JP2001330412A - Visibility measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the detection of poor visibility at higher accuracy with a lower cost within a tunnel or the like by facilitating the measurement of visibility. SOLUTION: In a tunnel where lighting is set, a video signal taken by a monitoring camera installed therein is converted into image data to observe luminance information obtained by the lighting in an image data. Thus, a conversion function to the visibility from the luminance information is previously held to calculate the visibility from the luminance information observed using the conversion function.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a visibility measuring device for measuring visibility in a tunnel or the like.

[0002]

2. Description of the Related Art The degree of visibility in the atmosphere can be measured by detecting the reflection or attenuation of light, whereby visibility is measured, and when visibility is poor, an alarm such as fog or snowstorm is issued depending on the degree. Or ventilate in tunnels. FIG. 5 is a diagram showing a schematic configuration of a VI measuring device which is a conventional visibility measuring device based on a transmitted light amount measuring method for measuring light attenuation. In the figure, reference numeral 51 denotes a light projecting unit that emits light from a reference light source, 52 denotes a reference detecting unit that keeps the luminance level of the light source constant, 53 denotes a light receiving unit that receives light from the light projecting unit 51, and 54 denotes a light receiving level. This is an arithmetic control unit that measures the attenuation rate of the data. The conventional VI measuring device is configured as described above, emits a thin beam of light from the light projecting unit 51, receives the light, and measures the attenuation of the light. For this reason, due to the straightness of the light, it is necessary to install the device in a straight and well-visible place. Usually, the light projecting unit 51 and the light receiving unit 53 are 100
m apart.

[0003]

In a tunnel, the visibility is deteriorated due to the exhaust gas of the vehicle and the like. Therefore, the visibility is measured and the ventilation in the tunnel is performed when the visibility is poor. The conventional VI measuring instrument can be installed only in a straight line with good visibility in the tunnel, and its installation position is different from the visibility in actual vehicle running on an inspection road or the like provided at the end of the road. Becomes Further, since the VI measuring instruments themselves are expensive, it is costly to install them in large numbers. For these reasons, it has been difficult to accurately detect the visibility defect in the tunnel.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is possible to easily measure the visibility at a place other than a straight line where the visibility is good. It is an object of the present invention to provide a visibility measuring device capable of detecting a defect.

[0005]

Means for Solving the Problems Claim 1 according to the present invention.
The visibility measurement device described above captures an image of the direction in which the visibility is measured and outputs a video signal, and converts the video signal from the imaging means into two-dimensional image data, and from luminance information in the image data, A visibility calculating means for calculating a visibility using a preset conversion function, wherein the luminance information is a difference between high luminance information and low luminance information in predetermined high luminance parts and low luminance parts in the image data. This is luminance information.

According to a second aspect of the present invention, there is provided a visibility measuring apparatus according to the first aspect, wherein differential luminance information in image data obtained by converting a video signal from the imaging means and light attenuation provided separately. The regression curve between the difference luminance information and the visibility data calculated by statistically processing the collected data previously collected in association with the visibility data measured by the VI measuring device by detection is set as a conversion function. .

According to a third aspect of the present invention, in the visibility measuring apparatus according to the second aspect, means for inputting visibility data from a VI measuring device, and collecting the visibility data and difference luminance information to obtain a regression curve. Means for determining a conversion function by calculation.

According to a fourth aspect of the present invention, there is provided a visibility measuring apparatus according to any one of the first to third aspects, wherein the visibility measurement measures a visibility in a tunnel, and is installed at a predetermined position in the tunnel. Imaging means is constituted by the monitoring camera.

According to a fifth aspect of the present invention, in the visibility measuring apparatus according to the fourth aspect, the illumination is installed at a predetermined position in the tunnel, and the high-brightness information and the low-brightness information correspond to the illumination in the image data. These are high luminance information near the center and low luminance information around the illumination.

According to a sixth aspect of the present invention, in the visibility measuring device according to the fourth or fifth aspect, a conversion function is set for each illumination mode based on a combination pattern of ON / OFF of a plurality of illuminations installed in the tunnel. An illumination mode determination unit that determines the illumination mode by observing luminance information of the illumination from image data obtained by converting a video signal from the imaging unit, and a conversion function corresponding to the determined illumination mode. Is used to calculate the visibility by the visibility calculation means.

