Why not hemispheres as Internal Reflection Elements?

Occasionally, we’ve been asked if our cells are able to be used with hemispherical ATR elements. While there is no fundamental reason preventing the use of a hemisphere in one of our cells, the performance of hemispheres is substantially worse than the Face-Angled Crystals (FACs) that we recommend. This performance difference is apparent by looking at the single channel energy spectrum of the two elements:

Figure 1: Energy curves to compare throughput of internal reflection elements.

Both spectra were collected with the same 2 mm aperture and 128 co-additions. The spectrum collected using the FAC had 12 times the spectral power of the spectrum collected with the hemisphere. Both spectra were collected at an angle of incidence of 60 degrees, which was the face angle of the FAC.

The main reason for the difference in throughput is that the hemisphere is a focusing optic, while the planar surfaces of the FAC do not alter the shape of the wavefront. The stock VeeMAX III configuration is designed to focus the beam onto the surface of a flat mirror or the face of a prism, and the extra optical power provided by the curved surface of the hemisphere reduces the amount of light which is directed onto the detector by the mirrors of the VeeMAX. The anti-reflection coating further helps to boost the throughput compared to an uncoated hemisphere by minimizing reflection losses at the two air/Si interfaces.

For more additional information comparing the performance of a hemispheres and FACs using the VeeMAX II, see the following article:

Sigrist JA, Lins ES, Morhart TA, Briggs JL, Burgess IJ. Optimization of a Commercial Variable Angle Accessory for Entry Level Users of Electrochemical Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS). Applied Spectroscopy. 2019;73(12):1394-1402. 10.1177/0003702819858353