Realization of the optical pascal has been limited by systematic errors caused by distortion of the optic. In this work, distortion error is circumvented via synchronous measurement of helium refractivity at two optical frequencies. The resultant pressure realization achieves combined standard uncertainty of $5.7\times {10}^{-6}p$, chiefly limited by ab initio knowledge of helium dispersion. Two-color measurements are also presented for neon, argon, and nitrogen, which enable semiprimary realization of the pascal in a more practical embodiment. For argon dispersion, measurement and ab initio calculation barely agree within mutual expanded uncertainty; experiment is about 16 times more accurate than theory.

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• Equations of state
• Metrology
• Atomic gases
• Polarizability
• Fabry-Pérot interferometry
• Precision measurements
• Pressure techniques
• Virial theorem