1 Introduction
In foggy weather, visible light propagates in the fog, and its visibility is reduced due to scattering and absorption, which is not conducive to road traffic safety. With the development of science and technology, there are more and more types of road illumination sources. Due to the different illumination principles, the spectral differences of various light sources are very different, and the absorption of light by different wavelengths is different. Therefore, it is necessary to study different light sources. Translucent.
In 2008, Lu Zheng et al. of the Institute of Metrology of China studied the optimal wavelength of fog light penetration ability, and compared the propagation of light signals in mist, medium fog and dense fog at the same propagation distance. The optimum wavelength for use as a fog lamp is 578 nm. Since most of the light sources are complex color light in actual road lighting, it is more practical to study the fogging property of the complex color light. In 2009, ZAINI and MFb used subjective evaluation methods to observe and score different monochromatic light sources under different fog concentrations, and the best yellow-yellow light transmission was obtained. Since the direct observation by the human eye is greatly influenced by subjective factors, the experimental results are not objective. In 2011, Guan Xuefeng selected four monochromatic LEDs of red, yellow, green and blue as test light sources to measure the illuminance values ​​under different fog concentrations, and obtained four kinds of monochromatic light. At any concentration, the yellow light has the best fogging performance. Second, red light, the worst fogging is green and blue, but the difference between them is not big. Since these results are based on clear vision, no intermediate vision is considered, and foggy weather often occurs in the morning and evening. At this time, most of the road lighting belongs to the intermediate vision category. If the function is still calculated by the visual visual function, there may be some Deviation. Therefore, this experiment introduces intermediate vision to study the fog permeability of visible light, which more accurately reflects the actual situation of the human eye and provides guarantee for road traffic safety. In road illumination, since the amount of information carried by the scattered light relative to the direct light is small, although the scattered light can be received by the human eye, the human eye cannot judge the position of the object based on the scattered light, so this experiment studies the direct part of the light source. Translucent.
This experiment studied the fog permeability of tungsten filament lamps, LEDs, metal halide lamps and high pressure sodium lamps under bright vision and intermediate vision. The spectra of four light sources with different fog concentrations were measured in a black box, and the four kinds of light sources were compared to see the fog. The latest recommended model MES2 of CIE was used to study the relationship between the fogging of four light sources and the brightness. The fog is compared.
2 experiment
2. 1 Experimental purpose
Tungsten lamps, LEDs, metal halide lamps and high pressure sodium lamps were used as experimental objects to compare the transmittance of four light sources under bright vision and to study the relationship between transmittance and brightness.
2. 2 experimental objects
Spectra of tungsten filament lamps, LEDs, metal halide lamps and high pressure sodium lamps at different fog concentrations.
2. 3 Experimental conditions
This experiment requires the measurement of the spectra of four sources at different fog concentrations. The requirements for fog are: uniformity, stability, and a large concentration range. In this experiment, the artificial fogging method was adopted. The whole experiment was carried out in a self-made dark box to avoid the influence of the external light source on the experiment. After the black box was closed, the internal illumination was below 10 - 3 lx. Keep the room temperature at 20 ° C and close the doors and windows to prevent external airflow from affecting the experiment. In this experiment, the direct fogging of the light source is studied. Therefore, three black baffles are placed in the dark box and black cloth is applied around the dark box to absorb stray light as much as possible; a small dark wall and baffle on the same horizontal line The hole allows the light with only the direct aperture to be received by the optical head. The entire experimental setup is shown in Figure 1.
2. 4 Experimental steps
(1) Light up the LED and adjust the LED position so that the photometric head is in the center of the spot, record the position of the LED; illuminate the tungsten lamp and adjust the position of the tungsten lamp, also make the photometric head in the center of the spot, fix the tungsten lamp position.
(2) After waiting for 30 minutes for the light source to stabilize, measure the initial spectrum of the LED and the tungsten lamp separately.
(3) Turn on the humidifier for 15 minutes, then turn it off and wait for 5 minutes for the mist to spread evenly and start measuring. The first measurement of the tungsten filament lamp, the second measurement of the LED, the third measurement of the tungsten filament lamp and the like, a total of 120 measurements, the measurement interval of 10s, and the placement of the LED is the same as the initial recorded value.
(4) Open the black box and wipe off the moisture in the black box after the fog has been dispersed.
(5) Repeat steps (1) to (4) 10 times. Then repeat the steps (1) to (5) using a metal halide lamp and a high pressure sodium lamp instead of a tungsten lamp to measure the spectrum at different fog concentrations.
3 Experimental results and analysis
Step (2) The measured spectra of the four light sources are shown in Figure 2. It can be seen that the spectra of the four light sources are quite different, and the blue light of the tungsten filament lamp is less red, and the LED light consists of two peaks of blue and green. There are few red light parts, and the high-pressure sodium lamp has more yellow light, while the peaks of high-pressure sodium lamps and metal halide lamps are more.
The partial luminous flux transmittance η received by the optical head is used as an evaluation index of the fogging performance:
Where K1 is the maximum spectral luminous efficiency when fogging, K2 is the maximum spectral luminous efficiency when no fogging, S1(λ) is the absolute spectral power distribution of the light source when fogging, and S2(λ) is the absolute spectrum of the light source when not fogging The power distribution, V1(λ) is a function of spectral light efficiency at fogging, and V2(λ) is a function of spectral light efficiency at unfogging.
According to the measured spectrum, the transmittance of the LED and the tungsten lamp at different fog concentrations is calculated using the formula (1), and the data points with the transmittance in the range of 10% to 70% are taken, and the matlab is used for fitting. Use a third-order fit:
The transmittances of LEDs and tungsten lamps were fitted as a function of time, and data points larger than 2 standard deviations were excluded during the fitting process. The study shows that the determination coefficient R2 is greater than 0.99. Then, the transmittance of the LED was 10%, 5%, 20%, 25%, 70%, and the transmittance of the tungsten lamp was determined. After all the transmittances of the 10 experimental tungsten lamps were obtained, the maximum and minimum values ​​were excluded, and the average of the remaining 8 times was taken as the transmittance of the tungsten lamp. The calculation method of the transmittance of the metal halide lamp and the high pressure sodium lamp is the same as that of the tungsten lamp. After all calculations are completed, as shown in Figure 3.