According to a seventh aspect of the present invention, there is provided a visibility measuring apparatus which captures a direction in which visibility is measured and outputs a video signal, and converts a video signal from the capturing means into two-dimensional image data. , The distance from the imaging means is different 2
And a visibility calculating means for calculating a transmittance as a visibility from brightness information of the two constant light sources in the image data.

According to an eighth aspect of the present invention, in the visibility measuring device according to the seventh aspect, the transmittance by the visibility calculating means is such that two constant light sources (distances L ₁ , L _{2 from the} image capturing means) in the image data. Assuming that the luminance of (L ₂ > L ₁ ) is E ₁ and E ₂ , (E ₂ / E ₁ ) ^K where K = 100 / (L ₂ −L ₁ ).

According to a ninth aspect of the present invention, there is provided the visibility measuring apparatus according to the seventh or eighth aspect, wherein the visibility measurement measures the visibility within the tunnel, and the monitoring camera is installed at a predetermined position in the tunnel. Constitutes imaging means by
Illumination at two locations where a fixed light source is installed at a predetermined position apart from the tunnel.

According to a tenth aspect of the present invention, in the visibility measuring apparatus according to any one of the fourth to sixth or ninth aspects, the surveillance camera controls an output signal level according to light irradiation or the like. A camera status detecting means for detecting a control operation for suppressing the output signal level of the monitoring camera is provided, and the visibility calculation by the visibility calculating means is canceled based on the detection result.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of a visibility measuring device according to Embodiment 1 of the present invention.
In the figure, reference numeral 11 denotes an image pickup means for picking up an image of a moving object and outputting a video signal; 12, an image data storage means for storing image data obtained by digitizing the video signal from the image pickup means 11; 13, luminance information in the image data , And input visibility data measured by a VI measuring instrument by separately installed light attenuation detection, collect it in correspondence with the luminance information, and statistically process these to convert the luminance information to visibility data. Conversion function determining means for obtaining a function, 14 is a conversion function storing means for storing the determined conversion function, and 15 is a visibility calculating means for observing luminance information in image data and calculating a visibility using the stored conversion function. It is. Here, the visibility calculated using the visibility data measured by the VI measuring device and the conversion function is a visibility index represented by 0 to 100%, and the best visibility state is defined as 100%.

In this embodiment, a case where the visibility in a tunnel is measured will be described. In the tunnel, the visibility deteriorates due to the exhaust gas of the vehicle. Therefore, the visibility is measured, and when the visibility is poor, ventilation in the tunnel is performed. In a tunnel, lighting for illuminating the inside of the tunnel is usually arranged so as to be regularly arranged. In a tunnel of a certain size, the traffic flow of the vehicle, for example, the number of vehicles, the degree of congestion, the vehicle speed, etc. In order to monitor the traffic, a CCTV camera (hereinafter referred to as a surveillance camera) is provided at an appropriate interval (normally 20
(0 m pitch). Here, the surveillance camera installed in the tunnel is used as the imaging unit 11, a video signal from the surveillance camera is converted into image data, and luminance information by illumination in the image data is used.

Next, the operation will be described. First, prior to actual visibility measurement, a process of determining and storing a conversion function from luminance information to visibility data is performed in advance, which will be described below. A VI measuring device based on light attenuation detection is separately installed within the imaging range of the surveillance camera, and visibility data from the VI measuring device is input to the conversion function determining means 13 of the visibility measuring device. The video signal from the surveillance camera is converted into image data and stored in the image data storage unit 12. The conversion function determining means 13 detects, as luminance information from the image data, difference luminance information between the high luminance information of the highest luminance part near the center of the captured illumination and the low luminance information of the dark part around the illumination. Then, the difference luminance information is collected for a predetermined period together with the visibility data from the VI measuring device measured at the same time. From the collected difference luminance information and visibility data,
The conversion function determining means 13 performs a statistical process to calculate a regression curve between the difference luminance information and the visibility data, and determines a conversion function from the difference luminance information to the visibility data. The collection of the luminance information from within the image data by the conversion function determining means 13 is performed even if the conversion function determining means 13 observes the luminance information in the image data as needed. Either may be collected from the accumulated data. The determined conversion function is stored in the conversion function storage unit 14 by, for example, storing parameters for determining the function.
To memorize. For example, when the regression curve is a straight line, two parameters are stored.

