How Is the Refractive Index of Glass Measured?

Scientists measure the refractive index of bulk optical glass by observing how light bends through a prepared sample. Two established approaches are prism spectrometry and V-block refractometry, described in SCHOTT's guide to refractive index and dispersion.

The useful result is a value with its measurement conditions attached. Before comparing two glasses, ask which wavelength was used, how temperature was controlled, and what uncertainty accompanies the result.

What does a refractive-index value describe?

Refractive index enters Snell's law, which relates the angles of incoming and refracted light at a boundary. Its dependence on wavelength is called dispersion. The familiar catalog symbol n_d specifies the index at 587.56 nm. A value at that wavelength provides one point on the dispersion curve. SCHOTT TIE-29 explains these conventions.

For someone comparing material reports, the immediate implication is simple: check the wavelength before comparing the last decimal places.

Two ways to measure optical glass

Prism spectrometry: measuring beam deviation

A sample is prepared as a prism with a known apex angle, the angle between its refracting faces. A narrow-band beam passes through it. Rotating the prism reveals an orientation where the outgoing beam has its smallest angular deviation from the incoming direction. That minimum deviation and the apex angle determine the sample's refractive index relative to the surrounding medium.

The geometry is set out in Burnett and colleagues' description of minimum-deviation refractometry. Their research example concerns germanium, but the same measurement principle is used for optical glass.

Sample preparation matters. NIST's instrument description identifies prism dimensions, surface flatness, material homogeneity and stress-induced birefringence as factors affecting achievable uncertainty. Birefringence means that the optical response depends on polarization. A suitable measurement plan therefore includes requirements for the sample as well as the instrument.

V-block refractometry: using a known reference

Here, a small sample sits in a V-shaped prism whose refractive index is accurately known. The beam's deflection depends on the index difference between the sample and the reference block. SCHOTT describes stacking multiple samples for efficient comparison during production. See the measurement section of TIE-29.

Temperature and the surrounding air belong in the result

The refractive index of glass changes with temperature. The size and direction of that change depend on the glass and wavelength. Precision work also distinguishes an absolute index, referenced to vacuum, from an index referenced to air. Air itself has a temperature- and pressure-dependent index, so these conventions matter when converting or comparing measurements. SCHOTT TIE-19 explains the distinction and the temperature corrections.

If reports disagree, first align their stated conditions. A difference between two numbers alone is insufficient to establish a difference in glass composition.

How accurate is the measurement?

Accuracy depends on the method and the specimen. As a concrete example, Japan's National Metrology Institute lists an expanded uncertainty of 2.2 × 10^-6, with coverage factor k = 2, for its specified BK7 or equivalent glass-prism calibration at 632.99 nm in vacuum. The listed prism size is 40–80 mm. These conditions belong with the number. The same service lists a different uncertainty at another wavelength. NMIJ/AIST calibration details.

Treat this as an example of a defined calibration capability. For your own sample, request an uncertainty statement covering the actual wavelength, geometry and measurement procedure. Counting digits on a display cannot establish that uncertainty.

Check what a supplier's certificate represents

A delivery certificate can describe a lot using representative samples. SCHOTT's TIE-04 explanation of optical-glass test reports describes this approach and emphasizes preserving batch identifiers. It also distinguishes measured quantities from values calculated using dispersion data.

For the fine-annealed glass covered by that document, the reporting assumptions include subsequent processing conditions. Additional heat treatment near the glass transition can change the spread of refractive indices within a lot. If a component has undergone further thermal processing, establish which material state its certificate describes before using the values in a comparison.

A useful record for comparing glass samples

For a research project or a discussion with a measurement laboratory, keep the following together:

• Sample identity: glass type, batch, specimen shape and processing history.
• Optical conditions: wavelength and whether the index is referenced to air or vacuum.
• Environment: sample temperature and relevant atmospheric conditions.
• Measurement: method, reference material where applicable, and which values were measured or calculated.
• Uncertainty: its stated value, definition and coverage factor where supplied.

This record makes it easier to decide whether a reported difference warrants further investigation. For the complementary question of how much light passes through a glass, see absorption, scattering and transparency.

Sources and further reading

• SCHOTT TIE-29: Refractive Index and Dispersion.
• Burnett et al. (2020): Index of refraction of germanium, Applied Optics. Includes the minimum-deviation measurement geometry.
• NIST: Minimum-Deviation-Angle Refractometry System.
• SCHOTT TIE-19: Temperature Coefficient of the Refractive Index.
• NMIJ/AIST: Refractive Index of Glasses calibration service.
• SCHOTT TIE-04: Test Report for Delivery Lots of Optical Glass.