Lighting : LIGHTING Oct-Nov 2018
34 LIGHTING MAGAZINE | October/November 2018 October/November 2018 | LIGHTING MAGAZINE 35 luminaires. Besides, the majority of LED pathway luminaires comply with type 6 requirements . There are no formal light technical parameters (LTP) specified for the glare control in category P lighting design [8, 9]. Veiling luminance Figure 7 shows how a low angle sun can cause disability glare when it appears in the field of view but the visual target became clear when the glare is reduced (Figure 8). In Figure 9, the LED installation appears much more visually comfortable than the HPS installation while providing better illumination, which could be concluded as multiple reasons: efficient light source, optimum optical design, lower wattage and larger mounting height. The magnitude of disability glare can be estimated via the veiling luminance [10, 11, 12]: (4) where ����veil is the veiling luminance; ���� is a constant that is age dependent; �������� is the vertical illuminance at the observer’s eye, on the plane perpendicular to normal line of sight, contributed by the nth luminaire; and �������� is angle of the arc between the line of sight and the line from the eye to the centre of the luminaire [10, 11]. The reduction of visibility (contrast) due to veiling luminance [5,12] is: (5) where ���� is the luminance contrast; �������� is the target luminance; �������� is the background luminance and �������� is the veiling luminance. Formula (5) shows how the veiling luminance reduces the visibility. To enhance the visibility, veiling luminance ����veil should be minimised. This can be achieved by either reducing the vertical illuminance ���� or by increasing the angle ���� between the line of sight and the line from the eye to the centre of the luminaire. Luminaire mounting height and glare Figure 10 is an illustration of the vertical visual field of the human eye [13, 14]. For the lighting task, a large portion of light rays from the cycleway luminaire appear within the highlighted zone 0° to 25°, which means a greater chance of glare being perceived in this zone. In other words, this guides us to look for a reduction in luminous intensity in the 65° to 90° vertical angles to minimise glare. The zone from 25° to 50° (upper visual field limit) is not a major concern because it’s unlikely for a pedestrian to keep looking in this direction while walking. Likewise, the zone from 0° to 70° (lower visual field limit) is not a major concern either because it’s uncommon to have the low mounting lights such as lighting bollards “shining up” into pedestrian’s eyes. In fact the cycleway/pathway width is relatively narrow (typically 2-4m). Figure 9. Glare from light sources at different heights. Figure 10. Vertical visual field of the human eye. GLARE CONTROL Glare is caused by a significant ratio of luminance between the task and the glare source . It can be classified as discomfort glare or disability glare, if the viewer’s vision is impaired. In category V road lighting, both illuminance and luminance contribute to the lighting compliance, so the glare could come from light source itself or the illuminated road surface; it is assessed via threshold increment (TI). However in the category P road lighting design, the glare mainly comes from the luminaire itself. The lighting standard AS/NZS1158.3.1 has introduced a check table for luminaire glare control, categorising the luminaires into six types according to the distribution characteristics and level of Upward Waste Light Ratio (UWLR) . As per the table, a type 1 luminaire is close to a clear sphere and it has the highest UWLR without any glare control. In contrast, a type 6 luminaire requires zero intensity at ���� =90° at any angle of azimuth so the UWLR is close to zero. Other types from type 2 to type 5 lie between these two. Some organisations have used the glare requirement for type 4 luminaire in the luminaire selection process, which emphasises that the intensity at any angle of azimuth shall not be more than 720 cd in total or 180 cd/1000 lumens, whichever is the greater, at a vertical angle of 80°. This could be a practical guidance, however it might not be the most appropriate approach because AS/NZS1158.3.1, issued in 2005, was mostly based on HID Figure 7. Vision impaired due to disability glare. Figure 8. Vision became clear when glare is diminished. The lighting standard AS/NZS1158.3.1 has introduced a check table for luminaire glare control, categorising the luminaires into six types according to the distribution characteristics and level of Upward Waste Light Ratio (UWLR) .
LIGHTING Aug-Sep 2018