{c}

\b[John S. Beers] is a NIST <dimensional metrology> researcher and coauthor, with <Ted Doiron>, of <The Gauge Block Handbook by Ted Doiron and John S. Beers>. Its preface explicitly attributes the handbook's single-wavelength <interferometry> discussion to Beers's work. The official publication record retains the name John S. Beers.<The Gauge Block Handbook by Ted Doiron and John S. Beers>[Preface, printed p. iii; official NIST publication record, authors]{ref}

A <gauge block> provides a physical <length standard>, but its calibrated length depends on a stated convention. The handbook defines length using a wrung auxiliary plane and discusses mechanical comparison as well as <interferometry>.<The Gauge Block Handbook by Ted Doiron and John S. Beers>[Sections 2.2.2 and 6.1, gauge-block length and wringing convention; Section 5.3, comparative principle]{ref} This makes contact geometry and <gauge block wringing> part of the measurement, rather than treating the artifact as an abstract number printed on a block.

An original comparison example uses a reference and unknown of nominal length $50\,\mathrm{mm}$. Let the reference's certified deviation from nominal be $+0.120\,\mathrm{\mu m}$, and let the corrected unknown-minus-reference difference be $-0.035\,\mathrm{\mu m}$. The inferred unknown deviation is then $+0.085\,\mathrm{\mu m}$.

For independent reference and comparison <standard uncertainties> $u_r=0.025\,\mathrm{\mu m}$ and $u_d=0.015\,\mathrm{\mu m}$,
$$
u_x=\sqrt{u_r^2+u_d^2}\simeq0.0292\,\mathrm{\mu m}.
$$
If five independent repeat comparisons reduce only the second contribution, the result becomes
$$
u_x=\sqrt{u_r^2+u_d^2/5}\simeq0.0259\,\mathrm{\mu m}.
$$
The common reference contribution does not disappear by repeating the comparison. This <gauge-block reference uncertainty floor> illustrates why high <measurement repeatability> alone cannot establish the same numerical <measurement uncertainty> as a reference-free measurement. Correlated inputs require the corresponding <covariance> terms.<Evaluation of measurement data — Guide to the expression of uncertainty in measurement by JCGM>[Sections 5.1–5.2, independent and correlated input quantities]{ref}

These chosen numbers explain a calibration principle; they are not reported performance values for Beers's apparatus. His documented contribution concerns the measurement methods and their controlled realization, including the link between an optical observation and a defined physical length.


 John S. Beers (source code)