JPS5942417B2 - Automatic contrast adjustment device for video signals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic contrast adjustment device for a video signal used in a scanning electron microscope.

Image signals obtained by detecting secondary electrons, reflected electrons, etc., which are information sources in a scanning electron microscope, are usually a superimposition of DC and AC components, but the amplitude of the AC component is Determine the contrast of the image on the cathode ray tube. Therefore, conventionally, the difference voltage Vp between the maximum value and the minimum value of the peak (amplitude) in the video signal Vs as shown in FIG. 1 is calculated, and the ratio with the appropriate amplitude value is calculated based on the difference voltage. In some cases, the degree of amplification of the video signal is set. However, in such a method, if there is a charge-up of the sample, an abnormally large peak appears in the video signal, and the amplification degree must be adjusted so that the peak value becomes the appropriate amplitude value on the cathode ray tube. The force to be determined and other normal peaks other than the abnormal peaks will be below the appropriate amplitude value, and as a result, it will not be possible to obtain optimal contrast on the cathode ray tube.

Moreover, such drawbacks are particularly problematic when contrast adjustment is automated. The present invention provides a device that can prevent such inconveniences, and the principle of the present invention will be explained in detail below.

That is, the present invention calculates the average value (hereinafter referred to as the average contrast value) of the amplitude of the AC component in one period (for example, one horizontal scan or frame) of the video signal, and uses this average value as the appropriate value for providing the optimal contrast. This is to change the amplification degree of the intensifier so that the amplitude value is reached.

Here, the above-mentioned average contrast refers to the voltage (V, which may be a current) that changes in the positive direction (the rising edge is D) and the negative direction (the falling edge is V) in the video signal Vs shown in FIG.
l, IV21, lV3l, ......1Vnl are added and the added voltage is divided by the number of changes n,
The value Vm is expressed by the following equation. Σ V・i=1 Vm=・・・・・・・・・・・・・・・ (1) Figure 3 shows an example according to the above principle, and 1 is the lens barrel of a scanning electron microscope. It is.

Reference numeral 2 denotes an electron gun installed above the lens barrel, and the electron beam 3 generated from the electron gun is irradiated onto the surface of the sample 5 as a microprobe through a focusing lens 4, and is also directed onto the sample 5 through deflection coils 6X and 6Y. The surface is scanned two-dimensionally.

Signals such as secondary electrons generated from the sample by the probe irradiation are detected by the detector T, and the detected signal is amplified by the amplifier 8 and then used as a brightness modulation signal for the cathode ray tube 9. Deflection coils IOX and IOY of the cathode ray tube 9
Since the deflection coils 6X and 6Y in the lens barrel are supplied with deflection currents for horizontal and vertical scanning from the scanning signal generating circuit 11, a scanned sample image is displayed on the screen of the cathode ray tube 9. Reference numeral 12 denotes a gate circuit for introducing a part of the detection signal from the detector 7 into the circuit section 13, and a horizontal blanking pulse is supplied from the scanning signal generation circuit 11 to the gate circuit. The last blanking pulse after closing the gate opens the gate, and the next blanking pulse closes the gate.

Therefore, video signals corresponding to one arbitrary horizontal scanning period are introduced into the circuit section. The circuit section 13 is for measuring the average value of the contrast of the introduced video signal, that is, for calculating the above-mentioned formula (1). It is comprised of a circuit 15 that adds the voltage that changes in direction, a circuit 16 that measures the number of changes, and a division circuit 17 that divides the output signal from the circuit 15 by the output signal from the circuit 16. The average value of the contrast measured by the circuit section 13 is sent to a gain control circuit 18 that measures the degree of amplification of the amplification device 8, and here the appropriate amplitude is determined to provide the optimum contrast on the cathode ray tube 9. The value is compared with the average value of the contrast, and the amplification degree of the amplifier 8 is set so that the average value of the contrast becomes the appropriate amplitude value. For example, if the current optimum contrast amplitude value is 10V and the average value of the contrast of the video signal during one horizontal scanning period is 1V, the amplification degree of the amplifier 8 is set to 10 times. As described above, the present invention sets the degree of amplification of the input video signal based on the average value of the amplitude change, so if there is an abnormally large peak such as noise in the input video signal, However, since the peaks are averaged, the amplitude of the input video signal can be processed to the normal optimum, and therefore an image with an appropriate contrast can always be automatically displayed on the cathode ray tube. Note that the present invention is not limited to the embodiment shown in FIG. 3, and the circuit configuration can be modified and developed depending on the purpose. For example, the signal for calculating the average contrast value is not limited to one horizontal scanning signal, but signals from a plurality of horizontal scanning signals may be used. In addition, the gate circuit was synchronized with horizontal scanning by supplying a horizontal blanking pulse, but if it was synchronized with one field of view scanning (-frame) by supplying a vertical blanking pulse, the average value of the contrast of the entire field of view could be calculated. Therefore, a more suitable contrast can be obtained. In addition, when measuring the average value of contrast, we have shown the case where voltage changes in both the positive and negative directions are added to the video signal, but only voltage changes in either the positive or negative direction are added. You can.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining a conventional example, FIG. 2 is a diagram for explaining the principle of the present invention, and FIG. 3 is a schematic diagram for showing an embodiment of the present invention. In Figure 3, 1 is the lens barrel of the scanning electron microscope, 2 is the electron gun, 5 is the sample, 6X, 6Y, 10X and 10Y are the deflection coils, 7 is the detector, 8 is the amplifier, and 9 is the cathode ray tube. , 11
1 is a scanning signal generation circuit, 12 is a gate circuit, 13 is a circuit for measuring the average value of the contrast of an input video signal, 14 is a start button, and 18 is a gain control circuit.

Claims (1)

[Claims] 1. Means for measuring the average amplitude of the AC component of the input video signal, and means for setting the degree of amplification of the input video signal based on the value measured by the means. Automatic contrast adjustment device for video signals.