JPH0415669B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an imaging device that obtains point-sequential imaging signals from an imaging section formed of a solid-state imaging device such as a CCD (Charge Coupled Device), and in particular,
False signals due to spatial sampling of the checkered pixel structure are suppressed using an optical low-pass filter.
The present invention relates to an imaging device designed to improve the quality of captured images.

In general, in an imaging device using a solid-state imaging device such as a CCD having a discrete pixel structure, in order to spatially sample a subject image and obtain an imaging output, the frequency components of the spatial sampling generate false signals and output the imaging output. It is known that image quality is deteriorated due to so-called aliasing distortion. The aliasing distortion that occurs in the electrical signal processing system can be suppressed by adjusting the brightness level in the signal processing system, but the aliasing distortion that occurs in the electrical signal processing system can be suppressed by adjusting the brightness level in the signal processing system. The resulting aliasing distortion is
It cannot be suppressed by electrical signal processing and must be removed optically in the optical system, and is suppressed by an optical low-pass filter that provides a trap point at the spatial sampling frequency. .

For example, in the case of a discrete imaging system having an isotropic lattice pixel structure, as shown in FIG. An optical low-pass filter 2 using a plate is provided. The optical low-pass filter 2 has a plane defined by the optical axis of the crystal forming the birefringent plate and the normal to the wavefront of light (hereinafter referred to as the main cross section).
As shown in Fig. 1, the incident light has the property of being divided into a normal component and an abnormal light component and passing through.
If the separation distance between the optical axis through which the normal light component passes and the optical axis through which the abnormal light component passes is P _t , then H _OLPF0 = 1/2 (1+e ^-jptW )...In the first equation, Low pass filter characteristics shown
H Give _OLPF0 to the subject image. Then, the above-mentioned separation distance P _t is calculated as _{P t =}
By setting it to 1/2P _x , a trap point can be given to the spatial sampling frequency s in the imaging section 1.

By the way, as shown in FIG. 2, spatial sampling of the subject image is carried out in the imaging unit 11 in which each pixel 11A is arranged in a _checkered pattern with a horizontal pixel pitch of P _x and a vertical pixel pitch of P y. In this case, the two-dimensional space shown in Figure 3 is defined by the horizontal axis P _x u/2π, where the spatial frequency in the horizontal direction is u, and the vertical axis, P _y v/2π, where the spatial frequency in the vertical direction is v. On the coordinate plane,
Frequency components (spectrum) due to the spatial sampling are generated at positions as indicated by black dots.
Therefore, conventionally, the imaging section 11 with the above-mentioned checkered pixel structure
, the sideband component of the carrier at the coordinates (1/2, 1/2) on the spatial coordinate plane shown in FIG. 3 has zero frequency, that is, the coordinates (0,
H _OLPF1 = 1/2 (1 + e ^-jpyV ) ... as shown in the second equation or H _OLPF2 = 1/2 (1 + e ^{- jpxU} ) ... as shown in the third equation An optical low-pass filter providing low-pass filter characteristics was disposed on the front side of the imaging section 11. Here, the low-pass filter characteristic H _OLPF1 of the above-mentioned second equation is shown by the line density of solid parallel lines in FIG. 4, and the low-pass filter characteristic H _OLPF1 of the above-mentioned third equation is shown by the broken line It is shown by the line density of parallel lines. However, for the imaging unit 11 with the checkered pixel structure, the H _OLPF1 or
H If the low-pass filter characteristics of _OLPF2 are given, the coordinates (0, 1/2) or (1/2, 0) where few false signals occur due to spatial sampling will be suppressed, so the resolution in the vertical or horizontal direction will be reduced. There is a problem that it deteriorates.

Therefore, in view of the above-mentioned problems, the present invention optically suppresses false signals caused by spatial sampling sufficiently without deteriorating the resolution in a discrete imaging system in which picture elements are arranged in a checkered pattern. An object of the present invention is to provide an imaging device with a novel configuration that enables imaging with good image quality.

Hereinafter, one embodiment of the present invention will be described in detail.

