http://numb3r23.github.io/GLSLShaderLib/examples/webgl/ModifiedLambert.html

You can clamp the illumination into the “brighter” colors that is expanded into the (usually unlit) back areas and thus allows you lit the whole object. The light can be toggled between a (fixed) lightsource and the camera-position. This is usefull if you (cheaply) don’t want to show “dark parts” of 3d-models while still showing the structure. It can be further tweaked (not implemented yet!) by adding gamma or a two-tone mapping.