Performance of low-coherence and confocal refractometry with reduced index contrast

This paper looks to evaluate the performance of combined low-coherence and confocal refractometry under conditions of reduced refractive index contrast. The instrument measures the refractive index and thickness of transparent objects using a fibre-based low-coherence interferometer with a line-scan spectrometer. A sample was designed that mimics the on-axis structure of the eye, consisting of a lens tube holding a pair of windows surrounded on either side by fluid chambers and sealed at the far end by a diffuse black metallic plug. Sucrose solutions with a range of concentrations were injected into the fluid chambers, providing a linear variation in refractive index of 1.3330 to 1.4416. The instrument was used to simultaneously measure both the phase and group refractive indices n_p and n_g, as well as the physical thickness t of the windows and the fluid in the chambers. Although the measurement accuracy is shown to decrease with reduced refractive index contrast, it remains better than 0.6% over all measured components for sucrose concentrations of up to 60.3%, which is close to the saturation limit.