Abstract
Current standards for interior lighting design are discussed and an alternative design methodology proposed. Cuttle has previously suggested a new criterion be defined as perceived adequacy of illumination (PAI), and that the metric for specifying minimum illumination standards becomes mean room surface exitance (MRSE). This metric specifies the overall brightness of illumination, enabling its distribution to be planned in terms of target/ ambient illuminance ratio (TAIR). This new methodology is explained, analysed and discussed along with on-going research at the Dublin Institute of Technology. Introduction Lighting designers exercise their creativity against a backdrop of codes1,2,3, standards4, and recommended practice documents5, each specifying a range of lighting parameters for compliance. Foremost among this is a schedule of minimum illuminance values related to various indoor activities. While it is accepted that standards are necessary for general lighting practice, it has been quite common in the past for experienced lighting designers to sometimes disregard these standards as being irrelevant to their work. That attitude has become untenable due to the growth of regulations6 governing energy efficiency and sustainability. The practice of specifying indoor illumination in terms of workplane illuminance has been firmly established by the Commission internationale de l'eclairage (CIE) and the engineering-based lighting societies, and the energy regulators have followed this practice pretty strictly. This paper will discuss current standards and their relevance, introduce a new methodology for designing lighting within interiors, and briefly describe some ongoing research that is examining the suitability of the newly-proposed method. Illumination schedules Although specifying bodies have added various lighting quality criteria to their pronouncements7,8, the central factor remains the workplane illuminance, and it is claimed that this quantity is determined primarily by the category of the visual task. The IESNA Lighting Handbook1 states that “Changes in visual performance as a function of task contrast and size, background reflectance, and observer age can be calculated precisely”. Cuttle has previously9 applied the referenced procedure10 to examine how the illuminance required for a high standard of visual performance relates to various reading tasks. 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 generally accepted as the highest practical RVP level for lighting applications. It can be seen that the font size would have to be reduced to 6-pt for the required illuminance to exceed 100 lux, 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. SDAR Journal 2014 16 Figure 1: As previously applied by Cuttle, the illuminance necessary for high levels of RVP under varying illuminance levels, text size and background contrast. 10 10
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