ASTM E766
Definition
The ASTM International standard practice for image-scale calibration in microscopy, extended by OSAC to govern optical microscope calibration in US forensic trace-evidence laboratories.
- Full title
- Practice for calibrating the magnification of a microscope
- Application
- Image-scale calibration
- Extended by
- OSAC, for US forensic trace-evidence labs
- Field
- Optical microscopy
Common questions
Why does microscope magnification need a formal calibration standard at all?+
A microscope's actual field-of-view scale can drift from its nominal specification due to optics, camera adapters, or eyepiece changes, so calibrating against a certified stage micrometer under E766 ensures measurements taken through the microscope, such as fibre diameter, are accurate rather than just consistent.
What happens to prior measurements if a lab discovers its microscope was miscalibrated?+
Any size-dependent measurement taken before the calibration error was found and corrected may need to be re-evaluated or flagged, since a scale error propagates proportionally into every measurement made on that instrument during the affected period.
Related terms
- Abbe Diffraction Limit
- The minimum period of a periodic grating structure that can be resolved: d = lambda / (2 NA). Sets the hard optical...
- Depth of Field
- Axial thickness of the specimen that appears acceptably in focus at one time. High in stereoscopic microscopes, very low in oil-immersion compound...
- Empty Magnification
- Magnification beyond approximately 1000 times the objective NA; enlarges the image without adding resolvable structural information, producing blurred detail that can be...
- Infinity-Corrected Optics
- An objective design in which the back focal plane produces a collimated (afocal) beam rather than a converging beam, allowing optical elements...
- ISO 8576
- The ISO standard governing the accuracy of stage micrometers and reticle eyepieces, specifying Class I and Class II tolerance levels traceable to...
- Koehler Illumination
- The standard illumination method for research microscopy in which the light source is focused at the condenser back focal plane, producing uniform...
- Numerical Aperture (NA)
- The product of the refractive index of the medium between objective and specimen and the sine of the half-angle of the maximum...
- Parfocality
- The property of an objective nosepiece turret such that switching objectives keeps the specimen approximately in focus without re-focusing; defined by ISO...
- Plan Objective
- An objective corrected for field curvature so that the entire image field is simultaneously in focus; required for photomicrographic evidence documentation to...
- Rayleigh Criterion
- The condition that two point sources are just resolved when the central maximum of one Airy disc coincides with the first minimum...
- Stage Micrometer
- A calibrated glass slide with a ruled scale of known spacing (typically 0.01 mm divisions), used to establish the pixels-per-micrometre calibration factor...