A filter spectrometer using dielectric bandpass filters eliminates Rayleigh scattering down to 10-5 while passing more than 90% of Raman signal from 50 to 2000 cm-1. It improves the optical throughput about 8-9 times in comparison to a double-stage subtractive monochromator, and about 20% in comparison to a holographic notch filter. Moreover, transmittance of the filter spectrometer does not depend on polarization of the incident light at the optimum angle. Raman spectra from powder sample of bismuth oxide and copper phthalocyanine and liquid sample of carbon tetrachloride in a glass capillary were easily observed down to 50 cm-1 with the use of the filter spectrometer combined with a singlestage monochromator.
Masayuki Futamata, "Dielectric Filter for Highly Sensitive Raman Spectroscopy," Appl. Spectrosc. 50, 199-204 (1996)