Illumination Calculation Method of New Street Lamp
You can select several representative points on the road surface and calculate their illuminance. The method is a bit more complicated, but the calculation accuracy is high. In practice, usually only the points with the highest and lowest illuminance are calculated. Calculate the illuminance at two points AB as shown in Figure 18-11. The illuminance value of point A is proportional to the light intensity component of the light source in the PA direction, and inversely proportional to the square of the distance PA, that is
E＝I(Q, φ)cosQ/(PA)2
Calculate the illuminance of points A and B
Because PA·cosQ=H, PA=H───cosQ
So PA2＝H2───cosQ(18-2)
That is, E＝I(Q, φ)/H2·cos2Q
In the formula, I(Q, φ)─The luminous intensity of the light source in the PA direction can be obtained according to the light distribution curve;
H - the height of the light source/m;
Q─The angle between the vertical line from the light source to the ground and PA. the
In the above example, if the illuminance of the lamp on the ground point B is calculated, find out ∠BPO in the right triangle BOP,
Then: tg(90°－∠BPO)＝H/B＝7/13＝0.54 can be calculated ∠BPO＝62°, cos∠BPO＝cos62°＝0.47 and then find out the point B according to the light distribution curve of the lamp DDY400 Light intensity I (or check out from the iso-intensity curve diagram). Find out from the graph that the light intensity when ∠BPO=62° is 66cd. Because this light distribution curve is based on the luminous flux of the light source as 1000lm, and the luminous flux of the light source GGY400 we choose is 21500lm, so 66cd has to be multiplied by 21.5 times. Therefore, the illuminance at point B is
E＝I(Q, φ)/H2·cos3Q
＝I/H2·∠BPO
=66×21.5/72×0.473
=6.40(lx)
