WINDOW TECHNOLOGIES: Glass

Visible Transmittance Center-of-glass visible transmittance values of double pane units.

Visible Transmittance (VT or Tvis)

Visible transmittance) is the amount of light in the visible portion of the spectrum that passes through a glazing material. A higher VT means there is more daylight in a space which, if designed properly, can offset electric lighting and its associated cooling loads. Visible transmittance is influenced by the glazing type, the number of panes, and any glass coatings. Visible transmittance of glazing ranges from above 90% for uncoated water-white clear glass to less than 10% for highly reflective coatings on tinted glass. A typical double-pane IGU had a VT of around 78%. This value decreases somewhat by adding a low-E coating and decreased substantially when adding a tint (see figure to the right). VT values for the whole window are always less than center-of-glass values since the VT of the frame is zero.

Light-to-Solar-Gain Ratio
In the past, windows that reduced solar gain (with tints and coatings) also reduced visible transmittance. However, new high-performance tinted glass and low-solar-gain low-E coatings have made it possible to reduce solar heat gain with little reduction in visible transmittance. Because the concept of separating solar gain control and light control is so important, measures have been developed to reflect this. The LSG ratio is defined as a ratio between visible transmittance (VT) and solar heat gain coefficient (SHGC).

LSG

The image illustrates the center-of-glass properties for the options used in the Facade Design Tool. A double-glazed unit with clear glass (B) has a visible transmittance (VT) of 0.79 and a solar heat gain coefficient (SHGC) of 0.70, so the LSG is VT/SHGC = 1.12. Bronze-tinted glass in a double-glazed unit (C) has a visible transmittance of 0.45 and a solar heat gain coefficient of 0.50, which results in an LSG ratio of 0.89. This illustrates that while the bronze tint lowers the SHGC, it lowers the VT even more compared to clear glass. The double-glazed unit with a high-performance tint (E) has a relatively high VT of 0.52 but a lower SHGC of 0.29, resulting in an LSG of 1.80—significantly better than the bronze tint. A clear double-glazed unit with a low-solar-gain low-E coating (H) reduces the SHGC significantly, to 0.27, but retains a relatively high VT of 0.64, producing an LSG ratio of 2.4—far superior to those for clear or tinted glass.

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