In the actual visibility measurement, a video signal from the surveillance camera is converted into image data and stored in the image data storage means 12, and the visibility calculation means 15 uses the same data used to determine the conversion function from the image data. The difference luminance information is detected, and the visibility is calculated from the difference luminance information using the conversion function stored in the conversion function storage unit 15.

Next, the relationship between luminance information and visibility data will be described. Normally, the luminance value on the image data of a bright part such as illumination decreases when the visibility deteriorates, and conversely, the luminance value on the image data of the dark part increases when the visibility deteriorates. This is considered to be because the influence of the scattered light is present in both cases, but the luminance of the dark portion is more susceptible to the scattered light. For this reason, when a difference obtained by subtracting the luminance of a dark part from the luminance of a bright part is used, the range of change is larger than when only one of them is used, and a highly accurate correlation with the visibility is obtained. This is considered to be because the influence of the scattered light can be removed by subtracting the luminance of the dark part. In this embodiment, the luminance information on the image data of the illumination installed in a range in which the change in visibility can be captured as a change in luminance is observed, and the high luminance information of the highest luminance point near the illumination center and the illumination Difference luminance information from the low luminance information of the surrounding dark part is detected and used.

As described above, the visibility can be determined by processing the image of the surveillance camera installed in the tunnel, so that the visibility measurement can be easily performed at the installation locations of a large number of surveillance cameras arranged in the tunnel. This makes it possible to measure the visibility in the same range as the driver's field of view, so that the detection of poor visibility in the tunnel can be performed at low cost and with high accuracy.

It is to be noted that only a plurality of surveillance cameras may be provided, and the other means 12 to 15 may be gathered at one place and connected to each surveillance camera to transmit and receive data. In this case, the conversion function may not be determined for each surveillance camera, and a conversion function determined at another monitoring camera installation location under similar or similar conditions may be used. As a result, even in places where measurement with a VI measuring instrument is actually difficult,
Visibility measurement can be performed.

In this embodiment, the conversion function determining means 13 is provided inside the visibility measuring device. However, the brightness information in the image data and the visibility data from the VI measuring device are collected in advance by an external device, and the conversion function is determined. The conversion function may be determined and input to the conversion function storage means 14 for setting.

In addition, image data obtained by image processing of a video image of a normal surveillance camera is subjected to a conversion process using an exponential function called a γ correction for a brightness value for brightness adjustment.
When luminance information is detected from image data, if luminance information is detected by performing inverse correction of γ correction, accurate luminance information is obtained, and the accuracy of visibility measurement is improved.

Embodiment 2 FIG. Next, a second embodiment of the present invention will be described with reference to FIG. In FIG. 2, reference numeral 21 denotes an illumination mode determination unit. The lighting status of the plurality of lights installed in the tunnel may change depending on the time zone or the like. In this embodiment, the visibility measurement device according to the first embodiment includes the illumination mode determination unit 21 and the tunnel. It detects the lighting status of the lighting inside. An illumination mode based on a combination pattern of ON / OFF in a plurality of illuminations is determined, and a conversion function determining unit 13 determines a conversion function for each illumination mode in advance and stores a conversion function storage unit 14.
Remember. The lighting mode determination means 21 observes luminance information on the image of the lighting whose lighting state changes due to the switching of the lighting mode, and detects the lighting state. For example,
The ON / OFF of lighting is determined by comparing the observed luminance information with a preset threshold value. In the determination of the lighting mode, the lighting mode is determined by comparing the lighting state detected by observing the luminance information with a previously stored combination pattern of ON / OFF for each lighting mode. The visibility calculating means 15 detects difference luminance information from the image data as in the first embodiment, and extracts a conversion function from the conversion function storage means 14 based on the illumination mode determined by the illumination mode determination means 21. Then, the visibility is calculated from the difference luminance information using this conversion function. As a result, it becomes possible to measure the visibility according to the change in the lighting environment in the tunnel.

