Lighting : LIGHTING Oct-Nov 2018
30 LIGHTING MAGAZINE | October/November 2018 to the ground plane. The illuminance at point P can be calculated using inverse square law: , (1) where I is the luminous intensity towards the calculation point and ���� is the distance from the light source L to calculation point P. and where ���� is the azimuth angle, ���� is the elevation angle and h is the height of the light above the ground plane. Substituting into Equation (1) gives: (2) Figure 2 shows the vertical illuminance calculation geometry for the point P on the vertical plane. The light leaves light source L and arrives at point P on the vertical plane. The magenta arrow indicates the direction of the light whilst the blue arrow is the normal to the vertical plane. In this case, and d = (h-1 .5)/cosγ, where h is the mounting height and 1.5 is the nominal height of calculation point P (also regarded as the observer point). So, the vertical illuminance is: (3) There are two arrays of lights in Figures 3 and 4 showing the newly installed LEDs complying with both horizontal and vertical illuminance requirements and the old HPS installation meeting only the horizontal illuminance requirement. A pedestrian standing at the right side can be easily distinguished from the observer position whereas the same pedestrian standing at the left side at similar distance is difficult to see, due to lack of vertical illuminance and poor colour rendering. The bright pools under the HPS installation suggest that a large portion of light has been wasted at the front- and back-sides of lantern. Which path would the pedestrians/cyclists choose? Figure 5 shows a path lit with an LED installation achieving full compliance. The entire path as well both its verges can be seen, giving reassurance to the users that the path is safe to use . Figure 3. Pathway lighting with vertical illuminance compliance – right. Figure 4. Pathway lighting without vertical illuminance compliance – left. Figure 5. LED pathway/cycleway lighting in full view.
LIGHTING Aug-Sep 2018