A structurally robust beamsplitter that has substantially constant transmittance values over a wide range of wavelengths includes a freestanding monolithic synthetic polycrystalline diamond optical plate that has a thickness in the range of about 1 .mu.m to about 1 mm. The optical plate further typically has a transmittance in the range between 55% and 75% for incident radiation over almost all wavelengths of radiation from the infrared to the visible. A beamsplitter in accordance with the present invention is readily adapted for use in a interferometric spectroscopy system, such as a system using infrared and far infrared radiation for spectroscopic analysis.