1 Introduction
The driver of the motor vehicle mainly pays attention to the road condition in the front 60-160 meters. When driving at night, the purpose of the road lighting is to create a bright and uniform road visual environment for the driver, thereby reducing the night accident rate. Since the publication of CIENO.12.2 in 1977, the brightness standard of road lighting has been adopted and widely promoted in Europe and the United States, and as the main standard of road design, China has also introduced the brightness standard in CJJ45-1991 standard, and in CJJ45- The brightness uniformity requirement is enhanced in the 2006 standard. However, as many non-lighting professionals have turned into the LED road lighting industry, many practitioners have insufficient understanding of the differences in road surface brightness and illumination, which has led to the gradual weakening of brightness standards in China and the tendency to be replaced by illuminance standards. To promote the brightness standard in China, the first task is to study the reflection performance of different pavement materials, to understand the effect of the simplified brightness coefficient table (ie R-Table) on the light distribution of road lighting fixtures, and to analyze the light distribution of road lamps based on illumination standards. The problem that comes with it.
2 opaque material reflection performance characterization method
The reflective properties of the material are related to the angle of incidence of the light, the direction of observation, the spectral distribution of the incident light, and the location of the incident point. Nicodemus et al. proposed a bidirectional reflectance distribution function (BRDF) to describe the reflection performance of a material, which is represented by the formula (1). Figure 1 shows the meaning of each geometric quantity in the formula. The BRDF data can generally be obtained from experimental data, or it can be approximated from many mathematical models such as Phong and Blinn-Phong. For the uniform diffuse reflection model, since the reflection of the material is independent of the incident angle and the reflection angle, and the brightness in all directions is equal, the mathematical model is very simple, and the brightness can be calculated according to the formula L=ÏE/Ï€ (ie, f= Ï/Ï€). For example, if the reflectivity Ï of the cement is about 30%, it can be calculated that the illuminance of the cement road surface is about 10 times of the brightness; and the reflectance Ï of the asphalt is about 20%, the illuminance of the asphalt road surface can be calculated to be about 15 times of the brightness. However, in fact, only a few materials have a reflective property consistent with uniform diffuse reflection, such as a paint used to paint a wall surface, the wall surface is flat, and the size of the gypsum powder particles in the paint is small and uniform in thickness. For the pavement material, the composition of the material is complicated, the thickness is uneven, and some materials also have reflective material additives. Therefore, the reflection performance is very different from the standard uniform diffuse reflection. As shown schematically in Fig. 2, although the reflectances of the two materials of Fig. 2A and Fig. 2B are the same, the difference in brightness in a particular viewing direction is large.
BRDF data is generally used for rendering rendering of 3D simulation software, or optical simulation software based on radiance calculation. These softwares require relatively accurate spectral distribution function calculations, which require high fidelity, color and precision. However, for road lighting, due to the single color of the road surface, the low overall brightness, and the reflection performance of the road surface are affected by many factors such as climate, wear level, ash thickness, uneven surface settlement, etc., therefore, when calculating the road surface brightness, The accuracy of the chromaticity of the road surface and the brightness of the road surface are not high. Therefore, the method of characterizing the reflection performance of the road lighting road surface is usually q (α, β, γ) stereo, which is much simplified compared with BRDF, as in formula (2). Shown, Figure 3 illustrates the relationship between the various geometric parameters.
Equation (2) ignores the influence of the spectral distribution function, so when testing the reflection performance of the road surface, standard light source or natural light is usually used as the test light source. The β in the formula (2) is the complementary angle of (θi + θr) in the formula (1), and the purpose of the simplification is to ignore the influence of the anisotropy of the material.
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