A new basis for lighting practice

The standards and recommendations that guide general lighting practice bear no sensible relationship to providing for human satisfaction. The conventional way of specifying indoor lighting levels in terms of workplane illuminance is defended as the means for ensuring appropriate levels of visual performance, but this claim has been challenged and shown to be unsupportable. Meanwhile, current lighting standards drive general practice towards installations which direct high proportions of the lamp lumens downwards onto the horizontal workplane, thereby dictating the lighting distribution that has to be provided to satisfy energy efficiency standards and sustainability ratings. Without a valid measure of how lighting provides for user satisfaction there can be no valid measure of lighting efficiency. The unavoidable conclusion is that lighting standards are in need of fundamental revision.

It is argued that the prime factor determining how much light needs to be provided for a given category of indoor space is meeting peoples’ expectations for that space to appear adequately illuminated. On this simple basis, an entirely new lighting criterion is required to replace visual performance. The concept of perceived adequacy of illumination (PAI) is proposed as the basis for specifying illumination levels in general lighting practice, and this calls for a lighting metric that corresponds to visual assessments of the ambient level of reflected light arriving at the eye from surrounding room surfaces. Mean room surface exitance (MRSE), being the average luminous flux density (lm/m²) from surrounding room surfaces, is such a metric. Its adoption would recognize that direct light at the eye from luminaires and fenestration comprises glare, and this component of illumination detracts from, rather than adding to, assessment of illumination adequacy. It should be excluded from illumination measurements concerned with assessments of adequacy.

Adoption of a PAI-related metric for specifying illumination quantity in general lighting practice would lead to a fundamentally new understanding of how light is to be distributed within indoor spaces. It would be recognized that the role of luminaires is primarily to control the initial light distribution, after which interreflection between the room surfaces generates the light distribution presented to the eye. Room surfaces would be seen to be integral components of the lighting system, as influential as lamps and luminaires. This new understanding would have profound implications for lighting designers. Instead of being coerced into applying “efficient” downlighting distributions in order to comply with sustainability targets, they would have freedom to devise lighting distributions that interact with room surfaces and objects of interest, creating visual hierarchies and removing conflicting aims that currently differentiate lighting design and illumination engineering objectives. New lighting standards based on PAI may be seen as the means for changing everything from lighting education through to luminaire and fenestration controls, but before such standards can be prescribed some good human factors research is needed.

For more background on this topic, refer to:

Cuttle C. Towards the Third Stage of the Lighting Profession. Lighting Research & Technology, 2010; 42(1): 73-93.

Section 2.1, Ambient Illumination, from Cuttle, Christopher. Lighting by Design, Second edition. Oxford, Architectural Press, 2008.

Sunday 15 May 2011