Concentrate the force on two small faces
A narrow heel presses harder on a floor than a broad sole carrying the same weight. Concentrating force on a smaller area raises pressure. A diamond anvil cell brings two small, polished diamond faces together. Diamond supports the concentrated load and also transmits visible light, leaving an optical window into the compressed region.
A thin metal gasket sits between the faces. A hole in its centre holds the specimen, a pressure-transmitting medium and ruby grains. Closing the anvils seals this small chamber. The gasket deforms and the medium transfers the load, so an external force reading does not directly give the pressure at the specimen. A local indicator must share its surroundings.
Let the ruby turn compression into light
Ruby contains a small amount of chromium. Its chromium ions absorb energy from green light and release some of it as red fluorescence. Compressing the surrounding crystal changes the energies available to those ions. Under the conditions used for ruby pressure measurements, increasing pressure shifts the main emission line toward longer wavelengths.
All of this light still looks red to the eye. A spectrometer separates the wavelengths and resolves two narrow peaks. Their positions can be measured much more precisely than an apparent colour change. Brightness determines how clearly a peak can be located; its displacement supplies the pressure-sensitive observation.
The excitation enters through a diamond. Fluorescence returns through that window, and microscope optics and an optical fibre carry it to the spectrometer. The chamber stays sealed throughout the reading.
Measure the same grain before compression
First the spectrometer needs a wavelength ruler. Neon and argon lamps provide independently known lines that calibrate that ruler and reveal drift. The compressed ruby then has a reliable horizontal coordinate against which its peak can move.
Next, researchers measure the actual grain at ambient pressure and a known temperature. Grains can have slightly different starting positions because of internal stress and other sample differences. Following the same grain after loading gives a well-defined shift from its own starting point.
Converting that shift into pressure requires an experimentally established relation. A widely used scale published by Mao, Xu and Bell in 1986 compared ruby shifts with pressures referenced to metal compression. The grain thus acquires a pressure ruler tied to reference experiments.
Compare locations inside the chamber
A ruby reports conditions near its own position. Several grains distributed across the chamber can reveal spatial differences when each is compared with its ambient spectrum. A larger redshift on one side may indicate stronger compression there.
The location of the pressure reading therefore matters. A clear spectrum from a grain farther away may describe different conditions from those at the specimen. Sampling several positions helps researchers examine their loading and improve how the chamber is prepared.
Check temperature and the character of the load
Temperature also moves the ruby lines. Strong laser illumination can even heat a grain locally. Interpreting a shift as pressure requires a known temperature and a pressure scale appropriate to it. Weak, brief illumination helps limit heating caused by the measurement.
Unequal compression in different directions can broaden or distort the lines. A room-temperature pressure scale has specified loading conditions; quasi-hydrostatic means approaching equal compression from every direction. With temperature and loading suitably characterized, the ruby line carries the pressure of a microscopic chamber into an accessible spectrum.