The UK law regarding window film states that the front side windows (i.e. the windows either side of the driver’s head) must have a VLT (visible light transmission) of greater than 70%. That means that at least 70% of light must pass through the window. Most modern car windows are made of glass with a 80-70% VLT, so even a very light film applied to the front windows will unfortunately take the VLT the wrong side of 70% and therefore will not be legal. The windscreen by law must have a VLT of greater than 75%, even lighter than the sides, so again, even a very lightly tinted window film would not be legal.
All windows behind the driver have no such law applying to them, so all levels of tint are perfectly legal.
An instrumented check is performed by a suitably trained officer with a ‘TintMan’ VLT meter. These checks are normally from VOSA roadside campaigns.
65%-46% VLT: Advise only – The driver will be advised that the legal requirements have been breached.
45%-30% VLT: Delayed prohibition – The driver will be given a prohibition notice and will usually have 10 days to have the film removed before going to a vosa testing station to have the VLT re-checked.
<30% VLT: Immediate prohibition – The vehicle is considered dangerous and cannot be driven until the film is removed.
An officer with no Tintman can make a subjective assessment by sitting in the driver’s seat with the doors closed. If the tints are very dark and restrict visibility, they will be considered dangerous and the officer will issue an immediate prohibition (as <30% VLT above). If the level of visibility is not obviously dangerous then the driver will be advised that the legal requirements may have been breached, thus putting the onus on the owner to investigate further.