a device for applying light to an inner surface of a cylinder , comprising a homogenizer , into which light can enter and from which the light can exit , wherein the homogenizer has a cylindrical internal surface , on which the light can be reflected after entering and before exiting , and also comprising ways for introducing light into the homogenizing means , and focusing arrangements , which can focus light exiting from the homogenizer onto the inner surface of the cylinder to which light is to be applied .