First, before explaining specific embodiments of the present invention,
Regarding the operating principle of the false signal suppression function by two-dimensional spatial sampling in the imaging device according to the present invention, the imaging unit 1 with the checkered pixel structure shown in FIG.
The case where two-dimensional space sampling is performed using 1 will be explained as an example.

That is, when performing secondary spatial sampling of a subject image using the image pickup section 11 having a checkered pixel structure in which the horizontal pixel pitch is _Px and the vertical pixel pitch is _Py , the image pickup section 11 is On a two-dimensional spatial coordinate plane indicated by the horizontal axis of P _x u/2π and the vertical axis of P _y v/2π, where u is the spatial frequency sampled in the horizontal direction and v is the spatial frequency in the vertical direction, The spectrum distribution obtained by the above two-dimensional spatial sampling is shown in one picture element 11A of the imaging unit 11.
The spatial position of is the coordinate origin (0,0), and each coordinate (1/2, 1/2), (-1/2, 1/ 2),
The aliasing distortion caused by the sideband components of the carrier at (-1/2, - 1/2) and (1/2, -1/2) being aliased to the coordinate origin (0, 0), that is, the zero frequency, affects the image quality. ,
It causes deterioration. Therefore, in the present invention, a straight line passing through the coordinates (1/2, 1/2) on the two-dimensional space coordinate plane and intersecting each coordinate axis, and a straight line parallel to the straight line and at the coordinate (-
1/2, -1/2), and a straight line passing through the coordinates (-1/2, 1/2) and intersecting each coordinate axis. A second optical low-pass filter is superimposed on the straight line and exhibits a trap characteristic along a straight line that is parallel to the line and passes through the coordinates (1/2, -1/2).

for example, The low-pass filter characteristic shown by the fourth equation is
a first optical low pass filter having H _OLPFa ; The low-pass filter characteristic shown by the fifth equation is
By superimposing a second optical low-pass filter having H _OLPFb , each coordinate (1/2, 1/2), (-1 /2, 1/2), (-1/2, -1/2), (1
/2, -1/2) Low-pass filter characteristic H _OLPFc where the trap characteristic is shown by each straight line passing through can be given in two-dimensional space.

When the above low-pass filter characteristic H _OLPFc is given, each coordinate (1/2, 1/2), (-1/2, 1
/2), (-1/2, -1/2), (1/2, -1/2) and the respective coordinates (1,0), (0,
1), (-1, 0), (0, -1) carrier sideband components to the coordinate origin (0, 0) can be sufficiently suppressed optically. Moreover, each coordinate (1/2, 0), (0, 1/2), (-1
/2, 0) and (0, -1/2) are not trapped, so there is very little deterioration in the resolution of the image in the horizontal and vertical directions. Note that the first and second optical low-pass filters ensure resolution and suppress aliasing distortion even if the low-pass filter characteristics H _OLPFa ′ and H _OLPFb ′ shown in FIG. 6 are provided. be able to.

Next, as shown in FIG. 7, one chip has picture elements G for green imaging arranged orthogonally, picture elements R for red imaging and picture elements B for blue imaging arranged in a checkerboard pattern.
A specific embodiment in which the present invention is applied to a color imaging device using a CCD image sensor 21 will be described. Here, the image sensor 21
is arranged in the same pixel arrangement for each two horizontal lines in consideration of interlacing, and the spectral distribution of frequency components obtained by two-dimensional spatial sampling of the subject image is shown on the two-dimensional spatial coordinate plane as shown in Figure 8. .

Therefore, in this embodiment, a straight line passing through the coordinates (1/4, 0), (1/2, 0) in FIG. 8 and the coordinates (-1/4, -1/4), (-1/2 , 0), and a straight line passing through the coordinates (-1/4, 1/4), (0, 1/2) and the coordinates (1/4). 4, -1/4), (0, -1/2) and a second optical low-pass filter 32 whose trap characteristic is shown by a straight line passing through (0, -1/2), as shown in FIG.
They are arranged in series on the front side of the image sensor 21. Each of the above optical low pass filters 31, 32
For this purpose, a birefringent plate having birefringent properties such as a quartz plate is used, and where the horizontal pixel pitch of the image sensor 21 is P _x and the vertical pixel pitch P _y , Z ^-1 = e ^-jpxU ω ^{- 1} = e ^-jpyV H _OLPFA = 1/2 (1 + Z ^-1・ω ^-1 ) ...The first optical low pass filter 31 has the low pass filter characteristic H _OLPFA of the seventh formula, H _OLPFB = 1/2 (1+Z ^-1 · ω ¹ ) ... The second optical low-pass filter 32 has a low-pass filter characteristic H _OLPFB expressed by the eighth formula.

