For the research glass house in Dookie, The point of glass and windows is to let light pass through. The percentage of visible light that passes through a window or other glazing unit is called the Visible Light Transmittance (T vis or VLT). An opaque wall would have a Tvis of 0%, while an empty opening would have 100%; many un-tinted glass and plastic materials have a Tvis of 90% or more. Tvis does not measure shorter-wavelength light like UV or longer-wavelength light like infrared--only visible light. More light is often not better, as it can cause glare and overheating. Tints, frits, and coatings can be chosen to produce any Tvis; common values are often 30 - 80%. Tvis is influenced by the color of the glass (clear glass has the highest
