Qualities of Light

"Of all the elements of a room, the window is the most marvelous. What slice of the sun does your building have?"

- Louis Kahn

For those of us working in stained glass, there’s a wonderful resonance to this quote. Light is life to a stained glass window; and every work we create must be made with a good understanding of the light that will animate it once it’s in place. Although the star that sustains our planet gives a bright and steady light, many factors, both distant and close by, determine what “slice of the sun” enters a given window in a building - and when.

It is a reality of working in stained glass that the one element most essential to a window - the light it will live in - isn’t present at the studio. This means that the artist must learn about the light and how it works in the building before she begins to design. This is not only a question of knowing how much light is there, but what has to be done with that light, on both on an artistic and a practical level, based on the intended uses of the room.

This adds a level of complexity to a window; at the same time, it is this variation of light, the cyclical changes in direction, intensity and spectral range over the days and seasons, that makes stained glass so interesting. It turns a single window into an ever changing experience; one that stays fresh, and even surprising, long after it is installed. (Artificial light is another story; we've all seen enough fluorescent-lit "ghost windows" to know that this usually doesn't make for an interesting and “living” stained glass work).

In planning for light, the two essential questions are: “What light will be coming through this window?,” and “What do I want to do with it?”

The first thing to understand is that natural light is ever changing. These changes in the light take place throughout the day, through the seasons, and in response to alterations in the building and its surroundings. Knowing this, how is the artist to design for all possible conditions? Day and night; winter and summer; volcanic eruptions that obscure the sky for weeks on end....? Of course one can't predict everything - nor should one try to. There has to be room in every window for some surprises, as long as the more unpleasant ones are foreseen and avoided. The shaft of morning sun focused directly into the minister’s eyes every Sunday morning won’t be appreciated - and should have been predicted. She may think more kindly of a spray of color from a prism, illuminating the chancel on Easter morning.

While it’s not practical to consider every possible light combination, it is possible to think in terms of ranges; that is, the span of most likely conditions, based on the building and its surroundings. For example, the height of the sun in the sky at a given time of day will vary according to the season. In planning the window, the artist needs to think about where the sun will be at critical times of the day in each season - its maximum and minimum heights - and design the window to work within that range.

In order to make these decisions, and to get a good picture of the way light works in the space, the artist needs to spend some time there. This usually requires more than one visit. If possible, the observations of the light and the space should take place at different times of the day, so that the movement of the light can be seen. On a large project, with a longer time line, there may even be opportunities to watch the changing of the seasons. The purpose of this observation is to see how the light falls in the room, and to get a sense of how the architecture and surroundings will affect this.

As part of the observation, the artist should note the direction that the window faces, as this will determine the amount of light and the angle it will enter from during the course of a day, and over the changing seasons. It's important to check that this is true direction of the window. It's not enough just to know the position of the window in relation to the city or town grid. Sometimes, the grid can be off by several degrees of the compass, and this can make a difference to the light.

Based on the observations from these visits, the artist may wish to draw a diagram of the sun’s path over the course of a day, noting how this will change throughout the year. The common weather conditions in the area should also be considered. Some locations tend to be sunny, while in others, an overcast sky is the norm. Mist or smog also change the quality of light, both dimming and diffusing it. Snow, too, has a major effect. It can not only double the available light in winter, but the angle of the light as well.

So far, we’ve looked at the “macro” factors of location, orientation and weather. However, it’s important to remember that it can be the local factors - the characteristics of the building and its immediate surroundings - that have an even greater effect.

A fundamental ingredient is, of course, the architecture of the building. This can affect the light in a number of ways. For example, there is the size of the window itself, compared to the size of the room. Light coming through a small bright window in a very dark interior will create “halation” (a halo effect around the window), and this can make the window seem much brighter than it actually is. The artist can compensate by choosing darker glass than would otherwise be used, but to do this, he or she will have had to recognize the problem in advance.

The height of the window should also be considered, as it will determine where the sunlight falls in the room. A extreme case is clerestory windows. Unless there are tall trees or buildings about, these windows will be unobstructed, and special attention has to be given to seasonal changes in the angle of the sun, and the overall light (and heat) the windows will transmit. In observing and working with the architecture, the artist may wish to consult with the architects themselves to see how they considered and designed for natural light in their plans.

Other constructional details are also important: Is the window in an alcove, and its light blocked by part of the building? What is the effect on the light of the other windows in the space? If they are clear, the light they transmit will affect the stained glass, and can make it look washed out. Following up on this, the artist should find out if there are plans to install stained glass in these windows later on, as that will affect the light conditions in the space over the longer term. Artificial lighting is also a factor. Although this can be controlled, some uses of the space may regularly require bright lights. Where this can’t be changed, the artist has to accept it and design around it.

Beyond the building itself are the window’s immediate surroundings. Common concerns here are nearby buildings and structures, and the local vegetation. Of these, the latter is most subject to change over the seasons, and over time. A healthy deciduous tree is very effective at catching its own slice of the sun, and the window it beside it that looked fine in December may suffer in the summer. Likewise, if the same window was designed to make the most of the available light in the summer, it may turn out to be dreadfully overexposed when the tree loses its leaves in autumn.

In considering local factors, the artist has to try to some extent to predict the future. Some of this can be done through simple logic: trees grow bigger, get old and die. If the tree shading the window is showing signs of age, the window design had better allow for the possibility a smaller tree taking its place. Likewise, it wouldn’t be surprising, in a fast-growing area, for a small rundown building next door to be replaced by a bigger one. Careful designing would try to make sure that that wouldn’t be a disaster.

Of course there's no way to be entirely certain of the future, but it’s sometimes surprising how much can be found out by asking questions (a useful one is: “Are there any major landscaping projects planned for the area outside the window in next few years?”).

Once the artist has observed the building, its location/orientation, and its surroundings, he or she should have a pretty good picture of the patterns of natural light through their window. The next step is to decide what to do with the light. Should it be focused? dampened? diffused? redirected?

The answer to this depends a lot on the space, who will use it, and what the users want the windows to do. Among the questions to be answered are: Where will people be when they use the space? Will they usually be sitting? or standing? Will they be using natural light for any of the tasks they are doing? Do they want a darker atmosphere or a lighter one? At this point, it will be necessary to have some discussions with the building’s users or their representatives, to get an idea of their needs and expectations. Although this will provide useful input, it shouldn’t be the solve determinant of the design. An experienced artist will listen to the clients’ wishes for the space, but will also contribute his or her experience of what works and what doesn't; what can be done, and what can’t. Whatever the final design, it’s essential to avoid any situation where the light transmitted by the window becomes uncomfortable to those within.

Once the artist knows what is needed with regards to the light, she has a wide palette of materials and techniques to choose from. In addition to the various colors and types of glass (ie flash glass, clear textures, bevels, and prisms), several of the common glass techniques can be used to change the characteristics of the light. In Figures 1-7, I give some examples of how design was used in several projects to resolve or work around some challenging lighting situations.

In deciding how to manage the light, and with what materials, the best approach is to visit the site with some glass samples, and see what happens when light passes through them. Keep in mind when doing so that managing the light is only part of the picture; it shouldn’t entirely determine the materials that are used.

The story of the light in a space is a complex one. Although lighting conditions can present challenges, these can often be transformed, with some imagination, into opportunities. The work of the stained glass artist is to take the light that is given, with all its complexities, and complicate it just a little bit more; to moderate and/or modify and/or enhance it; and at the same time to add beauty and meaning to the light, and to the space it illuminates.