New MOT guidelines issued in 2012 state that "products on the headlight or lens that obviously reduces the lights intensity or changes its colour will become a reason for failure". COVEX has no effect whatsoever on the light shape and colour. It has also been manufactured to have as minimal effect as possible on light output with a VLT (visible light transmission) of 80%. Please note however that light output may vary depending on vehicle model and age.
For further details on MOT Guidelines please view link below:http://www.ukmot.com/mot%20manual/index.asp?MOT=Headlamps&MOT_Number=1.7&MOT_Section=lights
See also AA notes at http://www.theaa.com/motoring_advice/car-servicing-repair/mot-changes-2012.html