Note that each of the optical low-pass filters 31, 3
A 1/4 wavelength plate 33 is disposed between the two to return the normal light component and the abnormal light component separated by the first optical low-pass filter 31 to normal light.

Imaging light is irradiated through the first and second optical low-pass filters 31 and 32 as described above.
In the CCD image sensor 21, as shown in FIG. 10, the frequency components due to the checkered spatial sampling cause problems as aliasing distortion to the zero frequency at each coordinate (±1/2, 0), (0, ±1/ 2), (±1
/4, ± -), (±1/4, 〓1/4) are all suppressed by the optical low-pass filters 31 and 32, so it is possible to perform an imaging operation with extremely little deterioration in image quality due to aliasing distortion. In addition, the optical low-pass filters 31 and 32 each coordinate ((±
Since the spatial frequencies of 1/4,0) and (0,±1/4) are not suppressed, horizontal and vertical resolution is extremely unlikely to be degraded.

Among the point-sequential imaging outputs read out from the CCD image sensor 21, that is, the red imaging output _ER , the green imaging output _EG , and the blue imaging output _EB , the _ER signal is
The E _B signal is supplied to a synchronization circuit 41 and the E _G signal is supplied to a vertical contour compensation circuit 42 .

The above-mentioned synchronization circuit 41 calculates H _IP (Z・ω) = 1/4 (4+Z ^-1 +Z+ω ^-2 +ω ² ) for the supplied E _R /E _B signal according to the ninth equation It acts as a post-filter that gives a transfer function H _IP expressed by , and synchronizes the E _R signal and the E _B signal. Here, the above transfer function H _IP (Z・ω)
The contour line of |H|=1/2 is shown in FIG. 11, and the carrier component at the coordinates (1/4, 1/4) is roughly suppressed by this post filter. Then, the E _R signal synchronized by the synchronization circuit 41 and
The E _B signals are horizontal contour compensation circuits 43 and 4, respectively.
4.

Each of the horizontal contour compensation circuits 43 and 44 provides a transfer function H _IE1 expressed by the following equation 10: H _IE1 = 1/4 {6-(Z ^-1 +Z)} for the E _R signal and E _B signal, respectively. This performs horizontal contour compensation. In other words, the E _R signal and the E _B signal are subjected to the synchronization processing in the synchronization circuit 41 so that the transfer characteristic is expressed by H _IP (z, 0) = 1/4 (6 + Z ^-1 + Z)...Equation 11. However, by performing each of the horizontal contour compensations described above, it is possible to balance the carrier at coordinates (1/2, 0) with a transmission characteristic equivalent to that of the E _G signal in terms of horizontal high frequency components. can.

Further _, ^the vertical _contour compensation circuit 42 provides _the above ^- mentioned optical Low-pass filter characteristics of low-pass filters 31 and 32 H _OLPF3 H _OLPF3 = H _OLPF1・H _OLPF2 = 1/4 (1+Z ^-1・ω ^-1 ) (1+Z ^-1・ω ¹ ) ...Deterioration of vertical resolution according to equation 13 , does not contain false signals
Compensate with E _G signal component.

The above-mentioned vertical contour-compensated E _G signal and horizontal contour-compensated E _R signal and E _B signal are converted into three-color simultaneous signals through synchronization circuits 45, 46, and 47, respectively, and then processed in each process. circuit 48,4
The signal is supplied to the matrix circuit 51 via 9 and 50. This matrix circuit 51 receives the above E _R signal,
A luminance signal E _Y is synthesized from the E _G signal and E _B signal, and each color difference signal E _I and E _Q are synthesized. Above luminance signal
E _Y is subjected to horizontal contour compensation via the horizontal contour compensation circuit 52 and then supplied to the encoder 55 . In addition, each color difference signal E _I and E _Q are each filtered through a low pass filter 5.
3 and 54, the signal is band-limited and supplied to the encoder 55.

