wavelength plate 7 is arranged in series on the back side of the first birefringent plate 6. Further, a second birefringent plate arranged in series on the back side of the first birefringent plate 6 and the quarter-wave plate 7 cancels the polarization of the first-order extraordinary light component 9 and the first-order normal light component 10. The thickness is adjusted so that the secondary incident light beams 9A and 10A are separated into secondary abnormal light components 9B and 10B and secondary normal light components 9C and 10C so that the distance between the optical axes is 1/2 Px. Therefore, (Ho ₂ )=1/2 (1+e ^-jPxU/2 )... gives the optical low-pass filter characteristic of the transfer function Ho ₂ expressed by the third equation. That is, the second
The birefringent plate 8 provides the first trap point at the spatial sampling frequency s.

Therefore, the optical low-pass filter 5
Since the first birefringent plate 6 and the second birefringent plate 8 are arranged in series, the low-pass filter characteristics are synergized (Ho ₃ )=1/4 (1+e ^-3jPxU/4 ) (1+e ^-jPxU/2 )...Gives the optical low-pass filter characteristic of the transfer function Ho ₃ expressed by the fourth equation.

FIG. 4 shows the characteristics of the optical low-pass filter 5 described above. FIG. 4 shows the low-pass filter characteristics shown by the transfer function Ho ₁ by the first birefringent plate 6 in the embodiment of the present invention, the low-pass filter characteristics shown by the transfer function Ho ₂ by the second birefringent plate 8, and the optical Transfer function Ho ₃ in low-pass filter 5
The low-pass filter characteristics shown in are shown respectively. As is clear from FIG. 4 above, the first birefringence plate 6 can prevent the sideband components from folding back into the baseband at 2s with respect to the spatial sampling frequency s, and can also obtain a band compression effect in the Nyquist region. It is possible. Furthermore, the second birefringence plate 8 can prevent the sideband components at the frequency s of the spatial sampling carrier from being folded back into the baseband. Therefore, in the optical low-pass filter 5 according to the embodiment of the present invention, it is possible to prevent sideband components from folding back into the baseband when the frequency of the spatial sampling carrier is s and 2s, and to prevent folding back into the Nyquist region. It is also possible to enhance the band compression effect.

As is clear from the embodiments described above, the present invention includes a first birefringent plate that provides trap points at least at 2s/3 and 2s with respect to the spatial sampling frequency s, a 1/4 wavelength plate, and at least Since this is an optical low-pass filter configured by arranging in series a second birefringent plate that provides a trap point at s and 3s, sideband components at frequencies s and 2s of the spatial sampling carrier are folded back to the baseband. It is possible to prevent this and enhance the band compression effect in the Nyquist region.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of a main part showing a conventional example of an optical low-pass filter, Fig. 2 is a characteristic diagram showing characteristics of the low-pass filter in the conventional example, and Fig. 3 is an example of an optical low-pass filter according to the present invention. FIG. 4 is a characteristic diagram showing the characteristics of a low-pass filter in an embodiment of the present invention. 5... Optical low-pass filter, 6... First birefringence plate, 7... Quarter wavelength plate, 8... Second birefringence plate, 11... Imaging section.

Claims (1)

[Scope of utility model registration request] a first birefringent plate providing trap points at least 2s/3 and 2s for a spatial sampling frequency s in the discrete imaging system;
An optical low-pass filter comprising a wave plate and a second birefringent plate that provides trap points at least at s and 3s, arranged in series.