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A forensic microscope is any microscope used to examine physical evidence so that two items can be compared or a material can be identified. In practice the term covers several instruments, and the one most associated with forensic work is the comparison microscope, which lets an examiner view two objects side by side in a single field of view.
Forensic microscopy, also called microscopy in forensic science, is the use of these instruments on evidence. For the wider discipline, see what forensic science is.
What a forensic microscope is
NIJ’s firearms examiner training describes the comparison microscope as a pair of compound microscopes joined by an optical bridge, so the scientist can observe and compare two physically separated but optically joined objects simultaneously in one field of view. NIST’s scientific foundation review of firearm examination describes the technique as comparing marks on bullets and cartridge cases under a comparison microscope with those from a suspected firearm.
Who uses it: firearm and toolmark examiners and forensic laboratory staff, as described in the NIJ and NIST material cited below.
How a comparison microscope is built
Per NIJ’s module on comparison microscopes, the core design is two compound microscopes whose stages are joined by lenses, prisms and mirrors forming the optical bridge. The field of view is split by an optical hairline. Calvin Goddard and Phillip O. Gravelle adapted the compound microscope for simultaneous bullet comparison in 1925, replacing a workflow of examining items one after another and comparing photographic prints, which are two-dimensional views of three-dimensional objects.
NIJ lists these features of modern instruments:
- Binocular viewing and rotating nosepieces holding a variety of objective lenses
- Illumination choices such as variable fibre optic, LED and high-intensity fluorescent sources
- Specialised specimen mounts and push-button focusing
- Motorised x, y and z control whose settings can be saved for reproducibility
- Image capture with file export and printing, and monitors for examination, training and capture
- Optical and digital image superimposition or conventional side-by-side comparison, plus digital reference marks to return to areas of interest
Types of forensic microscope
The two types below are the ones described in the NIJ and NIST sources cited on this page.
| Type | Typical use | Key difference | Source |
|---|---|---|---|
| Comparison microscope | Comparing marks on bullets and cartridge cases with those from a suspected firearm | Two microscopes joined by an optical bridge, one split field of view | NIJ firearms examiner training; NIST foundation review |
| Digital comparison system (for example the Vision X evaluated by NIJ) | Examining cartridge cases, bullets and toolmarks, and sharing digital images between agencies | Images can be displayed remotely; NIJ’s evaluation says this could save significant time in NIBIN hit confirmation | NIJ evaluation, NCJ 255945 (2020) |
Calibration and maintenance of a comparison microscope
NIJ’s training module reproduces the maintenance and calibration standards that the Association of Firearm and Tool Mark Examiners (AFTE) gives its members. These are not mandatory unless individual laboratory policy says so, and they may be used to help satisfy accreditation standards.
| Interval | What is done | Record |
|---|---|---|
| Annually | Cleaned, serviced and certified by a factory-certified technician | Maintenance and calibration logbook |
| Quarterly | Calibrated with a ruled glass slide (0.04 ruled to 0.001) that has a NIST-traceable certificate | Maintenance and calibration logbook |
| Each use | Functioning check: place two similar items on the two stages and observe agreement | Examiner’s case notes |
For the general principle of calibrating against a ruled standard, see microscope calibration with a stage micrometer.
Digital comparison systems
NIJ’s evaluation of the Vision X describes it as a powerful addition to the microscopic examination of cartridge cases, bullets and toolmarks. It notes that verifying all conclusions, not only identifications, reduces the risk of false eliminations and inconclusive results, and that sharing images between agencies requires both agencies to have the system plus authorisation.
Common mistakes
- Relying on photographs instead of the evidence. NIJ notes that photographic comparison gives two-dimensional representations of three-dimensional objects and that items were examined in sequence, which is the problem the comparison microscope addressed.
- Treating AFTE guidance as universal. NIJ states the intervals are not mandatory unless laboratory policy requires them.
- Skipping the each-use check or the log entry. The same module places the check in the examiner’s case notes and the other steps in the logbook.
- Treating the NIJ module as current. It is archived training material; check your laboratory’s own procedures and accreditation requirements.
Handling of the specimens themselves belongs to chain of custody in forensic science, and slide preparation for optical work is covered in what a microscope slide is.
FAQ
What is a forensic microscope?
It is a microscope used on physical evidence. The best-known type is the comparison microscope, two microscopes joined by an optical bridge so two items can be compared in one split field (NIJ).
What is microscopy in forensic science used for?
In the sources cited here it is used to compare marks on bullets, cartridge cases and toolmarks against those from a suspected firearm (NIST, NIJ).
How is forensic microscopy different from a laboratory microscope?
The distinguishing feature described by NIJ is the optical bridge and split field for simultaneous comparison. For general research instruments see what a microscope for research is.
Sources
Checked 2026-10-29.
- NIJ, Firearms Examiner Training, Module 07, Comparison Microscopes (archived training), checked 2026-10-29: https://nij.ojp.gov/nij-hosted-online-training-courses/firearms-examiner-training/module-07/comparison-microscopes
- NIST, Firearm Examination: A NIST Scientific Foundation Review, checked 2026-10-29: https://www.nist.gov/node/1848816
- NIJ, Evaluation of the Vision X comparison microscope (NCJ 255945, 2020), checked 2026-10-29: https://nij.ojp.gov/library/publications/evaluation-vision-x-comparison-microscope-share-digital-images-bullets-and
