Combined aircraft navigation and anti-collision light , comprising: at least one navigation light source ; a navigation light sensor , in operation outputting a light detection signal corresponding to an amount of light detected by the navigation light sensor ; a first optical system , associated with the at least one navigation light source , wherein the first optical system is configured for shaping a navigation light output from light emitted by the at least one navigation light source and is configured for directing stray light from the at least one navigation light source to the navigation light sensor ; at least one anti-collision light source ; a second optical system , associated with the at least one anti-collision light source and configured for shaping an anti-collision light output from light emitted by the at least one anti-collision light source ; a lens cover , arranged over the at least one navigation light source , the first optical system , the at least one anti-collision light source , and the second optical system for passing the navigation light output and the anti-collision light output therethrough, wherein the navigation light sensor is arranged with respect to the lens cover to detect light emitted by the at least one anti-collision light source that is diffusely reflected by the lens cover ; and a controller , coupled to the navigation light sensor and configured to determine a state of erosion of the lens cover from a pulsed signal component of the light detection signal , the pulsed signal component being caused by light emitted from the anti-collision light source that is diffusely reflected by the lens cover towards the navigation light sensor . at least one navigation light source ; a navigation light sensor , in operation outputting a light detection signal corresponding to an amount of light detected by the navigation light sensor ; a first optical system , associated with the at least one navigation light source , wherein the first optical system is configured for shaping a navigation light output from light emitted by the at least one navigation light source and is configured for directing stray light from the at least one navigation light source to the navigation light sensor ; at least one anti-collision light source ; a second optical system , associated with the at least one anti-collision light source and configured for shaping an anti-collision light output from light emitted by the at least one anti-collision light source ; a lens cover , arranged over the at least one navigation light source , the first optical system , the at least one anti-collision light source , and the second optical system for passing the navigation light output and the anti-collision light output therethrough, wherein the navigation light sensor is arranged with respect to the lens cover to detect light emitted by the at least one anti-collision light source that is diffusely reflected by the lens cover ; and a controller , coupled to the navigation light sensor and configured to determine a state of erosion of the lens cover from a pulsed signal component of the light detection signal , the pulsed signal component being caused by light emitted from the anti-collision light source that is diffusely reflected by the lens cover towards the navigation light sensor . <EOS>