When the surface feature height of the surface is larger than the wavelength of incident light, the incident rays are reflected at various angles because they no longer share the same surface normal.1 Any image carried by the incident light rays is destroyed as it interacts with the heterogeneously textured surface and is fragmented into many directions. The use of a surface as a secondary source of light is part and parcel of Hervé Descottes' "Six Visual Principles of Light".2 He aptly describes the power of this archetype through an example: "Consider the moon: it does not emit light on its own, but the sun's light reflected off the moon's surface creates what we recognize as moonlight."3 In Architectural Lighting Design, Descottes establishes that the process by which an object returns the light that it has received from a source of light to the eye of the viewer is quantitatively measured as luminance.4 He claims a sense of hierarchy can be developed within a space when surfaces of varying luminance levels are placed in proximity with one another.5
Architecture mediates the production of this archetype at two places.
The source of light must be framed and placed into a dialogue with a receiving surface before that surface can act as a seconday source of light. The architecture can express or conceal this relationship and can choose to display the secondary surface as a diffuser or conceal it and instead present it as a site of registration. The relative intensity of the light that is directed to the eye of the viewer can produce effects of both sharp and subtle layering of both surface and object.
It is interesting to note that the secondary surface can be used to itself perpetuate the production of silhouettes, one form of the simple image, within a space. A concealed source that washes a wall with light can reduce the appearance of a three-dimensional object to a crisp, dark outline of its potentially rich form. The secondary surface can thus be revealed as one of the most active and illusory of the seven archetypes. The range of luminance is so great as to include that amount of light information which imposes a kind of violence on our senses as well as a total absence of light information which reduces an object to an edge.
1. Duree, Optics, 48.
2. Hervé Descottes and Cecilia E. Ramos, Architectural Lighting: Designing with Light and Space (New York: Princeton Architectural Press, 2011).
3. Descottes, Architectural Lighting Design, 31.
4. Ibid., 30.
5. Ibid., 34.