  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.
