Legal tint limits - (the minimum percentage of light that must filter through windows).

Front side windows - must have 70% light visibility.

Your car may already have a manufacturers tint in the glass, in which case a film with 78% light visibility can possibly be installed and you will still be within the law.

B posts back - no restrictions on tinting.

Vehicles with tinted front windows can be stopped and tested for opacity and if a vehicle is found to be over the legal tint limit enforcement action may be taken.  In the most serious cases the vehicle will be taken off the road until the tint is removed.

 Motorists who do not rectify the problem will be prosecuted. 

VOSA are currently carrying out regular roadside checks in Norfolk.  Special equipment will be used to take an accurate measurement of the light passing through the glass.