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TECHNOLOGY / DEBATE


Following a lively response to his paper at PLDC in Madrid earlier this year, Christopher ‘Kit’ Cuttle became convinced that he needed to address the design community more directly about his thoughts on the current state of the lighting profession. Here are those thoughts...


A SHARED PURPOSE FOR THE LIGHTING PROFESSION


We have a divided profession. The first illuminating engineering societies, formed more than 100 years ago, set the initial agenda for professional involvement in lighting. The purpose for providing Illumina- tion was seen to be to satisfy human need for visibility, and the aim was to optimise illumination provision for efficiency and economy through application of science- based principles. This approach is entirely logical and has been adopted in many countries throughout the world, where the members of national IES or equivalent societies have been strongly influential in establishing lighting standards. However, around the middle of the 20th century it became apparent that there was growing dissatisfaction with this agenda as it failed to include the professional activities of a newer breed of lighting designers, and this has led to the emergence of the IALD in the North America, and in Europe, of ELDA, now superseded by the PLDA. Defining the pur- pose for providing illumination as seen by these groups is less straightforward, but one thing would seem to be unarguable: rather than thinking of illumination as the medium that makes things visible, these designers see lighting principally in terms of how it influences the appearance of peoples’ sur- roundings. This difference of visibility and appearance being envisioned as the purpose of lighting underlies the professional divi- sion that remains with us to this day.


VISIBILITY AS THE PURPOSE OF LIGHTING Rather than employing a scale of task vis- ibility, lighting standards make reference to relative visual performance (RVP) to specify illumination levels appropriate for various activities. The validity of RVP has been demonstrated many times for critical view- ing situations, such as nighttime illumina- tion of roadway signs, but its application to indoor general lighting practice lacks rigour. Visual performance research has been based almost exclusively on subjects view- ing two-dimensional, diffusely-reflecting


reading tasks, and researchers have chosen this viewing situation as it enables visual performance to be expressed as a function of illuminance. This has led to the wide- spread misunderstanding that prescribed levels of visual performance for all manner of activities can be assured by specifying task illuminance values. For example, in 2008 the IES of North America published Guide to Designing Quality Lighting for People and Buildings, which states that “The foundation for lighting design is ensur- ing that people have enough light to safely, efficiently and accurately perform predomi- nant visual tasks.” From this statement it may be inferred that “enough light” is all that is required to ensure a prescribed level of RVP for a given activity, but the reality is far more complicated. This because the two-dimensional diffusely-reflecting task is a special case, whereas the general case includes visual tasks that may be three-di- mensional and have quite different optical characteristics, so that predicting visibility has to take account of the form, texture, gloss, colour contrast, and perhaps, the transparency or translucency of the task materials. Application of RVP in everyday situations would require definition of both the eye-task geometry and the luminance distribution of the entire surrounding light field. To specify a lighting condition for a real visual task that will ensure satisfaction of a prescribed visual performance criterion is no mean undertaking, and for this reason, it is very seldom done outside a research laboratory. If such measurements were to be conducted in actual workplaces, those who claim that the currently recommended (or sometimes mandated) illuminance levels must be pro- vided to maintain workers’ productivity and health would find themselves confronted by some challenging data. Figure 1 shows that, for the typical reading task of 12-pt type on white paper, it requires just 20 lux to provide for the relative visual performance criterion of RVP=0.98, this value being


Figure 1: The task illuminance required to provide for relative visual performance RVP = 0.98 for a range of reading tasks. The reader is a normal- sighted 25 year-old with a viewing distance of 350mm. The reading matter is black print, ranging from 6- to 14-point size, on three types of paper: light (reflectance p=0.9); medium (p=0.6); and dark (p=0.3).


generally accepted as the highest practical RVP level for lighting applications. It can be seen that for the required illuminance to exceed 100 lux, the font size would have to be reduced to 6-pt, or alternatively, reduced to 10-pt but printed onto dark- coloured paper, which has the double effect of reducing the background luminance and the task contrast. However, even this value of 100 lux falls far short of the levels conventionally provided for applications where reading tasks are prevalent, which typically fall within the range 300 to 500 lux, and it is clear that such levels can be justified on the basis of visual performance only by presuming that either the users are partially visually defective, or that they are persistently required to cope with visual tasks equivalent to reading very small print with very low contrast on low reflectance backgrounds. It can be seen that for every- day reading tasks recommended illuminance values are far out of step with visual perfor- mance requirements. If we transfer our at- tention to real tasks involving complexities such as three-dimensional form, texture and gloss, the situation would be further confused and greater discrepancies could be expected. It needs to be recognised that the often-made claim that the illuminance schedules are research-based and must be enforced to maintain productivity levels is


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