Mettler FP-90
Definition
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 observation; a component of the semi-automated protocol, not a standalone RI analyser.
- Type
- General-purpose laboratory hot stage
- Application
- Forensic glass refractive index measurement
- Temperature control
- ±0.1 °C
- Role
- Component of semi-automated Becke-line protocol
Common questions
Why does a glass RI measurement need such tight temperature control?+
Refractive index of the immersion oil used in the Becke-line method changes measurably with temperature, so the ±0.1 °C control lets the analyst find the temperature at which the oil's RI matches the glass fragment's RI with the precision needed to distinguish glass sources.
Is the Mettler FP-90 itself an RI measuring instrument?+
No, it is a hot stage that controls sample temperature under the microscope. The RI value is determined by observing the Becke line disappear at a matched temperature, with instruments like GRIM providing the automated optical measurement component of the overall protocol.
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...
- 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,...
- 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...