1 Introduction
Highway tunnels are special structures on highways, and lighting has its own particularity. During the daytime, the brightness outside the tunnel is very high. When the driver enters the dark tunnel from the high-brightness hole, due to the visual adaptation characteristics of the human eye, it takes a certain adaptation time. In the entrance and transition sections of the tunnel, high illumination brightness is required to ensure a smooth transition of visual dark adaptation and ensure safe driving. Under the condition that the traffic volume of the motor vehicle and the design driving speed are certain, the design reference of the tunnel lighting system--the value of the outside brightness of the hole L20(S) will directly affect the brightness value of the inlet section and the transition section, that is, It has determined the energy saving of tunnel lighting, operating electricity charges, engineering investment and driving safety.
In China, when designing the L20 parameters, the tunnel lighting design usually refers to the relevant provisions of the tunnel tunnel external brightness in JTJ 026-1999 "Road Tunnel Lighting Design Specification", and directly checks the brightness L20 value of the tunnel hole with representative characteristics. Or use the environmental diagram method to calculate the brightness value outside the hole, the environment diagram calculation, you need to know the surface brightness of various scenes in the tunnel environment. The surface brightness reference values ​​of various typical scenes are given in "Code for Design of Road Tunnel Lighting". However, in fact, the brightness of the surface of each scene element of the tunnel mouth changes with the local light climate, season, time, tunnel orientation, reflection characteristics of the surface of the scene and reflection coefficient. The reference value of the surface brightness of the tunnel exterior given in the design specification is very rough.
In order to obtain the brightness of the tunnel mouth scene more accurately, it can be measured with a luminance meter. When the brightness meter measures the ambient brightness, the non-contact measurement method is adopted, and the brightness value of a certain point of the target is measured. The method for measuring the single point parameter is relatively accurate. However, the luminance meter measures the measurement error in the variable natural light environment condition caused by the positioning error and the long point-by-point test time, resulting in inaccuracy of the final test value.
The brightness and chromaticity information of the scene to be photographed are input into a digital image processing software such as Photoshop in the form of a digital image by a digital camera for analysis, and finally, the luminance values ​​at the respective measurement points of the road surface to be measured are obtained. This test method greatly compensates for the shortcomings of the conventional method. Its advantages are as follows: 1) High definition, and theoretically can obtain the brightness value on the micro-element area; 2) accurate positioning; 3) short test time, greatly Reduce the risk of testing; 4) Repeatability, will not change after repeated reproduction by computer. Therefore, using a digital camera to quickly measure the brightness of the tunnel opening is a good test method.
2 The principle of colorimetry of digital camera test scene brightness
In the color vision matching test of colorimetry: divide the field of view into two halves, half of which is the light to be measured, and the other half is the light of the three primary colors R, G, B, adjust the proportion of the three primary colors, when the boundary in the field of view When the two fields of view have the same color, the light to be measured and the three colors are matched to each other. The number of three primary colors required is called the three stimulus values ​​X, Y, and Z. In colorimetry we use the value of Y in the tristimulus value to represent the value of the brightness. The relationship between the tristimulus value of light and the gray value of light and the three components of R, G, and B can be expressed by the following formula:
As can be seen from the above equation, as long as we obtain the chromaticity R, G, B values ​​of the illuminating object, it is theoretically possible to calculate the tristimulus value of the illuminating object, and thus its brightness value can be obtained.
The spectral response of a CCD sensor in a digital camera is not the same as the color matching function in the CIE1931 standard chromaticity system, and there are differences between different digital cameras. Therefore, using a digital camera to approximate the brightness distribution of a scene, the key is to determine the relationship between the gray value ( Ir, Ig, Ib) of the digital image captured by the digital camera and the corresponding color stimulus value in the CIE1931 standard chromaticity system. , the so-called digital camera optical characterization. The characterization methods can generally be divided into two types: spectral response based methods and target color based methods. This paper adopts the latter characterization method, namely: using the standard gray board and swatch for photography as the reference target color, and taking a digital camera to take a digital image of a certain target, and using BM-5A chromatic brightness for this target. For measurement, read the X, Y, and Z values. The gray value (Ir, Ig, Ib) of the acquired digital image is read by Photoshop image processing software, and converted into the stimulus values ​​R', G', B' corresponding to the pixel points studied on the image, and then with the color The stimulus values ​​X, Y, and Z measured by the luminance meter were compared and analyzed to optically characterize the digital camera.
3 Experimental study on optical characterization of digital cameras
Experimental equipment: Canon EOD-5D digital camera, gray board and swatch according to national standard "Technical specifications for standard gray board and swatch for photography" (GB3556 - 83), calibrated BM-5 chromatic brightness meter
Experimental method: Place the standard gray board and swatch vertically on the stand. The digital camera is placed 2m in front of the standard gray board and swatch, and the center axis of the digital camera is at the same level as the center of the swatch gray board. The brightness meter is placed next to the digital camera so that it is at the same level as the gray board and swatch.
Test conditions: Unobstructed outdoor, typical summer sunny days.
3. 1 Study on the linear range of brightness reproduction of digital camera exposure
The linear recording brightness ratio range is usually represented by the dynamic range of a digital camera, and the dynamic range of a digital camera is equivalent to the latitude of the film. The dynamic range of the Canon EOD-5D camera is 8. 2 EV. Therefore, the maximum brightness ratio that the digital camera can record can even exceed 28. Since the recorded data is recorded in 8-bit jpg format, the brightness recording range can even reach 256:1. . But unfortunately, its brightness linear recording range is much smaller than the effective value of its dynamic range.
Therefore, firstly, according to the approximate range of the brightness of the general tunnel mouth scene, the appropriate digital camera exposure parameters are selected, and the linear range of brightness reproduction under certain exposure parameters is studied, and the camera exposure parameters studied in this paper are finally determined. According to the actual measurement of the tunnel entrance in Guizhou, the brightness value of various scenes is basically between 500cd / m2 ~ 10000cd / m2. According to the relationship between the photographing exposure amount and the camera aperture to H-L't and the measured range of the tunnel mouth scene brightness value, the appropriate exposure parameters when the photo is taken by the digital camera in the summer sunny state are determined.
Under the same exposure parameters, photograph the standard gray board and swatch under different sky conditions, and test the brightness of the specified point while taking the picture, then extract the gray level of the brightness test point on the picture for linear regression analysis. Observe its laws. Through regression analysis of a large number of data points, it is found that the logarithmic value of the exposure gray value has a good linear correlation with the logarithm of the luminance value (see Table 1).
According to the linear recording brightness range under different exposure parameters and the measured range of the measured value of the tunnel mouth scene brightness in the summer sunny day, in this paper, the F11, 1 /125 exposure parameters are used as the search camera optical characteristics and test verification.