  Understanding the optical transmission property of human hair, especially in
the infrared regime, is vital in physical, clinical, and biomedical research.
However, the majority of infrared spectroscopy on human hair is performed in
the reflection mode, which only probes the absorptance of the surface layer.
The direct transmission spectrum of individual hair without horizontal cut
offers a rapid and non-destructive test of the hair cortex but is less
investigated experimentally due to the small size and strong absorption of the
hair. In this work, we conduct transmission infrared micro-spectroscopic study
on individual human hair. By utilizing direct measurements of the transmission
spectrum using a Fourier-transform infrared microscope, the human hair is found
to display prominent band filtering behavior. The high spatial resolution of
infrared micro-spectroscopy further allows the comparison among different
regions of hair. In a case study of adult-onset Still's disease, the
corresponding infrared transmission exhibits systematic variations of spectral
weight as the disease evolves. The geometry effect of the internal hair
structure is further quantified using the finite-element simulation. The
results imply that the variation of spectral weight may relate to the
disordered microscopic structure variation of the hair cortex during the
inflammatory attack. Our work reveals the potential of hair infrared
transmission spectrum in tracing the variation of hair cortex retrospectively.
