Calculation of LED street light illuminance

Key words: Calculation of utilization coefficient of LED street light illuminance The average road surface illuminance is one of the evaluation indicators for motor vehicle traffic roads and pedestrian roads, and it is also an important reference indicator for street light configuration. To configure or replace an urban road with LED street lights the relevant indicators of lighting need to be calculated first. The "utilization coefficient" method is one of the simpler and more effective methods, and the formula for the "utilization coefficient" is directly derived from the conclusions of traditional lighting theory research. What we need to do is just to input some experimental or theoretical data LED street lights into the formula, compare the results with the requirements of road lighting design standards. It goes without saying that these calculations are also essential for the, development, improvement, enhancement, and marketing of LBD street lights.

     The utilization factor (U) is the ratio of the luminous flux directly illuminating the road surface to the total luminous flux emitted all light sources, which is related to the height of the street light fixtures, their elevation angles, arrangement methods, and the width of the road surface. Lous flux (in Chinese) is the total energy emitted by a light source and received by the human eye. It represents the amount of radiant light energy per unit, with units of lumens (lm). Other representation methods: cd.sr (cd is the unit of luminous intensity: candela. Sr is the of solid angle: steradian). Illuminance (B) is the proportionality coefficient between luminous flux and the illuminated area, with units of lux (). 1 lx refers to the energy of 1 lumen (lm) of luminous flux evenly distributed over an area of 1 square meter, i.e 1 m/m². Average road surface illuminance (Bav) is the average value of the illuminance at various points measured or calculated at predetermined points the road surface in accordance with relevant regulations.

The maintenance factor (k) is the ratio of the average illuminance or average luminance on a specified surface after a certain period of of the lighting fixture to the average illuminance or average luminance on the same surface when the fixture is newly installed under the same conditions. The mounting height () of the luminaire is the vertical distance from the light center of the luminaire to the road surface. The spacing (8) of the luminaires is distance measured along the centerline of the road between two adjacent luminaires. The overhang length (XL) is the horizontal distance from the light center of theinaire to the nearest curb, i.e., the horizontal distance that the luminaire extends or retracts from the curb. The effective road width (Weff) the theoretical road width used in road lighting design, which is related to the actual width of the road, the overhang length of the luminaires, and the of the luminaires, etc. The arrangement factor (N) is a value related to the arrangement pattern, for single-sided arrangement and staggered arrangement, N1, for opposite arrangement, N=2.

Some related concepts are shown in the following figure: II. Calculation formula According to the definition of illuminance, E = F/A1) Where, A is the illuminated area, m². The average road surface illuminance, Eav = F/(W*S) (2) Where F is the rated luminous flux of the street light source, lm; W is the road width, m; S is the installation spacing, m. Because height, elevation angle, arrangement method, and road width of street lights are different, the utilization degree of the rated luminous flux of the light source is also, so the rated luminous flux of the street light source must consider the "utilization factor" (U): This is the first point; Secondly, order to maintain the average illuminance on the road surface, the light decay of the luminaire itself, the protection level of the luminaire, and the cleaning must be considered, so the rated luminous flux of the street light source must consider the maintenance factor (k): Thirdly, because the arrangement methods of street, such as single row, double row symmetric, and double row staggered, also affect the average illuminance, the arrangement factor (N) must also be.

Therefore, formula (2) becomes: Hav=F*u*k*N/(W*S) (3) This is the formula for calculating the illuminance using the "utilization factor" method. In practical engineering calculations, the average illuminance standard for general road surfaces is usually specified, meaning Eav known. More often than not, it is necessary to calculate the rated luminous flux F of the streetlights to meet the requirements. By transforming formula (3 we get: F=Eav*W*S / (U*k*N) (4) Note that W in the formula should be the effective width of the road surface