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.

In advanced form, this comprises a similar arrangement to the simple form (a 50% beamsplitter at a 45 degree angle) but the beam splitter is positioned further up into the light in a reflection chamber so that no light can be projected straight onto the object.  As for the simple type, the camera looks straight down through the beamsplitter from above – this is arrangement is sometimes called an SCDI (Square Continuous Diffuse Illuminator). This technique can be used to illuminate highly reflective objects that have some profile (not flat).  Again, 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.