Becke Line
Definition
The bright optical halo at the edge of a solid fragment immersed in a liquid, visible under the microscope; moves toward the higher-RI medium when focus is raised. Its disappearance marks the RI match condition.
- Appearance
- Bright optical halo at a fragment's edge in immersion liquid
- Behaviour on raising focus
- Moves toward the higher refractive-index medium
- Match indicator
- Line disappears when solid and liquid RI are equal
- Use
- Glass and fibre refractive index determination
Common questions
Why raise the microscope focus rather than lower it to read the Becke line?+
Raising the objective moves the focal plane upward, which by convention causes the Becke line to migrate into the medium of higher refractive index, giving a directional cue that lowering focus would reverse, so the raising convention keeps readings consistent between analysts.
How does the Becke line method compare to GRIM for glass RI matching?+
The Becke line is a manual, visual immersion technique that finds the temperature or liquid at which the line vanishes. GRIM automates the same underlying principle by heating the immersion oil and using a video system to detect the match point instrumentally, giving more reproducible, quantitative results.
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...
- 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...
- Dispersion
- The variation of refractive index with wavelength; all glasses and immersion oils show dispersion, which is why RI must be specified at...
- 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...