A method performed by a luminance assessment system of a vehicle for monitoring of on-board vehicle image capturing device functionality compliance with a predeterminable requirement level, said vehicle comprising two or more image capturing devices adapted to capture surroundings of said vehicle, a first image capturing device having a first field of view and a second image capturing device having a second field of view with a primary region thereof at least partly overlapping said first field of view, said method comprising: obtaining at respective one or more time instants (t 1, t 2, t 3), a respective first image (311, 312, 313) with support from said first image capturing device and a respective second image (321, 322, 323) with support from said second image capturing device, a first section of said respective first image (311, 312, 313) and a second section of said respective second image (321, 322, 323) respectively covering said primary region; measuring for respective first image (311, 312, 313) a respective first luminance value (3111, 3121, 3131) of said first section, and for respective second image (321, 322, 323) a respective second luminance value (3211, 3221, 3231) of said second section; determining a luminance deviation based on comparing at least one of said respective first luminance values (3111, 3121, 3131) to at least one of said respective second luminance values (3211, 3221, 3231); determining with support from a positioning system a position of said vehicle; localizing based on said vehicle position, said vehicle in view of a digital map; determining whether there in said digital map is presence of an image luminance affecting element, such as e.g. a tunnel and/or overpass, within image capturing device range of said vehicle; and determining, when said luminance deviation exceeds a deviation threshold and when there is determined no presence of an image luminance affecting element, that said first image capturing device or said second image capturing device fails to function according to said requirement level. obtaining at respective one or more time instants (t 1, t 2, t 3), a respective first image (311, 312, 313) with support from said first image capturing device and a respective second image (321, 322, 323) with support from said second image capturing device, a first section of said respective first image (311, 312, 313) and a second section of said respective second image (321, 322, 323) respectively covering said primary region; measuring for respective first image (311, 312, 313) a respective first luminance value (3111, 3121, 3131) of said first section, and for respective second image (321, 322, 323) a respective second luminance value (3211, 3221, 3231) of said second section; determining a luminance deviation based on comparing at least one of said respective first luminance values (3111, 3121, 3131) to at least one of said respective second luminance values (3211, 3221, 3231); determining with support from a positioning system a position of said vehicle; localizing based on said vehicle position, said vehicle in view of a digital map; determining whether there in said digital map is presence of an image luminance affecting element, such as e.g. a tunnel and/or overpass, within image capturing device range of said vehicle; and determining, when said luminance deviation exceeds a deviation threshold and when there is determined no presence of an image luminance affecting element, that said first image capturing device or said second image capturing device fails to function according to said requirement level.