A feature of many national lighting standards is the recommendation that lamps with high correlated colour temperatures should not be used at low illuminances. The technical justification for this advice is limited so two experiments have been conducted to explore its validity. In the first experiment, fifteen observers carried out colour discrimination tasks and assessed the lighting of a small room lit to different illuminances using lamps with good colour rendering properties but with different correlated colour temperatures. The room decor was achromatic but natural colour, in the form of fruit and flowers, was introduced as another variable. In the second experiment, ten observers carried out colour discrimination tasks and made assessments of the lighting of the same room lit to 225 lx, but decorated with blue or pink walls and lit by lamps with good colour rendering properties but different correlated colour temperatures. Again, natural colour, in the form of fruit and flowers, was introduced as another variable. Analyses of the assessments of the lighting of the room showed that one major factor determining the impression given by the lighting was the illuminance. Increasing the illuminance made the lighting of the room appear more pleasant, more comfortable, clearer, more stimulating, brighter, more colourful, more natural, more friendly, more warm, more uniform, less hazy, less oppressive, less dim and less hostile. The correlated colour temperature of the lamps used had virtually no effect on the observer's impression of the lighting of the room. The other major factor influencing the impression of the lighting of the room was the presence of natural colour. Introducing natural colour, in the form of fruit and flowers, enhances the positive impressions created by the lighting, particularly at the higher illuminances. This enhancement occurs regardless of the correlated colour temperature of the lamps being used. It can be concluded that the advice limiting the use of high correlated colour temperature lamps in rooms lit to low illuminances is unnnecesarily restrictive. The results obtained suggest that provided the occupants are fully adapted to the lighting the correlated colour temperature of the lamp has little effect on people's impressions of the lighting of the room.

The notion of stages of the lighting profession is examined to identify the main themes that have directed the objectives of the lighting profession. It is proposed that the objective of the first stage was provision of uniform illumination over a horizontal plane, and that of the second stage has been to provide illuminance suited to human need, based on visual performance. This brings us up to the current era, and it is the author’s opinion that the second stage has failed to achieve its objective. While codes and standards pay lip service to visual performance, the reality is that for the vast majority of situations where lighting standards are applied, the aim is to meet user expectations for the spaces they occupy to appear adequately lit. The metrics currently used to specify, measure, and calculate lighting levels are inappropriate for this purpose. The concept of mean room surface exitance is proposed as a basis for lighting standards. Procedures for calculation and measurement lead to some startling conclusions. Familiar notions of lighting effectiveness and efficiency are turned upside down, and an entirely different way of thinking about interior lighting design is revealed. The essential difference is a switch from assessing light incident on planes to assessing light arriving at the eye. Such a change of thinking may be seen as a precursor for the third stage of the lighting profession.

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the basic purpose of general lighting practice is to enable performance of visual tasks is examined and found to be lacking in substance. It is proposed that the purpose of lighting should be redefined in terms of the visual experience of illuminated surroundings, and two criteria are proposed for this purpose, both of which represent significant departures from conventional practice. The first assesses the adequacy of illumination for an activity in terms of the density of reflected light from surrounding room surfaces and the second is concerned with how the luminaire luminous flux is directed onto selected target surfaces. Taken together, these criteria offer a quite new approach for designing lighting installations for general practice.

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