= S. G. Parker
{c}
\b[S. G. Parker] is the first named author of the 1983 primary study of optical radiation passing through human nails, coauthored with <Brian Diffey>. The byline uses these initials. The study measured total transmission through thirteen autopsy toenails at fifteen wavelengths from 300 to 600 nm and reported decreasing transmission towards the shorter wavelengths.<The transmission of optical radiation through human nails by S. G. Parker and B. L. Diffey>[Abstract and primary byline; British Journal of Dermatology 108, 11–16 (1983)]{ref}
The apparatus joined a <monochromator> to an <integrating sphere> and <photodiode>. Selecting an illumination wavelength and collecting the light that emerges answer different experimental questions. In particular, <light scattering> can redirect transmitted light outside a narrow detector acceptance angle without absorbing it. The collection geometry must therefore accompany a stated <transmittance>.
<Collection geometry in nail-transmission measurements> can be illustrated with an original accounting model. Let $T_{\rm direct}$ be directly transmitted fraction, $T_{\rm diffuse}$ the remaining transmitted fraction, and $\eta$ the fraction of diffuse transmission collected by a particular detector. Its idealized response is
$$
T_{\rm measured}=T_{\rm direct}+\eta T_{\rm diffuse}.
$$
For stipulated direct and diffuse fractions of 0.04 and 0.08, total transmission is 0.12. A detector collecting one quarter of the diffuse component gives $0.04+0.25(0.08)=0.06$. The sample has not changed; the reported value changes because the collection differs. An <integrating sphere> is designed to collect light over many directions, although its ports, calibration and losses still matter.
An isolated <nail plate> experiment also separates plate transmission from the optical behaviour of living tissue underneath. These measurements do not by themselves determine nail-bed <blood flow> or the response of every living nail. The work offers a concrete example of defining the specimen, wavelength and detector collection before interpreting an anatomical covering as an optical filter.
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