= Nonhydrostatic stress in ruby pressure measurement
A single optical shift represents <pressure> only to the extent that the calibration's mechanical and thermal conditions apply. Under <hydrostatic stress>, all directions have the same compressive normal <stress>. Under <deviatoric stress>, differently oriented <ruby> grains can respond differently; spatially varying <pressure> can also superpose shifted lines. These mechanisms can produce different optical signals at equal mean <pressure>.<Stress Measurements in Alumina by Optical Fluorescence: Revisited by Robert F. Cook and Chris A. Michaels>[Sections 1–2: stress-sensitive fluorescence shifts and crystallographic stress components]{ref}
The 2001 account describes scanning local line positions and measuring line width over an extended region. The former resolves spatial differences; the latter supplies a rapid warning.<High Pressure X-Ray Crystallography With the Diamond Cell at NIST⁄NBS by Gasper J. Piermarini>[Section 4.2, Methods I and II]{ref}
Neither a broadened line nor a narrow one uniquely determines the <stress tensor>. Instrument resolution, <temperature> variation and unresolved overlapping signals can also affect width. A small observed region can have a narrow line while differing from another region. Compare locations, reference traces and instrument settings before treating width as evidence for a particular mechanical explanation. These are proposed diagnostic checks, not additional historical controls attributed to the 1975 experiment.
<Temperature> is another independent input. A useful local expansion is $\Delta\lambda\simeq s_p p+s_T(T-T_0)$: one measured shift cannot determine both unknowns. Measure or control <temperature>, or supply a second independently calibrated observable. The coefficients need not remain constant across a large range. Even when a pressure sensitivity transfers over some <temperatures>, the reference line, line width and practical readout quality can change.
The historical high-temperature account reports degrading <pressure measurement> quality above about $300\,{}^\circ\mathrm C$.<High Pressure X-Ray Crystallography With the Diamond Cell at NIST⁄NBS by Gasper J. Piermarini>[Section 4.1, high-temperature cell and readout-quality discussion]{ref}
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