Integrating cavity spectroscopy is a highly sensitive technique for weak absorption measurements.  Basically, an integrating cavity is a closed container whose interior wall has a very high diffuse reflectance (Lambertian).  If light energy is supplied to an integrating cavity containing an absorbing sample, the equilibrium optical energy in the cavity provides a measure of the sample absorption.  Since the diffuse reflecting wall of the cavity produces an isotropic illumination of the sample, scattering in the sample cannot change this isotropic illumination.  Scattering also cannot change the amount of light energy in the cavity; as a result, absorption measurements are independent of scattering.  The many reflections of the light from the cavity walls mean the light also makes many many transits through the sample in this technique, i.e. the effective path length through the sample is much greater than the dimensions of the sample, again resulting in a high sensitivity to very weak absorption. 
Cavity ring down spectroscopy (CRDS) is a very different and and well-known technique for high sensitivity absorption spectroscopy.  In CRDS, the absorbing sample is placed between two high reflecting mirrors and a pulse of light is reflected back and forth through the sample many times.  Sample absorption reduces the pulse energy and the long effective path length (due to the multiple reflections passing through the sample) makes this an extremely sensitive technique for weak absorption.  But scattering also reduces the pulse energy and can lead to a significant systematic absorption error.  Nevertheless, this is a widely used and powerful technique when scattering is negligible. 
One could expect that combining these two absorption spectroscopy techniques would provide an extremely powerful and useful new technology - Integrating Cavity Ring-Down Spectroscopy (ICRDS).  But, ICRDS has not previously been exploited because the diffuse reflectivity of all known materials was simply not high enough to do ring-down spectroscopy in an integrating cavity.  We have now developed a new material (basically compressed quartz powder) that does have the high diffuse reflectivity (0.9992 at 532 nm) required for ring-down spectroscopy in an integrating cavity.  In a particularly exciting demonstration, the first successful spectral absorption measurements were made on retinal pigmented epithelium (RPE) cells; this is a biological sample whose absorption coefficient has proved difficult to measure due to its large scattering cross section.
ICRDS will open new research vistas by providing very sensitive and accurate direct spectral absorption measurements of both a sample and any particulates suspended in it while being unaffected by the scattering in the sample.

Dr. Edward Fry, Distinguished Professor and former Head of the Department of Physics & Astronomy at Texas A&M University, holds the George P. Mitchell Chair in Experimental Physics and has been with Texas A&M since 1969. He is Past-Chair of the Texas Section of the American Physical Society, and is a fellow of both the American Physical Society and the Optical Society of America. He received the Association of Former Students Distinguished Faculty Teaching Award (1993), the Texas A&M Distinguished Scientist Award of Sigma Xi (2001), and the Association of Former Students Distinguished Faculty Achievement Award (2012). Dr. Fry’s research interests cover the gamut from basic research to applied research. Some notable achievements include: (i) one of the first, and definitive, Bell inequality tests of the foundations of quantum mechanics – addressing questions first raised by Einstein; (ii) the first observations of Lasing Without (population) Inversion (LWI); (iii) a new integrating cavity technique for the measurement of optical absorption in the presence of even severe scattering, leading to what are now widely considered the standard reference data for pure water absorption; (iv) a new diffuse reflector whose reflectivity is so high that ring-down spectroscopy in an integrating cavity is now possible.