On-Axis lights are used to illuminate reflective objects.  The mail problem with imaging reflective objects is that reflections of the light and camera can often be seen in the resulting image.  On Axis illumination uses beam splitting techniques to enable the illumination to be positioned between the camera and the object so that the camera is situated behind the light. Cloudy Day type on-axis lights are the most capable of all on axis techniques.  They comprise of a hemi-spherical dome which produces light from all angles except from the hole in the top.  An on axis light with a beam splitter is positioned over the hole and this projects light from the position where the camera is situated.  This technique is sometimes called Cloudy Day Illumination (CDI) because it produces light from all directions without shadows but the correct name for it is Continuous Diffuse Illumination.  It can be used to illuminate very complex and highly reflective objects.  Light is optimised over an area covering 50% of the linear dimensions and the unit generally needs to be positioned as close as possible to the object.