[hennabm]

From the windscreen company website;

Tinted front windscreens and front side windows are subject to legal restrictions. Cars that were first used on or after 1st April 1985 must let at least 75% of light through the front windscreen and at least 70% of light through the front side windows. Older cars that were first used before 1st April 1985 must let at least 70% of light through both front windscreen and front side windows.

Many drivers and owners mistakenly calculate that this means a window tinting of 30% is acceptable. This is not the case as windows without tinting do not let in 100% of light; even clear glass only lets in around 70% to 80% of light.

Applying 30% window tinting will then drop the light passing through to well below the legal limit and this will be picked up by police or DVLA light meters.