Simple multi-wavelength imaging of birefringence: case study of silk

TL;DR

This paper presents a cost-effective, high-resolution polarised light imaging microscopy technique with liquid crystal retarders to accurately measure silk fiber birefringence across multiple wavelengths.

Contribution

It introduces a simple, multi-wavelength imaging method with liquid crystal phase retarders for precise birefringence measurement, including detailed hardware and numerical procedures.

Findings

Measured silk birefringence with 2% uncertainty.

Achieved high spatial resolution of 2 micrometers.

Demonstrated multi-wavelength analysis for thick samples.

Abstract

A polarised light imaging microscopy with an addition of liquid crystal (LC) phase retarder was implemented to determine the birefringence of silk fibers with the high 2 micrometers spatial resolution. The measurement was carried out with silk fiber (the optical slow axis) and the slow axis of the LC retarder set parallel (a perpendicular alignment can also be used). The direct fit of the transmission data provides a high fidelity determination of birefringence, $Dn = 1.63\times 10^{-2}$ (with 2% uncertainty) of the brown silk fiber (Antheraea pernyi) averaged over the wavelength range $l =$ (425-625) nm. By measuring retardance at four wavelengths it was possible to determine the true value of the birefringence of a thick sample when an optical path may include large number of wavelengths ($2p$ cycles in phase). The numerical procedures and required hardware are described for the do-it-yourself assembly of the imaging polariscope at a fractional budget compared with commercial units.