Lighting : LIGHTING Feb-Mar 2018
50 LIGHTING MAGAZINE | February/March 2018 February/March 2018 | LIGHTING MAGAZINE 51 area metric and, most interestingly, a graphics approach rather than a single number similar to that advocated by van Kemenade and van der Burgt7. There is little doubt that new colour metrics will soon be advanced although how many will depend on the balance struck between the complexity of colour perception and the simplicity required for widespread adoption. Other design metrics under question are illuminance uniformity and UGR. Illuminance uniformity is the great unexplored variable in lighting design. This is because the logistics needed to carry out investigations are inhibiting but a recent study by Narendran et al has shown the potential benefits in terms of the perception of safety and energy savings of enhanced uniformity in parking lots8. As for UGR, this has been shown to be incapable of accurately predicting discomfort glare for very non-uniform luminaires9. Consequently, a number of improved UGR models have been developed10,11 but I doubt if any of them have a future because the simplest solution to this problem is for luminaire manufacturers to hide any direct view of LEDs. When this is done UGR is an effective predictor. Another area of concern is the use of the horizontal working plane as a location for design. For many years attempts have been made to direct designers’ attention to the task plane, wherever that may be, and to the value of a task ambient approach12, but too little effect. Now Cuttle has come along with a more radical approach. He argues that current lighting standards are more than enough for good visual performance on virtually all tasks so rather than lighting the task, attention should be given to lighting the space13. This means the basic lighting criterion should be the perceived adequacy of illumination and for this the right metric is the mean room surface exitance. Adopting this metric would emphasise the illumination of the major room surfaces such as the walls and ceiling and make indirect lighting more efficient than direct lighting. The risk in doing this is that the room becomes a white box. To avoid this outcome, Cuttle suggests a second metric called the task/ambient illumination ratio which enable a hierarchy of lighting to be established14 for the sake of creating some interest15. For this approach to be adopted will require a demonstration of its value and some software developed to make it easy to use16. By now it should be clear that lighting is not short of ideas for improvement so it is interesting to ask why. I believe there are three reasons. First, there has been a steady development in understanding the subtleties of the impact of exposure to light on both the visual and non-visual systems. Second, we are in the middle a major transition to solid state lighting and sophisticated controls, offering much great flexibility in the amount, spectrum and even light distribution of light sources and luminaires. Third, there is a need to introduce objectives beyond good visual performance without visual discomfort if lighting is to avoid becoming a simple commodity. The first and last of these reasons deserve some discussion. The discovery of the intrinsically photosensitive retinal ganglion cells in the human retina17 has revealed a new field of lighting effects. The physiology behind these effects is partly understood but there is still much to learn. What is clear is that the response of the visual system to light is sensitive, fast, detailed and located while that of the non-visual system, or more correctly the non-image-forming system, is relatively insensitive, slow, crude and non-located. This division in characteristics does not mean that the two systems are completely separated. They are not. The rod and cone photoreceptors that dominate the visual system also have role to play in the non-visual system and the intrinsically photosensitive retinal ganglion cells influence some non- imaging forming aspects of vision such as pupil size. Of all the possible non-visual effects of light exposure the one that has been most extensively studied is that of melatonin concentration, this being a hormone circulating in the blood stream that is used to synchronise the timing of many different functions of the body. Exposure to a regular day/ night light pattern is necessary to entrain the circadian rhythm shown by melatonin but exactly what form that pattern should take is still under discussion. For example, there is no international agreement over the spectral sensitivity of the circadian cycle based on melatonin suppression or what constitutes the necessary amount of light or dark. Further, the consequences of light exposure are known to depend on the timing and duration of light exposure over the whole 24 hours as well as the prior photic history of the individual being exposed. Given this situation it is hardly surprising that there are few lighting applications devoted to improving the operation of the circadian system. Those that have been shown to be beneficial are typically concerned with people who have damaged circadian systems e.g., people with dementia18 or who have disrupted circadian systems e.g., people working rapidly rotating shift systems19 or who have very limited opportunity for a light/dark exposure pattern e.g., submarine crews20. It remains to be determined if people who regularly work by day and sleep at night benefit from lighting designed to stimulate the circadian system. Figure 3. The CIE luminous efficiency function V(λ) from Figure 1, dashed and the proposed universal luminous efficiency function, U(λ), solid line. The righthand vertical axis shows luminous efficacy in lm/W. Figure 4. The basic structure of the IESNA TM30 method of colour rendition. The discovery of the intrinsically photosensitive retinal ganglion cells in the human retina has revealed a new field of lighting effects. Other design metrics under question are illuminance uniformity and UGR. Illuminance uniformity is the great unexplored variable in lighting design.
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