If what you want is to imitate having specks on the lens that get brighter and then fade as the camera angle changes in relation to the light source, I’d be inclined to fake it using a separate Scene with some speck objects and a light source keyed to sweep across them in coordination with the camera move in the main Scene. Then in the Compositor they could be mixed with appropriate nodes, if you want it to render in a single pass. Same could be done with separately-rendered image sequences in a post-rendering setup in the Compositor. The Compositor will give you a great deal of extra control over how the final effect looks.
For additional effects like lens scratches, passes on variously textured objects could be composited in similar fashion.