Embodiment 3 Next, a third embodiment of the present invention will be described with reference to FIG. In FIG. 3, reference numeral 31 denotes a camera status detecting unit for detecting the control status of the output signal level of the monitoring camera as the imaging unit 11. The surveillance camera usually has a control function for controlling the output signal level when the headlight of the vehicle is directly irradiated, for example, when the headlight of the vehicle is directly irradiated. In this embodiment, the visibility measuring device according to the second embodiment is provided with the camera status detecting means 31 to observe, for example, luminance information on an image of illumination in a tunnel and detect a sudden change in the luminance information. . As the illumination for observing the luminance information, it is desirable to use illumination provided at a short distance, which is hardly affected by a change in visibility, which is a measurement purpose, on image data.
Luminance information on the image of such illumination is observed in advance when visibility is good and stored as reference luminance information, and luminance information on the image observed during actual operation is compared with the reference luminance information, When a change exceeding a preset threshold is detected, it is recognized that the camera control is ON. In the visibility calculation means 15, when the camera status detection means 31 detects the camera control as 0N, the visibility calculation cannot be performed because difference luminance information on which the visibility calculation is based cannot be accurately obtained. Thereby, the noise component can be removed by the operation of the monitoring camera, and the accuracy of the visibility measurement is improved.

Embodiment 4 Next, a fourth embodiment of the present invention will be described with reference to FIG. In FIG. 4, reference numeral 41 denotes transmittance calculating means as visibility calculating means. In this embodiment, unlike the above-described first to third embodiments, the transmittance of light that becomes a visibility is directly calculated from luminance information without using a conversion function. As shown in the figure, a video signal from a surveillance camera, which is an imaging unit 11, is converted into image data and stored in an image data storage unit 12, and is transmitted by a transmittance calculation unit 41 to two fixed light sources ( The luminance information of the illumination is observed, and the transmittance is calculated from the ratio of the luminance information of the two light sources.

Hereinafter, the calculation of the transmittance will be described. When light passes through the atmosphere, its brightness is attenuated by scattering, absorption, reflection, and the like. Assuming that the light attenuates from E to E-dE while the light passes through the gas layer dr at a distance r, dE / E is proportional to dr, so dE / E = −α · dr dE = − E · α · dr (1) α: attenuation coefficient for atmospheric light. Assuming that the brightness at the light receiving portion at a distance L by a substantially parallel light beam from the light source E ₀ is E _L , by integrating the equation (1) from 0 to L with respect to r, E _L = E ₀ · exp (−α · L) ····· (2) E L / E 0 = exp (-α · L) ····· (3) E 0: brightness of the light source E _L: the brightness at a distance L . Here, E _L / E ₀ is the light transmittance, and L is 1
The transmittance at the time of 00 m becomes the visibility.

Taking the logarithm of both sides of the equation (2), logE ₁ = logE ₀ −α · L (4) Lighting in the tunnel is used as a light source, and L ₁ m and L ₂ m from the surveillance camera, respectively. (However, L ₂ > L ₁ )
The brightness in the image data of the tunnel lighting is E ₁ , E ₂ ,
When both E ₀ the brightness of tunnel illumination itself, (4)
From the equation, logE ₁ = logE ₀ −α · L ₁ (5) logE ₂ = logE ₀ −α · L ₂ (6) Subtract equation (6) from equation (5) And logE ₁ −logE ₂ = α (L ₂ −L ₁ ), and the attenuation coefficient α for atmospheric light is represented by the following equation. α = (log E ₁ −log E ₂ ) / (L ₂ −L ₁ ) (7) Therefore, if L ₁ and L ₂ are given, α is obtained. From the equation (3), the transmittance is expressed by E _L / E ₀ = (E ₂ / E ₁ ) ^K (8) where K = L / (L ₂ −L ₁ ) In equation (8), the value when L = 100 is the visibility.

The transmittance calculating means 41 observes luminance information of two fixed light sources (illumination) in the image data, and
Assuming that the luminances of the two light sources (distances L ₁ and L ₂ (L ₂ > L ₁ ) from the imaging means) are E ₁ and E ₂ , (E ₂ / E ₁ ) ^K where K = 100 / (L ₂ − L ₁ ) is calculated.

This makes it possible to easily measure the visibility at the places where a large number of surveillance cameras are arranged in the tunnel.
Detecting a visibility defect in a tunnel can be performed inexpensively and with high accuracy, and it is not necessary to measure the visibility with a VI measuring instrument in advance to determine a conversion function, and it is easy to determine the visibility from luminance information detected from image data. Can be directly calculated.

