Light Transmission

The light transmission (LT) qualities of complete rooflights vary according to the glazing configuration and LT of the materials used. As it passes through each layer, the light transmitted is reduced cumulatively, as illustrated in the diagram.
The four charts illustrate the percentage reduction in available light within a building using various glazing materials and rooflight configurations. The intermediate and inner skins are assumed to be clear polycarbonate where appropriate.

Light Transmission




Solid Polycarbonate



Single skin - recommended only for unheated areas.
Double skin - non-preferred as it does not meet current Part L requirements.
Triple skin - meets Part L and gives improved sound insulation.
Quadruple skin - further improvement of performance characteristics.

Please note: there are substantial differences between various types of polycarbonate with ‘Opal’ giving only half the LT of clear as opposed to the popular ‘Diffused’ which transmits just 1% less than clear with a much improved light spread.


Structured Multi-Wall Sheet



Available in a range of thicknesses to suit thermal and other requirements – a selection shown.


Lumira® Sheet



The latest advanced glazing technology with excellent thermal, light diffusion and acoustic performance characteristics.