Daylight Design - Glazing
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When not reflected, light is either transmitted through the glazing or absorbed. Clear glazing absorbs very little visible light, while dark-tinted glazing absorbs considerably.
Typical 6 mm clear glazing absorbs only about 7% of sunlight at a normal angle of incidence (0°). The light absorptance of glazing increases by glazing additives that absorb solar energy. When they absorb visible light, the glazing appears dark. There will be little or no change in visual appearance when absorbing light in ultraviolet radiation or near-infrared wavelength ranges.
Visible transmittance refers to the percentage of light that can pass through glazing that has not been reflected or absorbed. While we are sensitive to light at wavelengths from about 400 nm to 700 nm, with peak sensitivity at 550 nm, we are less sensitive at the red and blue ends of the spectrum. We refer to this as the photopic sensitivity of the eye.
Visible transmittance determines the effectiveness of a type of glazing in providing daylight and a clear view through the glazing system. Influenced by the glazing type, the number of panes and any glazing coatings, tinted glazing has a lower visible transmittance than clear glazing.
Typical values range from above 90% for uncoated low iron clear glazing to less than 10% for highly reflective coatings on tinted glazing. A typical double-glazed unit without coatings has a visible transmittance of around 78%. Adding a low-E coating or a body tint to the glazing decreases this value. We typically aim for values between 45 – 60% for commercial office buildings and higher for residential buildings.
Glazing suppliers can also go one step further to alter glazing properties via high-performance body tints and low-E coatings. It is now possible, particularly with splutter coatings, to reduce solar heat gain with little reduction in visible transmittance. We call this high spectral selectivity, and it is defined by the light-to-solar (LSG) gain ratio.
LSG is a ratio between visible transmittance and solar heat gain coefficient (SHGC). In absolute percentage terms, a ratio greater than 1 signifies that the daylight passing through the glazing is more than the sun’s direct heat passing through it, with values of 1.25 and above indicating good spectral reflectivity and values around 2.0 reflecting high spectral reflectivity.
Some daylight will always be reflected on every glazing surface, interior and exterior. All materials lie somewhere between completely matte and totally gloss, with some portion of both diffuse and specular reflection subject to the roughness or smoothness of the finish.
The natural reflectivity of glazing depends on the type of glazing material, the quality of the glazing surface, the presence of coatings and the angle of incidence of the receiving light. This reflectivity is specular for glazing, where the angle of reflection is always equal to the angle of incidence mirrored around the surface normal. For a specular reflection to be noticeable, the amount of light reflected in the specular direction must be significantly greater than that which would be expected in that direction from a diffuse distribution.
Non-glazed matte surfaces reflect radiation, but as diffuse as well as specular reflections in which the reflected light or radiation is distributed over a wide range of directions rather than just a single direction. This occurs because the surface has small irregularities and is not completely flat, causing it to bounce off at many different angles.
A single pane of clear, uncoated glazing reflects 4% of visible light at each glazing-air interface or 8% in total when the angle of incidence is at 0°. The sharper the angle at which the light strikes, the more the light is reflected rather than transmitted or absorbed. Even clear glazing reflects 50% or more of the daylight, striking it at incident angles greater than about 80°. Therefore, while typical design limits of 20% solar reflectance at a 0° angle of incidence are often required, these are always exceeded due to the angle of incidence variability.
The reflectivity of various glazing types becomes especially apparent during low light conditions when the surface on the brighter side acts as a mirror because the amount of light passing through the glazing from the darker side is less than the amount of light being reflected. This effect can be noticed from the outside during the day and from the inside during the night. Special coatings can virtually eliminate this reflective effect, such as low iron glazing, for special applications when these surface reflections are undesirable (i.e., viewing merchandise through a store window on a bright day).
Most common coatings reflect in all regions of the light spectrum. However, coatings can now be applied to glazing to reflect only selected wavelengths of radiant energy preferentially. Varying the reflectance of far-infrared and near-infrared energy has formed the basis for higher-performance low-E coatings.
