Immersion Method
Definition
The forensic technique of matching a glass fragment's RI to that of a reference oil by adjusting temperature; the fragment becomes invisible in the oil at the match point, and the oil's calibrated RI at that temperature gives the fragment's RI.
- Match indicator
- Fragment disappears (Becke line vanishes)
- Variable controlled
- Temperature of the reference oil
- Automated variant
- GRIM (Glass Refractive Index Measurement)
- Application
- Glass fragment comparison
Common questions
Why is temperature varied instead of using many different oils?+
RI oils change refractive index predictably with temperature, so heating or cooling a single calibrated oil on a hot stage sweeps through a range of RI values without needing a large set of separate oils.
What can cause a false match point in the immersion method?+
Surface coatings, weathering, or dispersion mismatches between the fragment and oil can make a fragment appear to vanish at a temperature that does not correspond to its true bulk RI, which is why dispersion staining is used to cross-check 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...
- 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...
- 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...
- 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...