JPH02140066A - Aperture compensation method for image pickup device - Google Patents

Abstract

PURPOSE:To improve the photoelectric conversion characteristic with high accuracy by providing a preamplifier, a clamp circuit and a frequency correction circuit or the like, correcting phase distortion depending on the distortion of the current density distribution of the scanning beam, and applying aperture compensation. CONSTITUTION:An output signal (picture signal) of a photoconductive image pickup tube 1 based on beam scanning is amplified by a broad band preamplifier 2 without changing its characteristic, subject to DC clamp by a clamp circuit 3 and the result is inputted to a correction circuit 4. In this case, the response characteristic of the output signal of the tube 1 via the circuit 3 is made asynchronous depending on the asymmetry based on the self-sharpening effect of the current density distribution of the scanning beam or the like. Thus, the frequency characteristic of the input signal of the circuit 4 is deteriorated depending on the beam diameter of the scanning beam and the phase characteristic is distorted depending on the self-sharpening effect of the scanning beam or the like, but the circuit 4 applies the linear processing of the phase characteristic and the frequency correction. Thus, the output signal of the tube 1 is subject to aperture compensation, the photoelectric conversion characteristic is improved and high accuracy is attained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a beam aperture compensation method for an imaging device using a photoconductive image pickup tube. In an imaging device, in order to improve the resolution deterioration of the image (image) which depends on the spot diameter Cζ of the scanning beam of the imaging tube, for example, 97- As described on page 100, by beam aperture compensation that performs differential calculation and superposition, the level (luminance level) of the contour part of the output signal (image signal) of the image pickup tube is emphasized and frequency corrected, and the output The frequency characteristics (spatial frequency characteristics) of the signal are compensated to desired characteristics.

[Problem to be solved by the invention]

In the conventional beam aperture compensation method, only frequency correction is performed without taking phase characteristics into account, so as explained below, there is a problem that a signal with faithful photoelectric conversion characteristics cannot be obtained depending on the aperture compensation. be.

In other words, since the current density distribution of the scanning beam is distorted from a Gaussian distribution due to the self-sharpening effect and the output signal of the photoconductive image pickup tube becomes asymmetric, the output signal of the photoconductive image pickup tube is actually as shown in the diagram on page 97 of the above-mentioned "Image electronic circuit". It is not a linear phase characteristic as described in 3.43.

Therefore, even if frequency correction is performed by differential calculation and superposition without changing the phase characteristics, the phase distortion depending on the distortion of the current density distribution is not corrected, and the photoelectric conversion characteristics deteriorate.

In addition, in this type of photoconductive type image pickup tube, the amplitude of the output signal actually changes with nonlinear characteristics and is distorted depending on the brightness E of the incident light, based on the beam acceptance characteristic of the image pickup tube.

Based on the amplitude distortion of the output signal, the frequency characteristics of the aperture compensation for the output signal f deviate from the set characteristics, and depending on the brightness of the incident light, it becomes impossible to perform optimal aperture correction. There is a problem that a signal of photoelectric conversion characteristics cannot be obtained.

SUMMARY OF THE INVENTION An object of the present invention is to provide an aperture compensation method for an imaging device that performs aperture compensation by correcting phase distortion depending on distortion of current density distribution of a scanning beam, thereby improving photoelectric conversion characteristics. .

In addition, we have developed an aperture compensation method for imaging devices that corrects both the amplitude distortion that depends on beam acceptance and the phase distortion that depends on the distortion of the current density distribution of the scanning beam, and performs aperture compensation to improve the accuracy of photoelectric conversion characteristics. The purpose is to provide.

[Means to solve the problem]

In order to achieve the above object, in the aperture compensation method for an imaging device of the present invention, the output signal of the photoconductive imaging tube is
Frequency correction is performed using a nonlinear phase transfer characteristic that cancels out phase distortion depending on distortion of the current density distribution of the scanning beam of the image pickup tube, and the output signal is compensated for a predetermined frequency characteristic of the linear phase characteristic.

