Research Under Pressure: NIST Scientists Make Definitive Measurement of the Optical Pascal

Researchers at the National Institute of Standards and Technology (NIST) and their colleagues have overcome a key obstacle to measuring gas pressure with a novel method – using beams of light traversing through a gas.

Ultimately, the new work may enable industry to establish their own optical pressure scales directly traceable to the fundamental constants of nature, saving both cost and time by eliminating the need to send pressure-measuring devices to NIST for calibration.

Precision pressure measurements are essential to dozens of industrial applications including petroleum refining, as well as in aircraft altimeters, internal combustion engines and turbines, leak detection, microchip manufacture, and aerospace.

Scientists typically measure gas pressure, traditionally defined as force per unit area, with a pressure balance or a liquid manometer, techniques based on the principles of classical mechanics. In contrast, the optical method is based on thermodynamics and quantum theory. The optical technique relies only on the temperature of the gas and the amount by which it slows down, or refracts, different frequencies of light. Because these properties are tied to the fundamental constants of nature through the International System of Units (SI), the technique has the potential to decrease uncertainties in pressure measurements and shorten the number of steps required for calibration.

Pressure is measured in terms of a unit known as the pascal. In a new study, NIST researchers Patrick Egan and Jack Stone, along with Yuanchao Yang of the National Institute of Metrology in Beijing, realized a new measure of the pascal – the optical pascal – traceable to the SI unit of temperature.

To understand the optical method, consider a box filled with gas. For a fixed volume and known temperature, the pressure inside the box is simply given by the number of atoms in the box. (The greater the number of atoms, the greater the number of collisions between atoms and the sides of the box, and the higher the pressure.)

That sounds straightforward, but a container the size of a lunchbox would hold more than a billion trillion atoms, far too many to count. Instead, researchers rely on a proxy – an optical quantity proportional to the number of atoms. That quantity, known as the refractive index, provides a measure of how much the speed of light slows down when passing through a gas relative to its speed in a vacuum.

The slow-down arises when the electric field of the light beam polarizes, or stretches, the cloud of electrons that surround each atomic nucleus. Heavier atoms, such as argon, which have a greater number of electrons, are more easily polarized and lower the speed of light more than do lighter atoms, such as helium, which have fewer electrons.

Polarizability, however, isn’t always easy to calculate. In fact, researchers had only achieved an accurate calculation of polarizability in one atomic gas – helium – because of the relative simplicity of its electronic structure. That knowledge isn’t particularly helpful, however, because at low pressure, helium gas barely changes the speed of light, making it difficult to determine the number of helium atoms – and hence the pressure – in a fixed volume.

To provide a practical route to realizing the optical pascal, Egan and his colleagues concluded that they would have to accurately measure polarizability in a group of heavier atoms. In an article published online June 17 in Physical Review Applied, the NIST team and their collaborators have now accomplished that feat in argon gas.

In their study, the researchers overcame a challenge that had hindered the optical method over the past decade: The instrument used to measure refraction undergoes contraction or expansion due to the pressure of the gas it’s probing. If unaccounted for, the distortion of the instrument can reduce the accuracy of the measurement.

However, the deformation is independent of wavelength. Taking advantage of that property, the researchers measured the slowdown of light at two different wavelengths, which cancelled out the distorting effect. That enabled the team to accurately measure the polarizability of argon.

Because argon gas is both plentiful and easy to work with, the new measurement will allow industry and academia throughout the U.S. to adopt the optical pascal, Egan said.

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Paper:

Yang, Y., Stone, J.A., and Egan, P.F. Demonstration of dispersion gas barometry. Physical Review Applied, 23, 064041. Published online June 17, 2025. DOI: https://doi.org/10.1103/z9zz-lqzh