Until 2014 Association of Chief Police Officers (ACPO) guidance, supported at a local level,was for officers to ignore dark visors in bright sunshine, but to pull over and advise anyone riding in fading light with a dark visor.
The 2014 the ACPO ‘motorcycle safety strategy’ is silent on dark visors so it boils down to the constable’s discretion. The only authorised device for testing how much light goes through glass is the Tint Man’, which is calibrated for glass and for nothing other than a flat surface. Any test has to be administered by an officer trained in the use of what is a simple and portable device. The officer then needs to ask the CPS to prosecute you, and unless the Crown can show that the Tint Man’ is calibrated and authorised for curved polycarbonate as opposed to flat glass then the prosecution will fold. Curvature is important, because a curved clear visor will block 10% to 20% of available light without any tint at all.
In 30 years of riding I have never been pulled over for a dark visor, but on some forums it is suggested that if you are riding with a dark visor your insurance will be invalidated. This is not correct. Unless your insurer specifically asks you ‘Do you ride with a dark visor?’ and you answer ‘No,’ only then will your riding in a dark visor get you into insurance difficulties.
Notwithstanding all of the above, it is never a wise idea to goad the police. You have to prove your visor meets the British standard test and is properly marked as reaching the British standard, which no dark visor will do. You will have committed an offence under Section 18 of the Road Traffic Act 1988 and if you are riding at dusk or in darkness with a dark visor you could find yourself facing a charge of dangerous driving. Riding at night with a dark visor falls so far below the standards of a reasonably prudent driver that the mere fact of riding at night with barely any vision would be dangerous riding, which leads to a mandatory one year ban and a compulsory retest for all classes of licence.
Also, the police officer can confiscate your £60 iridium visor as its continuing use constitutes a road traffic offence. The police and British motorcyclists have come to an accommodation about dark visors: provided you’re not going to be a dick about it, the police won’t be either. In the summer I ride with a dark visor and keep a clear one with me. I am breaking a law which is difficult to prosecute, and which even the police hold in scorn.