In addition, nonlinear amplitude correction is performed to cancel the amplitude distortion of the output signal f of the photoconductive image pickup tube, which is dependent on the beam acceptance characteristic f of the image pickup tube, and the distortion lζ of the current density distribution of the scanning beam of the image pickup tube is After the output signal is corrected to a linear amplitude characteristic by sequentially performing frequency correction of the nonlinear phase transfer characteristic to cancel phase distortion, the linear phase characteristic is compensated to a predetermined frequency characteristic.

[Effect]

Because of the Cζ configuration as described above, in the aperture compensation method of the present invention, the output signal of the photoconductive image pickup tube is
By performing frequency correction using a nonlinear phase transfer characteristic that cancels the phase distortion that depends on the distortion of the current density distribution of the scanning beam, the output signal is compensated for the phase distortion and has a predetermined frequency characteristic, resulting in faithful photoelectric conversion. A signal with conversion characteristics is obtained.

In addition, nonlinear amplitude correction is applied to the output signal of the photoconductive image pickup tube to cancel amplitude distortion that depends on beam acceptance characteristics, and nonlinear phase transfer characteristics are applied to the output signal of the photoconductive image pickup tube to cancel phase distortion that depends on distortion of the current density distribution of the scanning beam. By performing the frequency correction in sequence, even if the output signal is compensated for the amplitude distortion and phase distortion to have a predetermined frequency characteristic, the photoelectric conversion characteristic becomes extremely accurate.

〔Example〕

Embodiment 1 will be explained with reference to FIGS. 1 to 7.

(Embodiment 1) Embodiment 1 will be described with reference to FIGS. 1 to 3.

In FIG. 1, (1) is a photoconductive type image pickup tube, (2) is a preamplifier, (3) is a clamp circuit, and (4) is a frequency correction circuit for aperture compensation.

The output signal (image signal) of the image pickup tube (1) based on beam scanning is amplified by a wideband amplifier (2) without changing its characteristics, and then DC clamped by a clamp circuit (3) and then corrected by a correction circuit. (4) is input.

At this time, the response characteristic of the output signal of the image pickup tube (1) via the clamp circuit (3) is based on the asymmetry caused by the self-sharpening effect of the current density distribution of the scanning beam, for example, as shown in FIG. 2(a). It becomes asymmetrical as shown in .

Therefore, the frequency characteristics of the input signal to the correction circuit (4) deteriorate depending on the beam diameter of the scanning beam, and the phase characteristics are distorted depending on the self-sharpening effect of the scanning beam.

In order to perform phase characteristic linearization and frequency correction by the correction circuit (4), the correction circuit (4) is formed of an asymmetric acyclic transversal filter shown in FIG.

In Fig. 3, (5) is the input terminal connected to the clamp circuit (3), (6),..., (6) are the input terminals (
5) are N@(N=2M (M is an integer)) delay devices connected in series, (7), ..., (7) are the input terminal (5) and the output terminal of each delay device (5). N+1 connected to each
It is a coefficient multiplier consisting of variable resistors. (8) is an adder that adds the output signals of each coefficient unit (7), and (9) is an output terminal connected to the adder (8).

Note that the delay time of each delay device (6) is τ(se
e) is set.

Further, the coefficient value of each coefficient unit (7) is input from the input terminal (5) to . . . -+1. -1 respectively.

Then, the output signal of the Nth delay device (6) from the input terminal (5), that is, the input signal of the central coefficient unit (7) is set as X (t), and the output signal of the Nth delay device (6) from the input terminal (5) is set as ) is the signal output to the clamp circuit (3
) to the input terminal (5) is X (t)
Since the signal is delayed by -τ, y (t) is expressed as (
1) It is shown by the formula.

Y(t)=KoX(t)+K, X(-τ)-1--
+ to No. 1 X(t--;τ)+ to X(t+τ)+...
Therefore, the transfer function H(Z) of the correction circuit (4) is expressed by the following equation (2).

Note that Z' represents a complex number of e-j"' (ω is the angular velocity).

