Usually, a surface of interest is lit by beams of light that strike the surface at 90 degrees or close to 90%. Tangential lighting is when he light beams strike the surface with a very small angle--the light source is placed to one side of the viewing area. This techniques highlights small contours and produces long shadows for small protrusions. Medical doctors have found some useful applications for tangential lighting.
Tangential lighting is when the light strikes the illuminated surface at a very low angle--the light is low to one side of the lighted surface. This is done to enhance any small imprefections on the otherwise flat surface. Shadows from the tangental lighting highlight any small protrusions or shallow contours.
Finding Small Objects
When a small object is lost in a dark inaccessible place (like underneath a bed) Tangential light can sometimes be used to find it, such as by putting a light source down on the ground and looking for the long shadows. Small objects tend to disappear when placed on a same color background. The tangential light produces a long black shadow; this will not work with a black object on a black background.
Tangential light is used for certain medical exams. For example, when examining the throat for beginning goiters and other burgeoning glandular disorders, physicians use tangential light. This lighting expose the small differences in surface that can mean impending trouble.
Minimizing Light Contamination
In Europe, many outdoor artifacts are lighted for night viewing. When it was realized that the continued exposure to artificial light was damaging the artifacts, the lighting was switched to tangential lighting, allowing the artifacts to be clearly visible with a minimum of direct lighting.
Plastic Surgery Pre-exams
When planning a plastic surgery, a contour area map is made with tangential lighting. There is no other way to get a clear picture of how the muscles lay underneath the skin. The clearer the surgeon's picture is of what lays under the skin, the less destructive the incisions will be.