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General principles for road lighting installation design

Whether the design is based on performance requirements described by road reflectivity data or on a table of regulations developed based on experience is a question related to the selected light source, the light distribution of the lamps, and the installation layout. When choosing highway lighting, it should be based on whether the driver feels that the road surface has a suitable and uniform brightness when passing. Installation is just a question of whether the "light spot" formed by each street lamp can be matched in the most economical way. The range of this "light spot" should be determined by the reflective characteristics of the road surface and the light distribution of the street lamp.

So, let's first discuss the reflection problem of the road surface

Road surfaces, including rough ones, have two forms of reflection:

One is a diffuse reflection surface, whose brightness is directly related to the illumination. The light spot formed by street lamps with symmetrical light intensity distribution on the road surface is circular when viewed from above, but elliptical when viewed along the road in perspective. If the light distribution is concentrated in any direction, the light spot will expand. If the road surface reflects some wavelengths of light better than other wavelengths of light, it can cause changes in the spectral distribution.

Under the illumination of street lamps, the light spot formed by diffuse reflection of light-colored road surface is similar to the directional diffuse reflection of mirror surface. It depends on the incident direction of light and the observation direction. This kind of reflection form has little effect on the formation of light spot. However, from the perspective of the driver, due to the change of viewing angle, a bright band can be generated between the driver and the street lamp. The spectral distribution of this light does not change much. The starting point of the bright band is where the brightness of the light spot is reduced due to diffuse reflection. The length of the bright band is determined by the smoothness and brightness of the road surface, and the luminous intensity between the angles of 75° and 85° between the street lamp and the vertical line. If one of the above two determining factors is insufficient, the length of the bright band will be greatly limited. The rougher the road surface, the wider the bright band and the lower the brightness. The plane width of the bright band is also related to the horizontal width generated by the street lamp beam and the relative direction of the beam and the observer. If it is a bright road surface, or a so-called "flood-light surface" road surface that looks like a wet road surface, the width of the bright band is determined by the width of the lampshade, and its brightness is determined by the maximum brightness of the light source. On this kind of road surface, the best effect is to use horizontally installed fluorescent lamps.

The head of the bright band formed by diffuse reflection and the tail formed by directional diffuse reflection combine to form a "T"-shaped light spot. The tail of the light spot formed by high-angle projection light and bright road surface (or pan-illuminated surface) is long. If the light intensity of the street lamp is limited in the direction of an angle exceeding 70°, or the road surface is rough, the tail of the light spot will be shorter. The head of this "T"-shaped light spot is determined by the illumination and brightness of the road surface. If the road surface is dark and shiny, the head of the "T"-shaped light spot will be very small.

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