Color By definition, the color of daylight is essentially perfect, with a color rendering index (CRI) of 100. However, almost every glazing system has a color characteristic, from very subtle hints of tint to strong variations of green, gray, bronze, aqua, and blue. Remember, just like any filter, the transmitted color is the opposite of the color that is most absorbed. Blue glass, for example, absorbs yellow light. While the color impact is often slight, a heavily tinted glass will cause the perceived color of daylight to change considerably, reducing the CRI of daylight considerably. For example, some green-glass systems have a CRI of 60, about as poor as old-fashioned “cool white” fluorescent.
An important, but far more subtle, color effect occurs in glazing with spectrally selective coatings, which is how low-E glazing is made. The purpose of these coatings is to cause the glazing to reflect infrared energy away from the building while permitting visible light into the building. If a glazing system has LSG>1.0, then it has a low-E coating. The challenge of low-E coatings is to pass visible red without passing invisible infrared energy. The wavelengths are nearly the same; the better the infrared heat rejection (LSG>1.0), the more likely that the low-E coating will also reject some visible red light. In other words, in order for a glazing system to have very high performance, it must, by nature, appear to be at least slightly bluish. When comparing glazing assemblies, do not be surprised that the highest performing products, even if sold as “clear,” lack the warmth of glazing systems with LSG<1.2.
Clarity The lack of coloration combined with a high VLT results in windows that appear especially clear. Premium, low-lead glass is used, usually as part of an assembly that includes coatings to reduce reflectance and solar heat gain. The key trait of low-lead glass is that it lacks the greenish tint of typical window glass, with results that are often considered spectacular and worth the extra cost. The key benefit is apparent sharpness and color dynamics; with minimum tendency to impart its own coloration, the color rendering of light is almost unaltered.
The larger question of whether a window appears adequately clear is often debated. In addition to low-lead glass, for clarity a high VLT is preferred. From my 30-plus years of work experience, I find that most architects feel that a minimum VLT of 50 percent is required for a window to appear adequately clear; personally, I believe the threshold is lower, with an absolute minimum of 35 percent before the window’s sense of clarity is lost. So among the challenges of choosing glazing, one is typically faced with choosing the highest possible VLT for clarity while keeping the lowest possible VLT for energy efficiency. A related consideration is brightness; even the brightest computer screen is less bright than the darkest window by day, and there is a genuine concern that windows that are too clear will affect office work, especially on sunny days.
Risking both significant solar gain and brightness problems, the new New York Times headquarters in New York City is clad in glass with VLT over 70 percent in order to achieve “transparency,” a key design statement of the building. The project relies heavily on interior shading systems to help control both heat and glare problems. Given the amount of publicity on the design of this critically important project, I hope that post-occupancy evaluations will test both energy and brightness aspects of the glazing system to help better understand the functional implications of high-clarity glazing.
Reflectance All glass reflects at least some amount of light. With typical window glass, the reflectance is often about 5 to 10 percent. Especially if the space behind the glazing is relatively dark, such reflectance appears as a mirror, defeating the sense of transparency. If the glass itself is also dark (VLT<.50), the effect is increased.
Glass can be coated to increase or decrease its reflectivity. Most architects and lighting designers are familiar with highly reflective glass used to create dramatic curtain walls; the reflectivity can also take on a tint to give the building a distinctive color. However, highly reflective coatings dramatically reduce the sense of transparency. Anti-reflective coatings, on the other hand, can be employed to decrease reflectivity to about 2 percent. Under almost any conditions, this glass tends to appear clearest.
Iridescence Iridescence is a quality associated with glass coatings, anodizing, and other special materials in which slight “rainbowing” or other three-dimensional color effects are seen, usually as a function of the viewing angle. In glazing, iridescence is caused by low-E coatings. Low-E is actually a dichroic coating, designed to reflect both long-wave (infrared) and short-wave (ultraviolet) light while passing the visible spectrum. As with any dichroic coating, there is a range of optimal angles, beyond which incident light can be reflected with secondary dichroic effects. When combined with reduced VLT, iridescence can cause strange three-dimensional reflectance, resulting in a glazing situation that feels more like a fishbowl than a window.
Fritting For use as a solar gain control element, frits are a pattern of white reflective shapes embedded into the interior surface of the exterior pane of a glazing system. Typically made of ceramic, their purpose is to reduce the transmission of the glazing by reflecting a desired percentage of total light away from the building. Frit patterns can be as simple as dots, or they can be elaborate works of art. Because frits reflect light, they can serve as an important solar gain control method. Note that frits are not diffusing, like frosting or acid-etching, which are generally used on interior glass to make the panes translucent. Rather, frits simply reduce the effective size of the glazing system, allowing larger actual panes of glass that optically behave like smaller panes.
A variation of fritting is to employ photovoltaic (PV) cells. In addition to reducing the amount of transmitted light, the PV cells generate electricity. For south-facing windows and clerestories, PV frits are a good way to get significant double benefit from a single investment. The Lillis Business Complex at the University of Oregon in Eugene is one such example of a project where photovoltaic cells were used as a fritting device. (See “Daylighting Gets to Work,” Jan/Feb 2005.)