The processes of interaction of light and matter described in the previous section cause the events in the luminous environment that are the stimulus for vision (Figure 1.1). The visual process informs the perceptual process, which causes the sensation of a perceived environment in the viewer’s brain. This perceived environment comprises recognized objects with distinct character- istics. Whereas a single pixel in the luminous environment can be specified completely in terms of luminance and chromaticity, the object that contains this pixel may be perceived to have charac- teristics of substance, utility, beauty, value, affection, and so forth. These interpretations, which occur during every moment of our waking hours, derive from recognition of perceived attributes. The perceived attributes that may be associated with any ‘thing’ that is seen depend on the mode of appearance in which it is perceived (Tables 1.1, 1.2).
It is important to appreciate that something that has the physi- cal properties of an object is not necessarily perceived in one of the ‘object modes’. Plate 2 shows two views of a luminaire. In case (a), the surface form, texture and lightness of the glass shade are all clearly visible. We could assess its lightness on a scale of 0 to 10. We could make a reasonable guess of its reflectance. We perceive this shade in object surface mode. However, our percep- tion of the shade is different in case (b). We recognize that it is still the same shade, but now that it glows we really have no idea of its texture, and it is quite meaningless to discuss its lightness. We could certainly discuss its brightness, and this lies at the heart of our changed perception of the shade. We are now perceiving it in illuminant mode, and as indicated in Table 1.2, the range of associated attributes is different, and furthermore, the number of attributes is reduced. Of course we know that the glass shade is not self-luminous, but nonetheless, the ‘thing’ that our intellect informs us is a trans-illuminated object is perceived as if it is the source of light.
Furthermore, an object may be perceived in more than one mode. The glass object shown in Plate 3 is perceived to have both surface and volume attributes. The two cases show how a change of background can give a different balance of the perceived attrib- utes. Without disturbing the light sources, the object can be presented to give emphasis to its internal colour or to the smooth glossiness of its surface. Differences of this sort are explored in the following chapters, but we should note that throughout these changes our understanding of the object’s fundamental nature remains intact. The differences of appearance may influence our sense of appreciation of the object, but basically it remains a coloured glass vase. Such is the power of the perceptual system to recognize object attributes that, providing there is sufficient light to enable the visual process to operate effectively, viewing condi- tions have to be severely constrained for viewers to be confused over object recognition. Consider for a moment; if we could present the glass vase in Plate 3 so that the surface attributes were completely invisible, what would a viewer perceive? Is it possible to imagine the volume attributes without a bounding surface? Fortunately the perceptual process very rarely presents us with such confusion.
The basic purpose of visual perception is to enable recognition of object attributes. Each attribute is associated with certain optical properties of the object, and is recognized by a characteristic inter- action with light. Generally, the prime purpose of indoor lighting is to enable recognition of stable environments comprising recog- nized objects within which people can orientate themselves and navigate with confidence. However, the perceptual process is very adept at doing this and it copes well over a vast range of visual conditions. On the one hand, this permits many lighting solutions that provide for no more than sufficiency of illumination to be found acceptable. On the other hand, it offers opportunities for design- ers to apply imagination to selecting object attributes for empha- sis without compromising the basic requirements that lighting for occupied space must fulfil.
It needs to be noted that the perceptual process involves placing interpretations upon the visible effects of optical interactions. Every ‘thing’ that we perceive in our surroundings is recognized to have certain attributes, and the range of attributes that may be associ- ated with a ‘thing’ depends upon the mode in which it is perceived. Objects perceived in the surface and volume modes have the greatest range of associated attributes, and this is where lighting designers often look for opportunities to influence the appearance of surroundings. In Plate 3, the light that is reflected towards the viewer from the surface has undergone a different reflection
process from the light that has been reflected or refracted within the volume of the object. While some lighting designers work on the basis of an intuitive understanding of this difference, a designer who understands the optical nature of this difference is in a stronger position to control the processes, and to select attributes for emphasis. To explore how this is done, we move on from characteristics of objects to characteristics of lighting, which opens up more opportunities for influencing the appearance of surround- ings.
Visual constancy requires that we are able to differentiate between changes of surface lightness and surface illuminance. According to the ‘modes of appearance’ model, ‘things’ that are perceived in surface or volume object modes have the attribute of lightness but not brightness. Helmholtz’s paper appeared white and the lump of coal black regardless of their relative luminances. Nonetheless, he would have been conscious that the coal was more brightly lit than the paper. While he perceived the stable attributes of these objects in surface mode, he perceived the different attributes of their light- ing in localized illumination mode. This chapter examines perceived attributes of lighting.