1 Introduction
The purpose of road lighting is to create a good visual environment for various vehicle drivers and pedestrians, to ensure traffic safety and improve transportation efficiency; to facilitate people's lives, reduce crime rates and beautify the environment. Road lighting design has long been based on the “illumination standard†of the road surface. Along with advances in human physiology research, it was found that the brightness of the road surface and the brightness of the light curtain caused by the stray light of the road lighting fixture at the eye can better evaluate and predict the visibility of objects on both sides of the road and the road. In particular, since the publication of CIE No. 12. 2, "Recommendations for Motor Vehicle Traffic Road Lighting" in 1977, the concept of road surface brightness has been gradually recognized and accepted in road lighting design standards, and gradually became a road clarification in the 1980s. Design criteria. China's "Urban Road Lighting Design Standards" CJJ45-2006 also stipulates the average brightness of road lighting, total uniformity of road surface brightness, longitudinal uniformity, glare limitation, etc. according to road use functions and traffic flow.
2 Traditional road pavement brightness measuring instrument requirements and testing methods
The "Lighting Measurement Method" GB / T5700-2008 has made corresponding provisions on the road surface brightness measurement method and technology.
2. 1 Luminance meter
In road illumination measurement, when it is necessary to measure the total brightness uniformity and the longitudinal uniformity of brightness, a illuminometer with a telescope head should be used, the angle of view in the vertical direction should be less than or equal to 2', and the angle of view in the horizontal direction should be 2 ' ～ 20'.
2. 2 brightness measurement method
The height of the observation point of the luminance meter should be 1. 5 m from the road surface. The longitudinal position of the observation point of the luminance meter: 60m from the first row of measurement points, 100m in the longitudinal direction (Fig. 1), or at least the area between the two poles on the same side.
In the longitudinal direction of the road, when the distance between the two poles on the same side is less than or equal to 50m, usually 10 measuring points should be arranged at equal intervals between the two poles; when the distance between the two poles is greater than 50 m, the spacing between the two measuring points should be less than The principle of equal to 5 m determines the number of points. In the horizontal direction of the road, five measuring points should be arranged in the horizontal direction of each lane. One point should be located on the center line of the lane. The two outermost points on both sides should be located at /10 of the boundary line on each side of each lane. Lane width; when the brightness uniformity is good or the accuracy of the measurement is low, 3 points can be arranged in the horizontal direction of each lane, one point should be located on the center line of each lane, and two points on both sides They should be located at a width of 1 / 6 lane from the boundary line on each side of each lane.
2. 3 Problems
Measuring with a traditional luminance meter requires measuring the brightness values ​​of nearly three hundred measuring points on the road surface, which makes the brightness measurement very complicated and difficult, and takes a lot of time. In particular, there is no need for vehicle interference on the road during the measurement process. With the development of social production, more and more motor vehicles are on the road. This is more difficult to achieve, and there are still major safety hazards.
3 Development and application of image brightness measurement system in road brightness measurement
3. 1 image street brightness meter advantage
CCD cameras are widely used in research fields where it is desirable to simultaneously obtain an array of digital radiation values ​​with good linearity and accuracy. Cameras that use CCDs can quickly capture images and turn them into digital electronic images. Image measurement is especially suitable for targets with intensive points to be measured, and is suitable for targets whose target environment is not stable or even drastically changed. Therefore, compared with the traditional point luminance meter measurement, the image street brightness meter can obtain the brightness value of all the road surface measurement points at one time. Under the premise of meeting the test accuracy requirement, the method can greatly improve the test efficiency. This advantage is even more pronounced in the case of increasing urban traffic.
At the same time, the traditional measurement method is very professional, so it is not conducive to widespread promotion. The image type brightness meter is easy to operate and easy to master, thus providing a safe and comfortable travel environment to provide a strong guarantee.
3. 2 image type street brightness meter focuses on solving problems
The emergence of image brightness testing technology provides a new technical means for light environment evaluation, but the development of evaluation methods is relatively lagging behind, and it is still based on traditional measurement methods. Therefore, the application research of current image brightness measurement technology firstly needs to improve the test efficiency and precision through the image brightness meter according to the requirements of the existing measurement method standards, so as to better guide the lighting practice, including the following three aspects:
(1) Image type street brightness meter brightness correction;
(2) Image type street luminance meter geometric correction;
(3) Image type street brightness meter operation and analysis software compilation.
3. 2. 1 Image type street brightness meter brightness correction
If a reasonable control method is used, by determining the value of any pixel of the camera CCD array, the brightness value of the point in the mapped environment can be determined, and the scene uniformity and contrast are obtained through correlation calculation.
As early as 2009, China Academy of Building Research completed the topic “Research on Image Brightness Measurement Technology Based on Digital Camerasâ€, which is the three major factors affecting the camera's measurement of brightness and chromaticity parameters (optical systems, electronic systems and image processing). Algorithm) has been carefully studied. Through experimental research and computer simulation analysis, the optical system error simulation test analysis, CCD characteristic experiment, the measurement range of the camera brightness measurement is obtained. In particular, through the research on the decoding technology of Canon camera RAW files, the loss of accuracy and data linearity caused by using jpeg compressed files as the measurement result carrier is avoided, and the test accuracy and the brightness of single picture can be further improved. range. These efforts have important guiding significance for the final development of image-based luminance meters with lower prices and good marketing value. Through the research of this technology, the traditional image brightness measurement technology has been greatly improved, as shown in Table 1.
By comparison, the main performance indicators of the research results of this subject are better than the main research results at home and abroad. At the appraisal meeting of the research results, the appraisal experts agreed that the research results have reached the international advanced level. In the project, based on the results of the research, the camera system used in the project will be brightness corrected to ensure that the brightness meter meets the test accuracy requirements.
3. 2. 2 Image type street brightness meter geometric correction
In order to record the brightness distribution in the field of view, the image brightness meter needs to accurately reflect the size and orientation of the light-emitting object. However, the wide-angle lens used in the image brightness measurement, especially the fisheye lens, has obvious distortion, but this aspect is still lacking. The study thus constrained the development of image brightness test devices. In particular, in road lighting, according to the existing measurement standards, to obtain the brightness value of the corresponding measuring point, it is urgent to perform geometric correction on the image street luminance meter.
According to the related theories of photogrammetry and other related theories completed by the China Academy of Building Research in 2011, a geometric correction method for image brightness measurement is proposed for camera lens distortion. Based on the linear mathematical model of camera lens, the eccentric distortion and thin prism distortion combined with radial distortion are used to establish the digital camera geometric mapping model with high precision for the first time, which doubles the geometric angle test accuracy.