introduction
LED light efficiency, long life, low operating temperature, non-toxic, no electromagnetic pollution, is recognized as the fourth generation of lighting sources. With the decline in the overall cost of LED lamps throughout their life cycle, LED lamps are gradually replacing traditional light sources as mainstream lighting sources. The high-power LED has a small light-emitting area, a large luminous flux, and a relatively concentrated light energy, but is directly used for indoor lighting glare. Although low-power LEDs can better solve the glare problem, in order to meet the higher illumination requirements, a large number of LEDs are often used, thereby sacrificing the light effect, and due to heat dissipation and the like, the shortcoming of light decay is widespread. At present, the highest high-power LED light effect in China is 170lm/W released by Epistar, and the commonly used high-power LED light efficiency is 120~150lm/W. For high-power LEDs, the use of reasonable optical design and luminaire surface diffusion materials can effectively reduce glare and improve illuminance uniformity while maintaining light extraction efficiency.
To achieve the light-emitting efficiency corresponding to the direct-type panel light, the edge-lit panel light needs to increase the number of LED beads to increase the cost. In this paper, the direct-type LED panel light is used. Although it needs a thicker thickness than the side light type to solve the uniformity problem, it is obviously superior to the edge-light panel light in the light-emitting efficiency. In this paper, through the optimal arrangement and arrangement spacing of LED arrays, combined with the design requirements of subway lighting fixtures, an LED panel light that can be used for indoor lighting in subways is designed. Through the simulation analysis, the optimal structural design and LED array design were obtained, and the illumination uniformity reached 99.8%, which was finally applied to subway lighting. Compared with the grille lamp for general lighting of the subway, it can theoretically save more than 40% of the electricity of the Guangzhou Metro Guanzhou Station.
1 High-power LED lighting optical design principle
Lighting uniformity requirements depend on the application environment. The illuminance uniformity is determined by the distance between the lamp bead and the target surface, the illumination angle of the lamp bead, the brightness of the background light source, the reflectivity of the target surface, and the color of the target surface. Applied to the panel light, this paper only considers the light intensity distribution of the LED and the distance between the target surface and the LED light source array.
It is generally believed that a single LED source is a Lambertian source and its illumination is subject to a cosine distribution. This approximation is very effective for bare and encapsulated lenses that are hemispherical. When the distance between the target surface and the light source is greater than 
When ALED is the LED illumination area, the LED can be regarded as a point source. The high-power LED light-emitting area is less than 4mm2, and the distance between the LED and the direct-light fixture target surface is much larger than 2cm, so it can be approximated as a point source, and the change of illumination is inversely proportional to the square of the distance. In model building, it is assumed that each LED in the array has the same wavelength of light, luminous flux, and intensity distribution.
For a triangular array of M rows and N columns (N is the number of LEDs in the longest 1 row), the total number of LEDs is [4]
The maximum flat condition d (D = d / z) is obtained from the equation (3), and d is a function with respect to m, N, M. When both N and M are even, the value of D = d/z is obtained from the zero crossing of the function f. When both N and M are odd, the maximum flat condition is given by the minimum value of the function f. For other cases, the maximum flat condition is given by the zero crossing of the function f or its minimum [4].
If the above-mentioned maximum flat condition can be satisfied in the arrangement of the lamp bead, it is theoretically possible to form a spot having no glare and uniform illumination distribution on the target surface.
2 Design model and its simulation
The scheme adopts a high-power 1W LED light source direct-type luminaire structure, and achieves the uniform brightness of the surface of the panel lamp through the cross-distribution of the LED light source. The diffused plate of the imported material is used, and the total light transmittance is above 90%, and the inner side of the luminaire is plated. Silver reflective film for improved light efficiency.
2.1 Lighting structure design
The size of the lamp panel is 1 000 mm × 300 mm × 80 mm. The LED lamp bead is arranged on 5 aluminum strips. The lamp bead is spaced 100mm apart, and each LED strip is equipped with 9 LED lamp beads. The strips are spaced 60mm apart. A thermal silica gel is attached to the bottom of the light bar to assist in cooling the LED lamp beads. The silver and high reflection coefficient reflective film on the side and bottom of the lamp has a reflectivity of 98%, which improves the utilization of light energy.
2.2 Light distribution design and simulation of the lamp panel
We built the lamp housing structure in the structural design software, imported it into the professional optical simulation software, and built the LED dot matrix model using the structure shown in Figure 1. Assuming that the luminous efficiency of a single LED is 120 lm/W, in order to achieve a uniform light mixing effect, a triangular lattice arrangement is used, when z=80 mm, M=5, N=9, and the maximum flatness is satisfied by the formula (3). The condition can be obtained as d0=50 mm, so d=100 mm cannot satisfy the maximum flat condition. This is a problem often encountered in actual design, and the actual model often fails to meet the ideal conditions. However, the arrangement method of the triangular lattice has a good light-mixing ability, and the effect of uniform light mixing can be achieved by attaching a reflective film to the inner wall and a diffusing plate to the light-emitting surface.
Figure 1 Schematic diagram of aluminum strip structure