In this embodiment, the camera status detecting means 31 shown in the third embodiment is provided.
When the camera control is detected as 0N, if the transmittance calculation by the transmittance calculator 41 is not performed, the noise component can be removed by the operation of the monitoring camera, and the accuracy of the visibility measurement is improved.

In each of the first to fourth embodiments, a monitoring camera is used to measure the visibility within the tunnel where the illumination is installed. Therefore, an imaging means and a light source are newly provided particularly for visibility measurement. The visibility measurement was easily performed by using the existing one without any limitation. However, the visibility measurement is not limited to this.

[0033]

As described above, the visibility measuring apparatus according to the first aspect of the present invention comprises: an imaging means for imaging a direction in which visibility is measured and outputting a video signal; and a two-dimensional video signal from the imaging means. And a visibility calculating means for calculating a visibility using a preset conversion function from the luminance information in the image data, wherein the luminance information is a predetermined high luminance in the image data. Part and low-luminance part, because it is difference luminance information between high-luminance information and low-luminance information,
Visibility measurement using the imaging means can be easily performed, and detection of poor visibility can be performed at low cost.

According to a second aspect of the present invention, there is provided a visibility measuring apparatus according to the first aspect, wherein difference luminance information in image data obtained by converting a video signal from the image pickup means and light separately installed are provided. The regression curve between the difference luminance information and the visibility data calculated by statistically processing the collected data previously collected in association with the visibility data measured by the VI measuring device based on the attenuation detection was set as a conversion function. The visibility measurement using the means can be easily and reliably performed, and the detection of visibility failure can be performed at low cost.

According to a third aspect of the present invention, in the visibility measuring apparatus according to the second aspect, means for inputting visibility data from the VI measuring device, and collecting the visibility data and difference luminance information to obtain a regression curve. And a means for determining a conversion function by the calculation of .times..times..times..times..times..times..times..times.

According to a fourth aspect of the present invention, there is provided a visibility measuring apparatus according to any one of the first to third aspects, wherein the visibility measurement measures a visibility within a tunnel, and is installed at a predetermined position in the tunnel. Since the imaging means is constituted by the surveillance camera, visibility measurement in the tunnel can be easily and inexpensively performed, and detection accuracy of visibility failure can be improved.

According to a fifth aspect of the present invention, in the visibility measuring apparatus according to the fourth aspect, the illumination is installed at a predetermined position in the tunnel, and the high luminance information and the low luminance information correspond to the illumination in the image data. , And the low-brightness information in the vicinity of the illumination, the visibility measurement can be reliably and reliably performed.

According to a sixth aspect of the present invention, in the visibility measuring device according to the fourth or fifth aspect, a conversion function is set for each illumination mode based on a combination pattern of ON / OFF of a plurality of illuminations installed in the tunnel. And an illumination mode determining means for determining the illumination mode by observing luminance information of the illumination from image data obtained by converting a video signal from the imaging means, wherein the conversion corresponding to the determined illumination mode is performed. Since the visibility is calculated by the visibility calculating means using the function, the visibility can be measured according to a change in the illumination environment in the tunnel, and the accuracy of the visibility measurement is improved.

According to a seventh aspect of the present invention, there is provided a visibility measuring apparatus which captures an image of a direction in which visibility is measured and outputs a video signal, and converts the video signal from the capturing means into two-dimensional image data. And a visibility calculating means for calculating a transmittance which becomes a visibility from brightness information of the two constant light sources in the image data, the image capturing apparatus comprising: The visibility measurement using the means is further facilitated, and the visibility failure can be easily detected at a low cost.

According to an eighth aspect of the present invention, in the visibility measuring apparatus according to the seventh aspect, the transmittance by the visibility calculating means is such that two constant light sources (distances L ₁ , L Assuming that the luminance of ₂ (L ₂ > L ₁ ) is E ₁ and E ₂ , (E ₂ / E ₁ ) ^K where K = 100 / (L ₂ −L ₁ ) Easy and inexpensive visibility measurement can be reliably performed.

According to a ninth aspect of the present invention, there is provided the visibility measuring apparatus according to the seventh or eighth aspect, wherein the visibility measurement measures a visibility in a tunnel, and a monitoring device installed at a predetermined position in the tunnel. The camera constitutes an imaging means, and a fixed light source is provided in two places at a predetermined position apart from each other in the tunnel, so that visibility measurement in the tunnel becomes easy and inexpensive, and detection of visibility failure is performed. The accuracy can be improved.

