Coherence refers to the potential for light rays to travel, closely packed in straight lines and parallel in a beam. The light rays hold together and seem to “cohere,” making what we usually call a sunbeam. This property of light is not common, but it occurs occasionally in our everyday lives. The clearest example of this is the laser,1 an instrument that emits visible radiation (light) with great temporal coherence (minimum divergence from the constant diameter of the beam) over a long distance. Laser light, its formal “coherence”, and its direction, are maintained by the introduction of a significant amount of energy. Other common examples of coherence are automobile headlights seen through fog or theatrical spotlights, in which the reflector portions of the lamp (light bulb) direct light rays so that they concentrate in a narrow beam and strike a well defined spot on a surface. Coherence, or the appearance of it, also occurs as a result of a third condition, in which intense, direct sunlight, is occluded by a defined opening—an opening in a wall, for example—defining a beam of light that streams into a space. As with the example of headlight seen in fog, this condition is dependent on the presence of particulates in the air, water droplets in the case of fog, or dust motes, small specks of dust. The particles catch light and reflect it toward our eyes. The specialness of this condition derives from that way in which the coherent beam of light assumes an almost material status. The light hangs in the air as if it were an object, frozen in place, with the dust sparkling and circulating in the air currents.
Seen in the atmosphere, this effect is often referred to as crepuscular light,2 the condition in which light and dark shafts emanate in a radial pattern from the sun. The term “crepuscular” is a reference to twilight; these effects are most common at sunrise and sunset. The distance of the sun from the earth is effectively infinite so that the beams of light are actually parallel, but appear to converge at the solar disk (the sun) due to perspective. As with so many of the effects of daylight, the perception of such sunbeams is as much a function of contrast, the presence of darkness, as it is the presence of light. The columns of light appear as sun shines between areas of cloud and the light is scattered by dust particles and air molecules. These columns are interspersed with shafts of darkness, the shadows of clouds. The condition of crepuscular light, or coherence, can be manufactured in architecture, however, the effect cannot be completely controlled, as it is a function of sun angle, weather, building design, time of day, and season.
The subject of coherence brings to mind the question of the substance of light. Some observers make the case that light has a material quality, but it may be that they are speaking metaphorically with the intention of saying that daylight is so important to architecture that it deserves to be considered, during the course of design, as having the significance equal to any of the construction or finish materials included in a composition. Undoubtedly, other observers are making a literal claim that light has substance, a view that gains some credibility from the notion that light behaves like a particle (a thing) as well as a wave (energy). But this particle-like behavior is exhibited at the scale of the photon. What we really see is more closely related to the behavior of light as energy or radiation in the sense that light never resides in any one location; it is always moving and its manifestation as substance is still a function of reflection, diffraction, and scattering.3 This is to say that light, even seen in its rare coherent state, is simply doing the things it always does.
Nevertheless, the appearance of coherent beams of sunlight in architecture instills awe,4 all the more so as it is simply a result of the way a building receives daylight. In Vilhelm Hammershoi’s5 painting, Dust Motes Dancing in the Sunlight, of 1900, the painter depicted sunlight streaming through a white-painted, wood window and into an empty room. We actually don’t see the sun, but on its way in, its light brightens the window jambs and the mullions. Then we see it slanting into the room, its beams derived from the pattern of the window sash. Finally, the sun falls on the floor of the room, replicating the configuration of the window divisions. But directly in front of the window, to the left of the sunshine, the floor is softly brightened, reminding us that skylight (without sun) is also present and available to illuminate the room.
In the case of the theatrical spotlight, the coherent beam of light focuses our attention on a particular performer or location. It aids in the process of storytelling by directing our eyes to the main point of the narrative. (In a grand theatrical production, Las Vegas-style perhaps, one might see a number of spotlights, some of them panning across the stage and implying that the entire stage or many things are important. Often, it is in the nature of celebration to dismiss focus or priority and to release energy every which way.) But the fact that, with one spotlight, everything else is left in relative darkness is also important. Much can be usefully obscured in the vast area of darkness, produced by contrast with a small area of bright light. For example, beams of light may also be used to define spaces or to distract our attention from irrelevant, uncontrollable, or dangerous information in the visual field. An important example of this, using electric light, is the 1936 display of anti-aircraft lights at Hitler’s Zeppelinfield rally in Nuremberg.
