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This paper presents a discussion of the errors involved determining the concentration of paramagnetic centers with the electron spin resonance method. The work is based on a theoretical analysis of the relation describing the amplitude of the recorded derivative spectrum. Experimental and computational work has been done on a dual cavity for comparing two samples different in size, shape, and material. Special attention is given to corrections for power saturation, and to the determination of the errors made in the evaluation of the recorded signals. Recommendations are given that will facilitate the evaluation of errors from each source in any individual case. It is concluded that errors involved in the comparison of two samples of the same size, material, and in equal tubes can be performed in a dual cavity with an error of less than 5%. If these requirements are not met, however, an over-all error of 10% will only be reached with extremely careful and extensive work.

The optical transmission matrix (TM) characterizes the transmission properties of a sample. We show a novel experimental procedure for measuring the TM of light waves in a slab geometry based on sampling the light field on a hexagonal lattice at the Rayleigh criterion. Our method enables the efficient measurement of a large fraction of the complete TM without oversampling while minimizing sampling crosstalk and the associated distortion of the statistics of the matrix elements. The procedure and analysis described here is demonstrated on a clear sample which serves as an important reference for other systems and geometries such as dense scattering media.

We report optical measurements of the spectral width of open transmission channels in a three-dimensional diffusive medium. The light transmission through a sample is enhanced by efficiently coupling to open transmission channels using repeated digital optical phase conjugation. The spectral properties are investigated by enhancing the transmission, fixing the incident wavefront and scanning the wavelength of the laser. We measure the transmitted field to extract the field correlation function and the enhancement of the total transmission. We find that optimizing the total transmission leads to a significant increase in the frequency width of the field correlation function. Additionally we find that the enhanced transmission persists over an even larger frequency bandwidth. This result shows open channels in the diffusive regime are spectrally much wider than previous measurements in the localized regime suggest.

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