Quality of lighting

The concept of quality in the field of lighting can be generally tought of as determined by three major aspects. In order of importance these are:

Each of these major aspects can be defined in more detail comprising features such as adequate illumnation of the task, freedom from unwanted reflections, avoidance of excessive illuminance variuations, etc. as summarized in the paper on lighting design presented by W.G. Julian (493) during the 23rd session of the CIE in New Delhi.

Energy efficiency is until today not yet regarded as an aspect of the quality of lighting. R. Wibom and A. Ottosson (140) highlighted in their presentation on the assessment of "Office workers reactions when changing old lighting to new optimized good quality energy efficient lighting", how much the perception of quality is determined by luminance in the field of vision and the essential role played by desk working area illuminance in this matter. Hence it can be concluded that in effective lighting design the requirements of good quality lighting ought to be fully matched with the conditions set forward by the requirements of energy efficiency.

As people are known to be inclined to higher lighting levels in case of poor quality lighting, good quality lighting is a prerequisite for achieving genuine energy savings.

To determine quality in lighting is however very difficult as it generally depends on personal preferences and taste. This was one of the conclusions of Yukinobu Minami et al (248) after they had assessed Cutlle's modelling index. They also found the occupants preference towards the modelling to change according to the usage of the space.

Through the course of time various attempts have been made to define a rating system for quality lighting . M.J. Perry et al (188) carried out a series of experiments aimed at verifying the validity of the so called CSP index (Consumer Satisfaction and Performance) developed by Bean and Bell and to extend the data base used to derive this index. M.J. Perry et al reported very low correlation between the subjective CSP scores and obejective scores derived from photometric measurements . Some believe that subjective impressions of a space rely significantly on the information content of the space. This information content can be estimated by the Fourier amplitude spectra gradient. M.J. Perry et al found the Fourier gradient and CSP objective scores (gathered through photometric measurements) to correlate significantly. They suggest to make use of this finding as a foundation for drafting a questionaire for use in developing a lighting quality assessment method.

Glare is generally recognized as a major factor of discomfort thus being a major quality determinant. The CIE unified glare rating system is the most practical method as is concluded by T. Pai and V. Gulati (184) in their paper presented at the 23rd session in New Delhi. The influence of the lamp, the luminaire, the room, and the lighting installation on discomfort glare are examined. The CIE glare rating sytem is valid in case of lighting by small electric sources and does not apply to day lighting systems.

Attempts to establish links between illuminated environments and occupant satisfaction or in other words to find correlation between objective photometric scores and subjective quality scores is very time consuming thus making appropriate automation tools high in demand. D. K. Tiller et al (294) describe two new software tools to faciliate post occupancy evaluation of lighting.

Also climatological and cultural differences play a role (S. Begemann et al (148)). This is especially true for the application of daylight as a source of lighting in the working environment. Begemeann et al report a relationship between daylight illuminance levels (on the desk) and preferred colour temperature. They found no correlation to exist between the preferred colour temperature of artificial light added tyo the available daylight and daylight colour temperature. One of their most interesting remarks is the conclusions that the CIE should start a serious discussion on the possibility that today's artificial lighting standards that are based on meeting visual needs may be totally inadequate to meet biological stimulation requiremnents.

It may be concluded that in many cases the use of daylight is an appealing way to reduce energy use for lighting while at the same time improving quality.

Given the uncertainties about what actually should be seen as good quality lighting, the best approach seems to be not to try to define a system that gives a rating in quality but to focus attention on what is not acceptable or irritating the user. A truly energy efficient system should neither disturb nor irritate the user, so he or she is not tempted to add extra light (e.g. through the use of desk lights) or even sabotage an installation fitted with electronic controls.