Then, as )Ll=,..., ni-2=nie, ~Lyu is set to the desired frequency correction coefficient F, and the transfer function H(Z) becomes only the cosine term of ω. Symmetrical correction circuit (
4), the phase transfer characteristic becomes a linear characteristic, and as in the conventional case, only frequency correction is performed without considering phase correction, and the output signal y(t) becomes, for example, similar to that shown in Fig. 2(a). Although the signal Fζ is asymmetric, in the present invention, the correction circuit (4) is configured as an asymmetric type, and frequency correction is performed using a nonlinear phase transfer characteristic.

That is, the phase characteristic 1 of the output signal of the image pickup tube (1) depending on the distortion of the current density distribution of the scanning beam (1), for example, FIG.
) has a desired frequency correction characteristic based on the phase characteristic obtained from the response characteristic of the image pickup tube (1), and corrects the phase characteristic of the output signal of the image pickup tube (1) to a linear characteristic by canceling the distortion of the current density distribution. Thus, the respective coefficient values K n and -Kn are set to, for example, the asymmetric value T 'T shown in FIG. 2(b), and the correction circuit (4) is configured in an asymmetric type.

Therefore, the output signal of the image pickup tube (1) via the clamp circuit (3) is subjected to frequency correction by the correction circuit (4) using a nonlinear phase transfer characteristic.

At this time, the output signal of the image pickup tube (1) is compensated for a predetermined linear phase frequency characteristic lζ aperture, for example, as shown in FIG.
As shown in c), frequency correction is performed by correcting the phase distortion based on the asymmetry shown in (,).

Therefore, the output signal of the correction circuit (4) is
), the phase distortion of the output signal is reduced and the frequency is corrected, resulting in a signal with faithful photoelectric conversion characteristics.

In the embodiment described above, the correction circuit (4) is configured with a one-dimensional transversal filter, and correction is performed in the horizontal or vertical direction based on the delay time τ.
It is composed of a correction circuit (4) & a two-dimensional transversal filter, and can also perform horizontal and vertical correction.

Furthermore, it goes without saying that the correction circuit (4) may be constructed from other than the transversal filter.

(Other Embodiments) Other embodiments will be described with reference to FIGS. 4 to 7.

In Fig. 4, the difference from Fig. 1 is that the clamp circuit (
The point is that an amplitude correction circuit α0 having the configuration shown in FIG. 5 is provided between the correction circuit (3) and the correction circuit (4).

In FIG. The emitter is grounded via an emitter bias resistor.

as, g6, α power, Q8), 4 is five variable resistors for slope setting, one end of which is connected to the emitter of transistor αυ, 翰, ■υ, (A), @, trout is the anode of variable resistor α9~ 5 connected to the other end of each other end of 0
diodes for linear approximation, and variable power supplies for bias correction with 5 cathodes (c), (a), @, (c), (
4) Each is reverse biased.

The output signal of the image pickup tube (1) via the clamp circuit (3) is subjected to amplitude correction of the nonlinear characteristics of the correction circuit OG before being subjected to frequency correction of the correction circuit (4).

That is, based on the beam acceptance characteristics of the image pickup tube (1), in many cases, the amplitude (level) of the output signal of the image pickup tube (1) relative to the brightness of the incident light (image plane illuminance)? ! For example, the property is nonlinear as shown in FIG. 6(a).

Then, the output signal of FIG. 6(a) is directly applied to the correction circuit (
4), the output signal of the bright nonlinear region of the incident light [On the other hand, the frequency transfer characteristics of the correction circuit (4) are small but optimal frequency correction characteristics due to the amplitude distortion f. If the aperture correction deviates from the correct position, it becomes impossible to perform high-precision aperture correction, and the force signal with high fidelity cannot be used, for example in Fig.
b), and corrects the amplitude characteristic of the input signal to the correction circuit (4) to a linear characteristic as shown in FIG. 6(C).

The amplification characteristics of the correction circuit OQ are based on the measurement results of the beam acceptance characteristics of the image pickup tube (1). , 1
! It is set by adjusting the sources (a) to 翰, etc.

Then, based on the amplitude correction of the correction circuit (IQ), the amplitude distortion of the input signal of the correction circuit (4) is improved.
As shown in FIG. 2(a), the response characteristics are better than in the case of FIG. 2(a).

