The average illuminance of the road surface is one of the evaluation indicators of urban motor vehicle traffic roads and pedestrian road lighting, and it is also an important reference indicator for street lamp configuration. A city road should be configured or replaced with an LED street light . Technically, it must first involve the calculation of relevant indicators of lighting. Using the "utilization coefficient" method is one of the simpler and more effective methods.
The formula calculated by the "utilization coefficient" method definitely affirms the conclusions of traditional lighting theory research. All we have to do is to enter some experimental or theoretical data of the LED street light into the formula, and get the result and compare it with the requirements of the road lighting design standard. It goes without saying that these calculations are also necessary for the design, development, improvement, improvement and marketing of LED street lamps.
1 concept
The utilization factor (U) is the ratio of the luminous flux directly on the road surface to the luminous flux emitted by all the light sources, and is related to the height, elevation angle, arrangement mode, and road width of the street lamp.
The luminous flux (Φ) is the sum of the energy emitted by the light source and received by the human eye. Indicates the amount of radiant energy per unit time, expressed in lumens lm. Other representations: cd.sr (cd is the unit of luminous intensity: Candela. Sr is the unit of solid angle sphericity.
Illuminance (E) is the proportional coefficient between the luminous flux and the area to be illuminated, in units of lux. 1lx means that the luminous flux of 1lm is evenly distributed over an area of 1 square meter of energy, ie lm/m2.
The average illuminance (Eav) of the road surface is the average value of the illuminance of each point measured or calculated at a predetermined point on the road surface according to the relevant regulations.
The maintenance factor (k) is the ratio of the average illuminance or average brightness on the specified surface after the illuminating device has been used for a certain period of time, and the average illuminance or average brightness obtained on the same surface when the device is newly installed under the same conditions.
The installation height (H) of the luminaire is the vertical distance from the light center of the luminaire to the road surface.
The installation spacing (S) of the luminaire is the distance between two adjacent luminaires measured along the centerline of the road.
The cantilever length (XL) is the horizontal distance from the light center of the luminaire to the adjacent side rim, ie the horizontal distance at which the luminaire extends or retracts the rim.
The effective width of the pavement (Weff) is the theoretical width of the pavement used for road lighting design. It is related to the actual width of the road, the cantilever length of the luminaire and the arrangement of the luminaire.
2 calculation formula
According to the definition of illuminance, E=Φ/A
(1) where A is the area to be illuminated, m2
Average illuminance of the road surface, Eav=F/(W*S)
(2) where the nominal luminous flux of the F-street light source, lm;
W—road width, m.
S—installation spacing, m;
Because the height, elevation angle, layout, and width of the street lamp are different, the degree of utilization of the rated luminous flux of the light source is also different. Therefore, the rated luminous flux of the street light source must consider the "utilization coefficient" (U); this is one;
Second, because the average illuminance of the road surface is to be maintained, the light fading of the luminaire itself, the degree of protection of the luminaire and the wiping period should be considered. Therefore, the rated luminous flux of the street light source must take into account the maintenance factor (k);
Third, because the street lights are arranged in a single row. Double row symmetry, double row staggering, etc., also have an effect on the average illuminance, and the alignment coefficient (N) must also be considered.
Therefore, after considering the above factors, formula (2) is:
Eav=F*U*k*N/(W*S)
(3) This is the formula for calculating the average illuminance by the "utilization coefficient" method.
In actual engineering calculations, the average illuminance standard for general road surfaces is required, that is, Eav is known. More is to calculate the rated luminous flux F that satisfies the need for street lamps. After the formula (3) is deformed, it is:
F=Eav*W*S/(U*k*N)
(4) Note that W should be the effective width of the road surface in the formula. The calculation of the effective width of the road surface is shown in Table 3.
3 related data
1, "utilization coefficient" U
The "utilization coefficient" of LED street lamps is shown in Table 1 below:
Note 1: W-road width, H-light fixture installation height, called lateral distance ratio.
Note 2: When there is a cantilever length, the total “utilization factor” is “roadside utilization factor” plus “houseside utilization factor”. (See Journal of Lighting Engineering, September 1994, Vol. 5, No. 3, p. 51).
Note 3: Data source: Xu Lianchengwen “Thinking of LED Street Light Design and Marketing Strategy Based on Market Demand”.
2, maintenance coefficient k
The maintenance factor k specified by CJJ45 is shown in Table 2:
Note 1: Maintenance factor = light attenuation coefficient of light source * light decay coefficient of lamp pollution.
Note 2: According to the quality and environmental conditions of road lighting sources and lamps in China, the data is premised on the lamp once a year.
3, street light array coefficient
The street lamp arrangement generally has four types of arrangement: one side, two sides, two sides, and the center of the road. N=2 in symmetric arrangement and N=1 in other arrangements.
4, the width of the road and the effective width
The calculation of the effective width of the road surface specified by CJJ45 is shown in Table 3:
Note 1: Ws—the actual width of the road surface, m; XL—cantilever length, m.
5, street light spacing
The relationship between the street lamp spacing S, the installation height H and the effective width W of the road defined by CJJ45 is shown in Table 4:
Note: Weff - effective width, m; curve section: radius below 1000m, spacing S by 50% -70% of the line.
6, the average illumination requirements of the road
The standard value (maintenance value) of motor vehicle traffic road lighting specified in CJJ45 is shown in Table 5:
Note: “/” is the low value on the left side and the high value on the right side. See CJJ45 for details.
Other types such as the standard value of the intersection area lighting and the standard value of the pedestrian road lighting are shown in CJJ45.
4 calculation examples
It is known that the LED street light is arranged in the main road, the low value Eav=20lx, the actual width Ws of the road is 15m, the LED street lamp protection level is IP54, the street lamp spacing S is 30m, the street lamp installation height is about 12m, the cantilever length is 1.5m, the lamp The elevation angle is 15°, and the two sides are symmetrically arranged.
Determined by Table 2, k = 0.65;
Calculate Weeff=Ws-2*XL=15-2*1.5=12m from Table 3;
It is checked from Table 4 that H=12m≥0.5Weff=6m; S=30m≤3H=36m is in compliance with the regulations;
It is known from the foregoing that when the road lamps are symmetrically arranged on both sides, N=2;
Looking up Table 1, the total "utilization coefficient" of 15° elevation angle corresponding to W/H=12/12=1 is 0.72;
Bring into formula (4)
F=Eav*Weff*S/(U*k*N)=(20*12*30)/(0.72*0.65*2)=7692(lm)
Choose an LED street light product with a lumen of 7902 or more to meet the illumination requirements.
Combine these data and data in the office software EXCIE, and automatically calculate the result by selecting and inputting the coefficients according to the actual situation.
Obviously, the lumen number of an LED street lamp is known, and the average illuminance can be calculated according to the conditions or setting conditions, and then compared with the requirements of CJJ45-2006 "Urban Road Lighting Design Standard", so as to carry out targeted configuration selection.