According to a tenth aspect of the present invention, in the visibility measuring apparatus according to any one of the fourth to sixth or ninth aspects, the surveillance camera controls an output signal level according to light irradiation or the like. A camera status detecting means for detecting a control operation of suppressing the output signal level of the monitoring camera,
Since the visibility calculation by the visibility calculation means is canceled based on the detection result, noise components can be removed by the operation of the monitoring camera, and the accuracy of visibility measurement is improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a visibility measuring device according to Embodiment 1 of the present invention.

FIG. 2 is a schematic configuration diagram of a visibility measurement device according to a second embodiment of the present invention.

FIG. 3 is a schematic configuration diagram of a visibility measuring device according to Embodiment 3 of the present invention.

FIG. 4 is a schematic configuration diagram of a visibility measuring device according to Embodiment 4 of the present invention.

FIG. 5 is a schematic configuration diagram of a conventional visibility measuring device.

[Explanation of symbols]

11 imaging means, 12 image data storage means, 13 conversion function determination means, 14 conversion function storage means, 15 visibility calculation means, 21 illumination mode determination means, 31 camera situation detection means, 41 transmittance calculation means as visibility calculation means.

Claims (10)

[Claims] 1. An image pickup means for picking up an image in a direction of visibility measurement and outputting a video signal, converting a video signal from the image pickup means into two-dimensional image data, and setting in advance from luminance information in the image data. Visibility calculating means for calculating a visibility using the converted function, wherein the luminance information is difference luminance information between high luminance information and low luminance information in predetermined high luminance parts and low luminance parts in the image data. A visibility measuring device, characterized in that: 2. A method of associating difference luminance information in image data obtained by converting a video signal from an imaging unit with visibility data measured by a VI measuring instrument by separately installed light attenuation detection. 2. The visibility measuring device according to claim 1, wherein a regression curve of the difference luminance information calculated by statistically processing the collected data and the visibility data is set as a conversion function. 3. A system comprising: means for inputting visibility data from a VI measuring instrument; and means for collecting the visibility data and difference luminance information and determining a conversion function by calculating a regression curve. The visibility measuring device according to claim 2. 4. The method according to claim 1, wherein the visibility measurement measures the visibility in the tunnel, and the monitoring means installed at a predetermined position in the tunnel constitutes an imaging unit. A visibility measuring device according to any of the claims. 5. A lighting device is provided at a predetermined position in a tunnel, and the high-brightness information and the low-brightness information are combined with high-brightness information near the center of the lighting and low-brightness information around the lighting in the image data. The visibility measuring device according to claim 4, wherein: 6. A plurality of lights installed in a tunnel.
A conversion function is set for each illumination mode based on a combination pattern of N / OFF, and an illumination mode for judging the illumination mode by observing luminance information of the illumination from image data obtained by converting a video signal from an imaging unit. The visibility measuring device according to claim 4, further comprising a determination unit, wherein the visibility calculation unit calculates the visibility using a conversion function corresponding to the determined illumination mode. 7. An image pickup means for picking up an image in the direction of visibility measurement and outputting a video signal, and converting a video signal from the image pickup means into two-dimensional image data, and two points having different distances from the image pickup means. A visibility measuring device, comprising: a constant light source; and a visibility calculating unit configured to calculate a transmittance as a visibility from luminance information of the two constant light sources in the image data. 8. The transmissivity calculated by the visibility calculating means is such that two constant light sources (distances L ₁ , L
_{2 where} (L ₂ > L ₁ )) is E ₁ and E ₂ , where (E ₂ / E ₁ ) ^K where K = 100 / (L ₂ −L ₁ ). Item 7. The visibility measuring device according to Item 7. 9. The visibility measurement measures the visibility in a tunnel, and a monitoring camera installed at a predetermined position in the tunnel constitutes an imaging means, and a constant light source is separated into the tunnel to a predetermined position. The visibility measuring device according to claim 7 or 8, wherein the illuminations are provided at two places installed in the apparatus. 10. A surveillance camera for controlling an output signal level according to light irradiation or the like, comprising: a camera status detection means for detecting a control operation for suppressing the output signal level of the surveillance camera; The visibility measurement device according to any one of claims 4 to 6, wherein the visibility calculation by the visibility calculation means is canceled on the basis of: