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 np and ng, 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.