The value of diffuse reflectance as an infrared or near-infrared spectroscopic sampling procedure has been limited by the low efficiency of accessories designed for it. In terms of signal-to-noise ratio, these average 2-6% for integrating spheres and 10-12% for various ellipsoidal mirror arrangements. Much better performances, up to 37% efficiency, can be obtained by optimizing a concentric confocal ellipsoidal mirror arrangement by using a very large central opening in the amular collector mirror, and adapting the throughput of the detector to the geometry of the collected beam.
T. Hirschfeld, "Optimization of Diffuse Reflectance Infrared Spectroscopy Accessories," Appl. Spectrosc. 40, 1082-1085 (1986)