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Critical angle refractometry is an established technique for determining the refractive index of liquids and solids. For transparent samples, the critical angle refractometry precision is limited by incidence angle resolution. For lossy samples, the precision is also affected by reflectance measurement error. In the present study, it is demonstarted that reflectance error can be practically eliminated, provided that the sample’s extinction coefficient is a priori known with sufficient accuracy (typically, better than 5%) through an independent measurement. Then, critical angle refractometry can be as precise with lossy media as with transparent ones.

We present a concept that enables the determination of the complex refractive index of attenuating media from two critical angles, measured sequentially at two interfaces between a single sample and two different prisms. The proposed method is general in that it applies with s and p polarisation states, thus it is suited for the characterisation of isotropic as well as anisotropic media. Uncertainty analysis indicates that relative error in the determination of the real (imaginary) index can be less than 10−4 (in the order of 10%), respectively.

At a critical angle of incidence, Fresnel reflectance at an interface between a fronttransparent and a rear lossy medium exhibits sensitive dependencies on the complex refractiveindex of the latter. This effect facilitates the design of optical sensors exploiting single (or multiple)reflections inside a prism (or a parallel plate). We determine an empirical framework that capturesperformance specifications of this sensing scheme, including sensitivity, detection limit, range oflinearity and—what we define here as—angular acceptance bandwidth. Subsequently, we developan optimization protocol that accounts for all relevant optical or geometrical variables and that canbe utilized in any application.