Guide to Protocols for Describing Lighting
|Publication Date:||1 January 2014|
The challenge of precise terminology is large and begins with the use of the word "light". The 1978 CIE document "Light as a true visual quantity" defined light as "...radiant power weighted according to the spectral sensitivity of the human eye" (CIE 1978). The CIE presently offers two international standard definitions (CIE 2011):
1: characteristic of all sensations and perceptions that is specific to vision
2: radiation that is considered from the point of view of its ability to excite the human visual system
The International Lighting Vocabulary goes on to specify that if there is a possibility of confusion between these meanings, the term "perceived light" is to be used in place of the first meaning. This second meaning is the sense in which the word "light" is used in this report. Light is the medium that carries information to the visual system (in the form of variations in radiant energy over time, wavelength and intensity). This electromagnetic radiation, both direct and reflected from objects in space, stimulates chemical reactions in the eye that, interpreted, lead to perception, emotion and action (Figure 2). The scale of information conveyed visually extends from the largest expanse of the surroundings through the smallest details.
Light is the stimulus, the physical quantity that initiates sensory, perceptual, cognitive and behavioural processes that researchers study and that are the outcomes that lighting designers seek to animate in the people who experience their creations. The aim of most behavioural or human factors research in lighting is to understand the specifics of this sequence. For example: What room surface luminance makes a room appear pleasantly bright? How does a dark ceiling make a space appear cave-like; or when might it appear intimate? It may be said that lighting designers use this sequence in reverse: they begin from understanding the desired end-point; knowing the viewer, understanding the emotions or actions that are wanted in response to the lighting installation. The job of the lighting designer is to work backwards, to develop a lighting scheme that will elicit the desired response.
There are three senses in which one might use the phrase "to describe lighting":
1) to specify the lighting equipment, controls and layout for the purposes of design and installation;
2) to report the physical parameters that produce the visual sensation in the person in the space; and
3) to describe the distribution of light as it affects the appearance of the space and the objects in it under a given set of physical parameters.
The focus of this committee report is on the specification of the physical quantities encompassed by items 1 and 2 above. This report draws a clear distinction between the physical and the experiential. We are concerned here with describing the stimulus, not the response; we find that in some respects the lighting literature confuses the two.
Consider, for example, complex mathematical formulae for combining physical quantities into indices, such as the Unified Glare Rating (UGR) or the Daylight Glare Index. The values in the index are not the same as the experience. If one has calculated UGR, one has applied a particular model about the inter-relation of adaptation luminance, source luminance, size and position, and has made a prediction concerning the likely degree of discomfort that occupants might experience. We tend to overlook or to forget that this is a prediction, not a measurement of discomfort, and that the prediction of a psychological phenomenon has random error. Although few people may believe that UGR or other glare prediction models fully account for the experience, we rarely discuss other variables that could influence whether or not that set of physical conditions will or will not lead to discomfort - for in some contexts, areas of high relative luminance that mathematical formulae would predict to be uncomfortable are perceived as interesting points of sparkle. Our imprecision leads us to think we fully understand the phenomenon.