Solar Spectrum
The solar spectrum, commonly referred to as sunlight, consists of ultraviolet light (UV), visible light and infrared light (IR). The energy distribution within the solar spectrum is approximately 2 percent UV light, 47 percent visible light and 51 percent IR light (see Figure). One aspect of the solar spectrum is its wavelength in which nanometer (nm) is the unit of length [1 nm = 10-9 m].

UV light is invisible to the human eye and has a wavelength range of ~300 - 380 nm. The damaging effects of long-term UV light exposure results in fabric fading and plastic deterioration.

Visible light is the only portion of the solar spectrum visible to the human eye. It has a wavelength band of ~380 - 780 nm. IR light is invisible to the human eye, has a wavelength range of ~790 - 3000 nm and has a penetrating heat effect. Short-wave IR light converts to heat when it is absorbed by an object.

Heat Transfer Methods
Heat transfers from one place to another via convection, conduction or radiation. Convection occurs from the upward movement of warm, light air currents. Conduction occurs when energy passes from one object to another. Radiation occurs when heat is sent through space and is capable of traveling to a distant object where it can be reflected, absorbed or transmitted (see Figure).

Solar Energy
When solar energy meets glass, portions of it are reflected, absorbed or transmitted-giving you the RAT equation (see Figure 1).

RAT Equation
The RAT equation accounts for 100 percent of solar energy, which is equal to the sum of solar reflectance, absorption and transmittance. For example, with a single pane of 1/8" (3 mm) clear glass, 83 percent of solar energy is transmitted, 8 percent is reflected and 9 percent is absorbed by the glass. Of the solar energy absorbed, portions are emitted back towards the exterior and towards the building interior (see Figure 2).

Solar Control
The visible light and IR portions of solar energy are an essential part of sunlight, since they represent nearly 100 percent of the solar spectrum. As a result, each plays an important role when glass is selected as a glazing material for commercial building applications. To enhance thermal performance, thin metallic films are applied to one or more glass surfaces.

Low-Emissivity Coatings (low-e)
Low-Emissivity coatings, which are applied to glass, reflect invisible long-wave infrared or heat. They reduce heat gain or loss in a building by redirecting the heat. In addition, they provide greater light transmission, low reflection and reduce heat transfer.