The Pantheon, with a large oculus centered at the top of its dome admits high sun, configuring a great beam that travels across the inner surfaces of the temple, grazing and clarifying sculptures, recesses, pediments, architraves, and all manner of Roman architectural detail. The oculus and its beam emphasize the singularity and centrality of the ancient temple at the center of the civilized world. The effect is not limited to the making of a single beam. A linear series of coherent beams can be seen, at the right time, of course, in the Cryptoporticus at Hadrian’s Villa. The Cryptoporticus is rectangular passage, mostly below ground, whose openings originally worked to receive sinking air that had passed over and been cooled by a large pool centered in the portico-surrounded courtyard above. The porticoes no longer existing to block some of the sun that now enters, somewhat altering the function of the space as one that captured and circulated cool air. Such spaces were used for the storage of perishable items.6
And in a somewhat more complex array, the cupolas atop the gable roofs at the Mosque at Cordoba (in Granada, Spain) create coherent beams of daylight. Perched high over the dark, multi-columned space of the Mosque, the cupolas are small, white plaster spaces punctured with small, squared windows in its sides. As the sun moves into the right location in the sky, its illumination enters the window at a high angle. The sun first brightens the sides of the monitors so that a region of diffused illumination is created directly below in the Mosque. When the sun angle corresponds with the cupola opening and a view to the floor, a beam of sunlight cuts through the orthogonal bays of stone columns and double-arches to create columns of light. The new array of tilted columns suggests an alternate building orientation, and superimposed on the given grid. The multiple openings low in the dome of St. Peter’s Basilica in Rome admit incoming high sun to create the effect of radiating columns of light aimed at the basilica floor. The scene is something like the occurrence of crepuscular sunlight seen though openings in clouds. The ability to capture the sun, to aim it, to establish a protective ring of solar power is entirely befitting of a religious space. One is reminded of artworks in which Jesus or rock stars7 are depicted with their faces at the hub of radiating spokes of sunshine.
The vast glazed openings of modern buildings illuminate broad swaths of space and large surface areas and as the great architects of the twentieth century have demonstrated, this can be done artfully and with meaning. But coherent beams of light are linked to the small apertures found in traditional stone and earth architecture, buildings with construction systems in which large openings are difficult. Only the small opening in an opaque enclosure both creates enough shadowed space to contrast with the narrow shaft of sunshine. The development of coherent beams also benefits from a sophisticated knowledge of astronomy, the geometric relationship between the sun and the earth. The great builders of ancient Egypt acquired this expertise and built temples in which they directed sunlight toward important statues inside otherwise dimly lighted rooms. They designed dark spaces, were proficient at spreading or absorbing light by the varying the reflectivity of stone surfaces, and could light rooms softly or uniformly through clerestory windows or grilles. Structures were designed to admit the rising morning sun, specifically identified with the sun god and his daily awakening, into an eastern facing doorway. Egyptian culture linked itself, through architecture and ritual, with daily and seasonal cycles of sunlight. “The Egyptians were well aware how to organize the interior lighting of their monumental buildings by creating axes of light, directing beams like theatrical spotlights on to a statue, leaving one room in shadow, and gradually increasing the darkness to suggest the rising mystery.”8 It is not surprising that, in ancient Egypt, the sun was worshipped as a god, named Ra, the creator of everything, a notion that is not so far from the truth as we now understand it.
1 “Laser” is an acronym for “light amplification by stimulated emission or radiation.”
2 David K. Lynch and William Livingston, Color and Light in Nature, New York: Cambridge University Press, pages 15-17.
3 “Crepuscular rays are near-parallel, but appear to diverge because of linear perspective. They often occur when objects such as mountain peaks or clouds partially shadow the sun's rays like a cloud cover. Various airborne compounds scatter the sunlight and make these rays visible, due to diffraction, reflection, and scattering.” http://en.wikipedia.org/wiki/Crepuscular_rays; retrieved June 16, 2011. See also John A. Day (2005). The Book of Clouds, Sterling Publishing Company, Inc.. pp. 124–127, retrieved June 28, 2011.
4 Awe: “a feeling of reverential respect mixed with fear or wonder.”
6 Chip Sullivan, Garden and Climate, New York: McGraw-Hill, 2002, pages 32-34; and http://en.wikipedia.org/wiki/Cryptoporticus retrieved June 28, 2011.
7 Jimi Hendrix Experience, Axis Bold as Love, released on various labels beginning in 1967. The well-known cover is a painting of the three musicians by Roger Law superimposed on a Hindu devotional painting featuring the god Vishnu, the preserver and protector of creation, in a variety of forms. Sources: retrieved June 28, 2011, http://en.wikipedia.org/wiki/Axis:_Bold_as_Love and http://www.sanatansociety.org/hindu_gods_and_goddesses/vishnu.htm
8 Jean-Louis de Cenival, Living Architecture: Egyptian, New York: Grosset & Dunlap, 1964, pages 91-92, 140, 173-174.
Pantheon: Martin Schwartz
Cryptoporticus: Terrence Goode
Mosque at Cordoba: Martin Schwartz
St. Peters Basilica: en.wikipedia.org/wiki/File:Crepescular_rays_in_saint_peters_basilica.JP