Dispersion
Definition
The variation of refractive index with wavelength; all glasses and immersion oils show dispersion, which is why RI must be specified at a defined wavelength (conventionally sodium D line, 589 nm) for comparison to be meaningful.
- Reference wavelength
- Sodium D line, 589 nm
- Measured as
- Change in refractive index per unit wavelength
- Common instrument
- Refractometer or GRIM (Glass Refractive Index Measurement)
- Reported alongside
- RI value at the D line
Common questions
Why does a glass comparison fail if dispersion is ignored?+
Two glasses can share the same refractive index at one wavelength but diverge at another, so quoting RI without specifying the wavelength and accounting for dispersion can make genuinely different glasses look identical, or vice versa.
How is dispersion measured in a forensic lab?+
The Mettler hot stage or GRIM system tracks the temperature or wavelength at which the glass fragment's RI matches the surrounding immersion oil, giving a dispersion curve rather than a single point value.
Related terms
- ASTM E1967
- The ASTM International standard for forensic glass RI measurement; specifies three acceptable methods (automated, semi-automated, manual), minimum precision of ±0.0001 RI units,...
- Bayesian Likelihood Ratio
- A statistical framework for expressing the evidential value of a comparison: the ratio of the probability of the observed RI similarity given...
- Becke Line
- The bright optical halo at the edge of a solid fragment immersed in a liquid, visible under the microscope; moves toward the...
- Cauchy Equation
- The empirical equation n(λ) = A + B/λ² + C/λ⁴ that describes the wavelength dependence (dispersion) of a material's refractive index; used...
- ENFSI ENG3-2013
- The ENFSI Glass Expert Working Group guideline for glass comparison casework; specifies the examination sequence, within-source variation requirements, statistical comparison framework, and...
- GRIM-3
- Glass Refractive Index Measurement, third-generation instrument by Foster + Freeman (UK); an automated phase-contrast hot-stage system that identifies the RI match point...
- Immersion Method
- The forensic technique of matching a glass fragment's RI to that of a reference oil by adjusting temperature; the fragment becomes invisible...
- Locke 1985 Protocol
- The calibrated immersion-oil series and operational procedure for forensic glass RI measurement described by Locke and Underhill (1985); specifies the oil series,...
- Mettler FP-90
- A general-purpose laboratory hot stage widely used in forensic glass RI measurement to provide ±0.1 °C temperature control for the manual Becke-line...
- Phase-Contrast Microscopy
- An optical technique that converts phase differences (RI differences at an interface) into intensity differences visible in the image; exploited by the...
- Refractive Index (RI)
- A physical property of glass measuring how much light bends as it passes through. RI is the primary discriminating characteristic used in...