A computer receives a first image of a plurality of lamps. The computer receives a second image of a plurality of lamps. The computer transforms the second image so a location depicted in a plurality of pixels of the first image is depicted in a corresponding plurality of pixels in the second image. The computer determines a brightness variation exists between a pixel of the plurality of pixels of the second image and a corresponding pixel of the plurality of pixels of the first image. The computer identifies a location corresponding to the pixel of the plurality of pixels of the second image where the brightness variation exists.