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Factors affecting the quality of road lighting
The main function of road lighting is to ensure night traffic safety and provide a comfortable and reliable visual environment for road users. In nighttime road users, since the speed of the motor vehicle is fast and the consequences of the accident are serious, it is necessary to first ensure the visual work demand of the motor vehicle driver. There is a close relationship between the quality of road lighting and the driver's visual reliability. If visual reliability is improved, accidents will be reduced. Therefore, to evaluate the quality of road lighting quality, we must first proceed from the basic factors and requirements that affect the driver's visual reliability.
Visual reliability consists of two aspects: visual function and visual comfort.
The visual function refers to the function of the driver to see the object and its changes in his field of vision, that is, whether the physiological function of the vision is in a normal state. Visual comfort refers to whether the driver feels comfortable in the visual environment generated by road lighting.
The basic factors that directly affect the driver's visual reliability are as follows:
The brightness of objects near the road or road;
General brightness of the road background;
Identify the size of objects and details;
Brightness contrast between object and background;
a contrast between the road surface and the observed environment;
Observe the effective time of the object;
The degree of glare, etc.
Based on the above factors, various relationships between the driver's visual reliability and road lighting parameters are discussed in several aspects to determine the factors and requirements that affect road lighting quality.
First, the basis for establishing lighting parameters - brightness
An object can be seen, causing a visual response. It is because the light emitted from the surface enters the observer's eyes, and the more light that enters, the stronger the visual perception. The illuminance on the surface of the road is the amount of light received per unit area of ​​the road surface as the object of observation, and it cannot directly give the observer the visual perception.
Only the brightness of the road surface can be directly proportional to the strength of the observer's visual perception. The actual road surface is a surface between diffuse and directional reflections. The brightness of a point on such a surface is a function of the illuminance and angle of incidence of the ray, the surface reflection characteristics, and the observed azimuth. Therefore, even if the illuminance is different at the same point on the road surface, the brightness differs depending on the position of the observer.
Therefore, for the user, the brightness of the road surface is not directly determined by the illuminance, but is determined by the brightness. The illuminance of the road surface only reflects the road surface receiving the luminous flux, and does not reflect the reflection of the light on the road surface, so it is not a direct reflection of the quality of road lighting. What directly affects the driver's visual function is the various brightness patterns that exist within his field of view.
It can be seen that only the various lighting parameters established based on the concept of brightness can satisfy the vision well.
Internationally, since 1940, the road lighting system has been evaluated with brightness. In the 1970s, visual indicators such as visibility and recognition probability have been added to comprehensively evaluate, in order to reflect the quality of road lighting more objectively, more completely and accurately.
Second, the visual function
1, lighting level:
To be able to see and distinguish an object, you need a certain brightness difference between the object and the background. On a general road, the level of illumination is low, so the color is less visually pleasing. For the driver, the visual information that must be obtained is that the obstacle can be found on the background (road surface). At the same time, the difference in brightness between the object and the background by the human eye is easier to distinguish than the color difference. So the concept of brightness contrast is introduced, which is defined as:
In the formula:
C—brightness contrast
Lb—background brightness
Lo - object brightness
If the object is darker than the background, this contrast can be called negative contrast, otherwise it is called positive contrast, and road illumination generally produces negative contrast. Because a black shadow on a bright background is easier to find than a bright object on a dark background, the driver only needs to see the outline of the obstacle and determine its direction, and does not need to know the details.
The brightness Lo of an object on the road surface is proportional to the product of its surface diffuse reflectance RO and its illuminance Ev on the vertical plane.
On a road with a brightness of Lb, an object that is Lo may visually appear in the following three situations:
A, Lo
B, Lo = Lb, the two merge, can not be distinguished;
C, Lo>Lb, you can see the bright shadow.
Obviously, the second situation should be avoided. Road lighting usually meets Lo Lb is necessarily high in Ev, and high Ev is easy to cause glare.
It can be seen that increasing the background brightness or reducing the vertical surface illumination can improve the brightness contrast. Thereby affecting the visual function. Under the condition of controlling vertical illumination, the average brightness Lav of the road surface can be used as an important parameter to evaluate the quality of road lighting.
2, uniformity:
In order to meet the driver's visual function requirements, not only must there be a certain average brightness of the road surface, but also a certain uniformity of the road surface brightness distribution. This is the second parameter that affects the quality of the illumination - the total brightness uniformity Uo, which is defined as:
In the formula:
Uo - total brightness uniformity
Lmin - minimum brightness of the road
Lav - average brightness of the road
In general, the worst contrast tends to occur in areas with low road surface brightness, making it difficult to identify obstacles in these areas. At the same time, when the driver observes the darker part of the road, the brighter part will also affect the visual function as a glare source. Therefore, it is necessary to determine the ratio between the minimum brightness and the average brightness of the road surface, that is, the total brightness uniformity Uo, which is also a very important parameter for visual reliability.
3. Glare:
Glare can cause a decrease in recognition ability due to scattering of light in the human eye. In the absence of glare, the image of the scene in the positive direction of the driver's field of vision is clearly focused on the retina of the eye, where the visual sensitivity is proportional to the brightness of the scene.
If there is a glare source at a position deviating from the positive direction of the field of view, the light from the glare source will also enter the eye, but this part of the light will not be focused and will scatter in the part of the eye. This part of the scattered light falls on the retina as if a layer of bright curtain is superimposed on the original clear image.
