Light spill or light pollution is excessive, misdirected, or obtrusive artificial light, often emitted from buildings. If there is too much light pollution, it can wash out starlight in the night sky, interfere with astronomical research and disrupts ecosystems.  

Reducing light spill, or Visible Light Transmission (VLT) from properties can be an important ecological consideration for some projects, for example where there is a requirement to mitigate the impact of artificial lighting on bats. The Bat Conservation Trust advises that low transmission glass may be a suitable option in achieving reduced illuminance targets. Depending on the height of the building and the position of the windows, and therefore predicted light spill, low transmission glass may not be required on all storeys. For projects where this is critical element of the design, a suitably qualified lighting professional may be able to provide guidance. 

To reduce light spill, tinted glass such as Pilkington Optifloat™ Tint tends to be used due to its low light transmittance; several of the products in the Pilkington Suncool™ Range also have low light transmittance. The lower the light transmittance of the glass, the less light will be transmitted through to the other side. This usually results in darker and/or more reflective glass, which may not be desirable from a visual appearance. Of course, even with a darker glass, the brighter side will tend to be more visible through the glass (like a ‘one way’ vision effect). If this a critical factor, then it may be worth considering additional shading (e.g. blinds, shutters, etc.) to cover the periods when it is dark outside and the lights are switched on inside.

If Pilkington Optifloat™ Tint is specified as the outer pane of an insulating glass unit, then this could be combined with a low-e inner pane such as Pilkington K Glass™ S or Pilkington Optitherm™ S1 Plus for thermal insulation.

If Pilkington Suncool™ is specified as the outer pane of an insulating glass unit, then it is not necessary to specify an inner pane of low-e glass.