One of the many examination techniques used when planning a paintings conservation treatment is raking light, where a light placed at one side of the painting and the light “rakes” across the surface.  It can reveal details that are not readily visible in normal lighting.  Here is a detail of an early 20^th c. painting, shown in normal light and again with a raking light from the left.  Though the “impasto,” or raised brushstrokes, are visible in normal light, the texture is enhanced by the raking light. 

More importantly, from a conservation point of view, the raking light also reveals the series of vertical cracks that are “cupped,” or slightly concave, on the paint surface (most easily seen on the left side of the image).  The cracks are virtually invisible in normal light.  In this painting they are an indication that the canvas is not under even tension on the stretcher.  It’s a discovery that our paintings conservator will plan to address during the conservation treatment.