The brightness of this screen is called the equivalent light curtain brightness and can be obtained by the following formula:
In the formula:
Lv - equivalent light curtain brightness
K-age factor (average = 10)
Ei—the illumination produced by the ith glare source on the eye (the plane perpendicular to the line of sight)
Θi—the angle between the direction of the light from the ith glare source and the direction of the line of sight
Due to the presence of this layer of light curtain, the recognition ability is reduced. This is because the equivalent brightness of the light curtain Lv is added to each of the original two brightnesses (Lo and Lb), resulting in a new contrast.
The following two modes can be used:
A. Since the effective background brightness is increased, the brightness contrast of the object, that is, the threshold contrast Cth, is reduced, and the recognition ability is improved.
Where: Lbeff - effective background brightness
B, effective contrast reduction
Where: Ceff - effective brightness contrast
On the one hand, the threshold contrast is reduced due to the increase in background brightness, and on the other hand the contrast is reduced, but the positive effect of the threshold contrast reduction is not sufficient to compensate for the loss of effective contrast reduction. That is to say, in the absence of glare, an object is just visible (threshold contrast).
Under the effect of glare, it is not clear, and the contrast must be increased to see it. This increased contrast is related to the original contrast, which is a measure of the reduction in visual function due to disability glare, called threshold contrast delta, referred to as threshold increment.
When the road surface brightness range is 0.05 cd/m2 < Lav < 5 cd/m 2 , the threshold increment TI is calculated as follows:
In the middle
Lav—observer's adaptive brightness (cd/m2)
Eeye—The illuminance (lx) produced by a luminaire in the observer's field of view at the surface of the eye that is perpendicular to the line of sight.
∑—The contribution of all luminaires in the observer's field of vision to TI
Θ—the angle between the line of sight and the line connecting the human eye to the light center of the luminaire (the line of sight is 1o below the horizontal line and passes through the longitudinal vertical plane of the observer's eye along the road axis).
This TI is the third parameter to evaluate the quality of lighting.
Third, visual comfort
The requirements for visual comfort are the same as those for visual functions. The purpose is to ensure the driver's visual reliability and thus ensure driving safety. Visual comfort is a psychological effect, which is mainly determined by subjective evaluation, but certain lighting quality can be guaranteed by limiting some parameters.
As with the visual function, visual comfort is also affected by the three lighting parameters of illumination level, uniformity and glare, which are described as follows:
1, lighting level:
Under the condition of keeping the glare level low, the high average brightness of the road surface will make the driver feel more comfortable and can delay the generation of mental fatigue to a certain extent. Therefore, the average brightness of the road surface is not only an important parameter for the visual function, but also an influential parameter for visual comfort.
2, uniformity:
When the vehicle is driving on the road, the bright and dark areas appearing on the road ahead will form a rhythmic flash. It is also called the phenomenon of "streak effect". This phenomenon can cause interference to the driver, causing visual and mental fatigue. This is also a cause of accidents that affect visual reliability.
Therefore, the fourth illumination parameter that affects visual reliability, longitudinal uniformity (UL), is defined, which is defined as the ratio of the minimum brightness to the maximum brightness on the centerline of the lane, namely:
In the formula:
UL - longitudinal uniformity
L, min—the minimum brightness on the centerline of the lane
Lmax—maximum brightness on the lane centerline
3. Glare:
Glare also has a great impact on the driver's visual comfort. According to experimental studies, the degree of discomfort glare can be measured by the glare control level (G). This is the fifth parameter affecting visual reliability, which is mainly influenced by both luminaire characteristics and device characteristics.
Its expression is as follows:
In the formula:
G-glare control level
I80, I88—the absolute intensity of the illuminator in the direction perpendicular to the vertical axis of the road axis and the vertical axis of the lamp at 80 degrees and 88 degrees;
F—the illuminating area of ​​the illuminator in a direction of 76 degrees from the vertical plane parallel to the road axis and the vertical axis of the luminaire;
C—light source color correction factor; (low pressure sodium lamp = +0.4; high pressure sodium lamp = +0.1; high pressure mercury lamp = -0.1; other bulb = 0)
Lav—average brightness of the road surface;
H—the vertical distance between the horizontal line of sight height and the illuminator height (lamp height – 1.5 m);
P—The number of luminaires per kilometer.
The above formula is an empirical formula, in which the part in square brackets represents the characteristics of the luminaire, called the luminaire index (SLI).
4. Inducibility :
The illuminator is placed along the road to an appropriate configuration, so that the vehicle and the pedestrian have a clear visual condition for the direction, route and related configuration of the road ahead. This function is called the inducibility of road lighting. Although it is not possible to give a quantitative parameter for this aspect, it is also an important factor that cannot be ignored for visual reliability.
In road lighting, in order to enhance optical inductivity, the following methods are generally used:
A. The configuration of the road illuminator in the straight section should be parallel to the road and arranged neatly;
B. The illuminator at the corner of the road should be placed at the outer edge of the curve;
C. The intersection is illuminated by a light source different from the color of the road;
D. Different streets use different types of light sources;
In addition to the above-mentioned elements, different light sources have certain influence on visual reliability due to their different emission spectra, but previous studies have been carried out based on traditional light sources.
In recent years, with the development of LED light source, its application in the field of road lighting has become more and more popular. However, the research on the influence of LED light source on the visual reliability of motor vehicle drivers in the road lighting environment is not comprehensive enough and deep, and it still needs to be on the road. Lighting workers further expand to promote the development of green light by scientifically promoting LED light source applications