Abstract: A heat spreader comprising a sheet of transparent diamond with an aperture therein that accommodates a solid-immersion lens (SIL). The heat spreader may be mounted within a clamp which allows the heat spreader to move freely across the Device Under Test (DUT) while maintaining a very high degree of planarity and contact between the diamond and the silicon substrate of the DUT. The DUT is secured to its electrical interface with a low profile clamp, the DUT may be held within the clamp by a mechanism that applies a pressure to the sides of the DUT package.