Calculation method for illuminance of street lamps
Several representative points can be selected on the road surface to calculate their illuminance, which is a bit complex and has high calculation accuracy. In practice, only the points with the highest and lowest illuminance are usually calculated. Calculate the illuminance at two points AB. The illuminance value of point A is directly proportional to the component of the light intensity of the light source in the PA direction, and inversely proportional to the square of the distance PA, i.e
E=I (Q φ) CosQ/(PA) 2
Calculate the illumination of points A and B
Because PA · cosQ=H, PA=H - medical cosQ
So PA2=H2 case - case cosQ (18-2)
I.e. E=I (Q φ)/ H2 · cos2Q
In the equation, I (Q φ) The luminous intensity of the light source in the PA direction can be obtained by plotting the light distribution curve;
H - height of the light source/m;
Q - The angle between the perpendicular line of the light source to the ground and PA.
In the above example, if we calculate the illuminance of the lamp on point B on the ground (as shown by the dashed line in Figure 18-11) and find ∠ BPO in the right triangle BOP, then:
Tg (90 ° - ∠ BPO)=H/B=7/13=0.54, which can be calculated as ∠ BPO=62 °, cos ∠ BPO=cos62 °=0.47
Then, based on the light distribution curve of the lamp DDY400, find the light intensity I of point B (or from the equal light intensity curve). According to the curve, the light intensity at ∠ BPO=62 ° is 66cd. Because this light distribution curve is based on the light flux of the light source of 1000lm, and the light flux of the light source GGY400 we selected is 21500lm, so 66cd needs to be multiplied by 21.5 times. Therefore, the illumination of point B is
E=I (Q φ)/ H2 · cos3Q
=I/H2 · ∠ BPO
=66 × 21.5/72 × zero point four seven three
=6.40 (lx)
