This is the second in a three-part series about the most important light-managing medium, glass. Part one, which appeared in the July/Aug 2007 issue, examined conventional building glass as it affects daylight and daylighting. Here, part two, discusses glass as a medium for lighting and lighting effects.
While the principal use of glass is to transmit light, glass (and similar materials) also can be used to modify light to accomplish a wide range of desirable effects. Some of these are ancient, from media with color such as stained glass to lenses, prisms, and acid etching that have become essential lighting tools. Modern technology contributes other effects, including one of lighting's most popular effects, dichroism. Together, these media enable the lighting designer to add depth, movement and glow. Once again, the term “medium” or “media” means light-transmitting material, whether glass, acrylic, polymer, etc.
COLOR MEDIA The coloring of light is probably the most commonly used lighting special effect. Fundamental to theatrical and entertainment lighting, coloration of the light source is increasingly in demand for architectural applications as well. Saturated and brilliant colors are obvious, but there are many other uses of color in architecture that are more subtle, such as removing the greenish tinge from an MR16 or fluorescent lamp.
There are two primary ways to “color,” light-filtering and dichroism. Filtering, the oldest and once most common, relies on a dye to absorb the energy of unwanted wavelengths. The absorbed energy is turned to heat. A newer method, dichroism, employs a coating on glass consisting of many very thin layers of a material like titanium dioxide. The effect is to selectively pass some wavelengths while reflecting others. By not absorbing the unwanted light, a dichroic lens may last much longer.
When discussing color, theatrical “gel” immediately comes to mind. The word “gel” comes from gelatin, which was used long ago for theatrical color. Modern gels, which are made of polycarbonate, are surprisingly robust and able to absorb an amazing amount of light but, depending on the application and rate of use, can have a very short life. Glass, on the other hand, can serve almost permanently without failure as long as the glass holder allows for expansion when hot. There are three principal ways to create a glass filter:
- Using glass with inherent color properties caused by specific chemical impurities, such as cobalt (blue) or cadmium (red);
- Coating or dyeing the surface of the glass; and
- Coating glass with thin multilayer coatings to cause dichroism.
When choosing color media, first consider the amount of energy absorption. In addition to unwanted visible light, media also can absorb or reflect ultraviolet (UV) and infrared (IR) rays. When using colored media with modest energy exposure, dyed or gel materials will work, but as the intensity and/or temperature of the installation increases, the use of high temperature glass, such as borosilicate, combined with long-term coatings, is important. Making durable saturated-color filters that can be used in architectural applications, especially outdoors, is not easy. When needed, consult with companies specializing in this type of work.
Special filter-making companies have become part of the lighting industry with the principal goal to create permanent filters with durability, superior color, and minimal side effects. For example, one company offers hybrid filters that are part dyed and part dichroic to minimize the off-axis color shift issues of the dichroic filter. Other companies make media such as crushed dichroic, a fused lens made of dichroic pieces that creates an amazing color effect. Artful uses of color are possible as well, such as dichroic glass block.
DIFFUSING MEDIA In current architecture, there is a clear trend to create self-illuminated glowing surfaces. A recent example is the Bloch Building, the new addition at the Nelson-Atkins Museum of Art in Kansas City, Missouri (see “Sculpting with Light,” Sept/Oct 2007). While projects like this use a complex mix of materials and achieve extraordinary effects, the basic idea of a glowing surface is something we use every day. It is the same technique whether designing backlighted signs, luminous ceilings, or evenly glowing walls.
Light transmission through any medium consists of clear and diffusing components. Being imperfect, all media have some of both, the percentage of each determining whether the material is seen as transparent (mostly clear), translucent (mostly diffusing), or obscure (similar amounts of each). As the light source moves closer to the medium, the medium must become denser and less transmissive in order to hide individual lamps. The design challenge often is to find the best combination of distance, transmission efficiency, and image-hiding capability.