Therefore, the frequency correction of the correction circuit (4) is performed with optimum characteristics regardless of the brightness of the incident light.

Therefore, the output signal of the correction circuit (4) is, for example, the seventh
As shown in FIG. 2(b), the response becomes sharper than that in FIG. 2(c).

That is, by providing the correction circuit qO before the correction circuit (4), the output signal of the image pickup tube (1) is subjected to amplitude correction that cancels the amplitude distortion based on the beam acceptance characteristics, and also performs amplitude correction that is dependent on the current density distribution of the scanning beam. Frequency correction of the nonlinear phase transfer characteristic to cancel out the phase distortion is performed in the order fζ.

Since the output signal of the image pickup tube (1) is corrected to have a linear amplitude characteristic and then a predetermined frequency characteristic of a linear phase characteristic, the aperture correction is always optimal regardless of the brightness of the incident light. This also improves resolution deterioration depending on the brightness of the incident light, resulting in extremely faithful photoelectric conversion characteristics.

In the embodiment described above, the correction circuit (4) is composed of a nonlinear amplifier approximating a diode broken line, but it goes without saying that the correction circuit (4) may be composed of an active filter or the like.

〔Effect of the invention〕

Since the present invention is configured as described above, it produces the effects described below.

Frequency correction is applied to the output signal of the photoconductive image pickup tube using a nonlinear phase transfer characteristic that cancels out phase distortion depending on the distortion of the current density distribution of the scanning beam of the image pickup tube, and the output signal is adjusted to a predetermined linear phase characteristic. As a result, it is possible to correct phase distortion and compensate for a predetermined frequency characteristic, improve resolution deterioration due to phase distortion, perform good aperture compensation, and obtain faithful photoelectric conversion characteristics. I can get a signal.

In addition, nonlinear amplitude correction is applied to the output signal of the photoconductive image pickup tube to cancel amplitude distortion that depends on the beam acceptance characteristics of the image pickup tube, and phase distortion that is dependent on the distortion of the current density distribution of the scanning beam of the image pickup tube. The output signal is corrected to a linear amplitude characteristic, and then the linear phase characteristic is compensated to a predetermined frequency characteristic, regardless of the brightness of the incident light. , it is possible to constantly correct phase distortion and compensate for the predetermined frequency characteristics, improve resolution deterioration due to amplitude distortion 9 phase distortion, perform extremely high-precision aperture compensation, and achieve extremely faithful and high-precision photoelectric conversion characteristics. signal can be obtained.

[Brief explanation of the drawing]

2(a) to (C) are characteristic diagrams for explaining the operation of FIG. 1, and FIG. 3 is a detailed wiring diagram of the main parts of FIG. 1.
FIG. 4 is a block diagram of another embodiment, FIG. 5 is a detailed wiring diagram of a part of FIG. 4, and FIGS. 6(a) to (e) are characteristic diagrams for explaining the operation of FIG. 5. FIG. 7(a), O)) is a characteristic diagram for explaining the operation of FIG. (1)...Photoconductive image pickup tube, (4)...Frequency correction circuit, CI(]...Amplitude correction circuit.

Claims (1)

[Scope of Claims] 1. Frequency correction is performed on the output signal of a photoconductive image pickup tube using a nonlinear phase transfer characteristic that cancels out phase distortion depending on distortion of the current density distribution of the scanning beam of the image pickup tube, and the output signal of the photoconductive image pickup tube is An aperture compensation method for an imaging device, the method comprising compensating a signal to a predetermined frequency characteristic having a linear phase characteristic. 2. The output signal of the photoconductive image pickup tube is subjected to nonlinear amplitude correction to cancel amplitude distortion that depends on the beam acceptance characteristics of the image pickup tube, and phase distortion that is dependent on the distortion of the current density distribution of the scanning beam of the image pickup tube. An aperture compensation method for an imaging device, comprising sequentially performing frequency correction of a nonlinear phase transfer characteristic to cancel the output signal, correcting the output signal to a linear amplitude characteristic, and then compensating the linear phase characteristic to a predetermined frequency characteristic.