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What Is VLT- Visible Light Transmittance?
Poor window design can reduce the amount of daylight entering your home.
The proportion of light transmitted by glazing is specified by the Visible Light Transmittance, which is recorded as either TVw (visible transmittance of the window) or VLT (visible light transmittance). These terms are interchangeable, but in Australia, the Window Energy Rating Scheme uses Tvw in its ratings.
While the VLT/Tvw rating varies between 0 and 1, most double glazed windows rate between 0.3 and 0.7 , which means that between 30% and 70% of the available light passes through the window. The illustration above shows you how the view changes, as the amount of light getting through the window changes.
The higher the value, the better. Why? It means more light is getting through the window.
A simple way to consider this is:
Low Tvw = too dark inside = lights on = higher energy bills.
High Tvw = brighter inside + better views outside.