The encoder 55 synthesizes a composite color television signal compatible with the NTSC system from the luminance signal _EY and the color difference signals EI _{and EQ} , and outputs the composite color television signal.

As described above, according to the present invention, the horizontal pixel pitch is
In a discrete imaging system in which _two -dimensional spatial sampling of a subject image is performed by an imaging unit having a checkered pixel structure with _P Let the frequency be u and the vertical spatial frequency be v
On a two-dimensional spatial coordinate plane indicated by the horizontal axis P _x u/2π and the vertical axis P _y v/2π, we draw a straight line that passes through the coordinates (1/2, 1/2) and intersects each coordinate axis. A first optical low-pass filter whose trap characteristic is expressed by a straight line that is parallel to the straight line and passes through the coordinates (-1/2, -1/2); A straight line that intersects each coordinate axis and a coordinate (1/2, -1/2
) A second optical low-pass filter having a trap characteristic expressed by a straight line passing through the image pickup section is arranged in series on the front side of the image pickup section. The optical low-pass filters described above can sufficiently suppress the aliasing distortion in the optical system caused by It is possible to obtain high-quality imaging output.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an imaging device provided with a general optical low-pass filter. Figure 2 shows
FIG. 3 is a schematic plan view showing a pixel structure of an imaging section applied to the present invention. FIG. 3 is a spectrum distribution diagram showing, on a two-dimensional spatial coordinate plane, the distribution state of frequency components resulting from two-dimensional spatial sampling of a subject image in the imaging section. FIG. 4 is a characteristic diagram showing, on a two-dimensional space coordinate plane, the low-pass filter characteristics provided by a conventionally commonly used optical low-pass filter. FIG. 5 is a characteristic diagram showing the low-pass filter characteristics provided by the optical low-pass filter in the imaging device according to the invention on a two-dimensional spatial coordinate plane. FIG. 6 is a characteristic diagram showing a modified example of the low-pass filter characteristics. FIG. 7 is a schematic plan view showing the pixel structure of the imaging section applied to one embodiment of the present invention. FIG. 8 is a spectrum distribution diagram showing, on a two-dimensional spatial coordinate plane, the distribution state of frequency components obtained by two-dimensional spatial sampling by the imaging section. FIG. 9 is a block diagram showing the configuration of a signal processing system of an embodiment of a color imaging device using the above imaging section. FIG. 10 is a characteristic diagram showing the characteristics of the optical low-pass filter in this embodiment on a two-dimensional spatial coordinate plane. FIG. 11 is a characteristic diagram showing the filter characteristics similarly provided by the synchronization circuit. 11, 21...Imaging section, 11A, 21A...Picture element, 31, 32...Optical low-pass filter.

Claims (1)

[Claims] 1. Let the horizontal picture element pitch be P _x and the vertical picture element pitch be
In a discrete imaging system in which two-dimensional spatial sampling of a subject image is performed using an imaging unit with a checkered pixel structure, P _y , the spatial frequency in the horizontal direction sampled by the imaging unit is u, and the spatial frequency in the vertical direction is On a two-dimensional spatial coordinate plane, where the frequency is v and the horizontal axis is P _x u/2π and the vertical axis is P _y v/2π,
The trap characteristic is expressed by a straight line passing through the coordinates (1/2, 1/2) and intersecting each coordinate axis, and a straight line parallel to the straight line and passing through the coordinates (-1/2, -1/2). 1 optical low-pass filter, a straight line that passes through the coordinates (-1/2, 1/2) and intersects each coordinate axis, and a straight line that is parallel to the straight line and passes through the coordinates (1/2, -1/2). An imaging device characterized in that a second optical low-pass filter exhibiting a trap characteristic is arranged in series on the front side of the imaging section.