The comparison microscope is a specialized optical instrument developed for the side-by-side examination of two separate objects at high magnification. Standard microscopy requires an observer to rely on memory when comparing two distinct samples sequentially. The comparison microscope overcomes this limitation by allowing the direct, simultaneous comparison of minute details. This capability makes it essential for establishing the common or non-common origin of evidence, particularly within forensic science laboratories.
Defining the Comparison Microscope
The comparison microscope is built from two independent microscopes, which can be either compound or stereo models, integrated by an optical bridge. Each microscope has its own stage, objective lenses, and focus mechanism. To ensure a valid comparison, the objective lenses on both sides must be precisely matched in magnification and optical quality. This guarantees that both specimens are viewed under identical optical conditions, preventing variations from being misinterpreted as differences in the evidence.
The optical bridge merges the separate light paths from the two microscopes using a system of prisms and mirrors to direct the images toward a single common eyepiece. This architecture allows an examiner to manipulate the lighting and position of each specimen independently while viewing them simultaneously. Separate light control is necessary when analyzing materials with different reflective properties, such as metal tool marks or various fibers.
The Principle of Split-Image Viewing
The operational principle centers on creating a split-field image within the eyepiece, delivered by the optical bridge. Light rays from the first microscope are directed to one half of the viewing field, and rays from the second microscope are directed to the other half. The operator sees a single image circle divided by a thin, adjustable hairline that separates the magnified view of Specimen A from Specimen B.
This simultaneous side-by-side display allows the examiner to align specific features, such as striations or markings, across the dividing line. Directly juxtaposing the patterns allows the human eye to perceive subtle differences or similarities in texture, width, and depth that would be impossible to recall from memory. Some models also allow for the complete overlapping or superimposition of the two images, which is useful when comparing impressed patterns. This direct visual comparison allows for the confident identification or elimination of a common source for the two samples.
Primary Applications in Forensic Science
The capability for side-by-side analysis makes the comparison microscope the primary instrument in firearms identification. When a bullet travels through a gun barrel, the rifling grooves leave unique striation patterns on the projectile. Examiners use the comparison microscope to match striations on a recovered crime scene bullet with those on a test-fired bullet from a suspect’s weapon.
The instrument also compares microscopic impressions left on a spent cartridge casing, such as marks from the firing pin, breech face, and ejector mechanism. Tool mark analysis relies on this technology to compare a mark left at a crime scene—for instance, from a screwdriver—with a test mark made by a suspect tool. The tool’s unique microscopic contours create a signature pattern that can be matched across the split field of view.
The comparison microscope is also used to compare trace evidence and handwriting characteristics on questioned documents. These applications establish a potential link between evidence and an individual or location. Trace evidence commonly compared includes:
- Hair
- Fiber
- Glass fragments
- Handwriting characteristics