Microscopy for Forensic Science: Polarizing, Comparison, Stereoscopic, Fluorescent and Electron

A polarising microscope is a compound microscope with two extra Polaroid filters. The polariser sits below the stage and converts unpolarised lamp light into plane-polarised light. The analyser sits in the tube above the objective, oriented at 90 degrees to the polariser. Between crossed polars, an isotropic material (glass, water, amorphous polymer) appears black. A birefringent material (most synthetic fibres, crystalline drugs, minerals, starch grains) splits the incoming ray into ordinary and extraordinary rays travelling at different speeds, and shows interference colours against the dark field.

For NET, three concrete uses carry:

1. Fibre examination. Nylon, polyester and acrylic are all birefringent with characteristic refractive indices and sign of elongation. The polarising microscope distinguishes them without destroying the fibre.
2. Drug identification. Cocaine, MDMA and many barbiturates form crystals whose polarisation behaviour matches reference standards. Useful in field-screening colour-test positives.
3. Mineralogy of soil and dust. Quartz, mica and feldspar each show diagnostic interference colours. Helps anchor a soil sample to a region.

The Indian anchor: every CFSL physics or trace division (Hyderabad, Chandigarh, Kolkata) keeps a Leica or Olympus polarising microscope as standard equipment for fibre and paint chip work.

The comparison microscope is the instrument forensic science gave the world. Invented by Philip Gravelle and popularised by Calvin Goddard in the 1920s, it joins two identical compound microscopes through an optical bridge so the analyst sees both specimens side by side in a single split eyepiece. A vertical hairline separates the two halves.

What it solves: a bullet from a crime scene and a test-fired bullet from the suspect weapon need to be compared on striation marks invisible to the naked eye. With two single microscopes the analyst would have to memorise the first image while looking at the second. The bridge eliminates that gap.

Forensic uses for NET:

• Firearms. Match striations on fired bullets, breech-face marks on cartridge cases, firing-pin impressions.
• Tool-marks. Match striations from screwdrivers, crowbars, wire-cutters to recovered marks at burglary scenes.
• Questioned documents. Compare ink lines, paper fibres or staple holes between two documents.

The Indian anchor: every CFSL ballistics division uses a Leica FS C or Projectina comparison macroscope (essentially a low-magnification comparison microscope) for routine bullet and cartridge case examination. GEQD Shimla uses comparison microscopes for questioned-document casework.

A stereoscopic microscope (also called a stereo microscope or dissecting microscope) has two separate optical paths, one for each eye, that converge on the specimen at a small angle. The brain fuses the two views into a true three-dimensional image. Magnification is low (commonly 7x to 40x, occasionally up to 90x) and the working distance is large, often 100 mm or more, which gives the analyst room to manipulate the specimen with forceps or a scalpel.

For forensic work the depth of field is the key advantage. A high-NA compound objective at 40x has a depth of field of about 1 micrometre, useless for a knot in a rope or a torn paper edge. A stereo microscope at 20x has a depth of field of hundreds of micrometres, so the whole feature stays in focus.

NET-favourite uses:

1. Trace recovery. Picking hairs, fibres, glass fragments and paint chips off tape lifts under stereo magnification before deciding which deserve polarising or SEM follow-up.
2. Document examination. Erasures, indented writing, paper-fibre disturbance, security-feature inspection on currency.
3. Forensic entomology. Identifying maggots and beetles recovered from corpses, anchoring time since death.
4. Gross pathology and odontology. Bite-mark photography, tool-mark casts, bone surface features.

The Indian anchor: AIIMS forensic medicine and the Anthropological Survey of India both use stereo microscopes for bone and bite-mark work; every state SFSL trace section keeps one on the bench.

A fluorescence microscope excites the specimen with short-wavelength light (UV or blue) and collects the longer-wavelength light the specimen emits in return (Stokes shift). The optical heart is the dichroic mirror in the filter cube, which reflects excitation light down onto the sample and lets the longer-wavelength emission pass up to the eyepiece. Excitation and emission filters before and after the mirror cut out everything except the wavelengths of interest.

Two modes of fluorescence in forensic casework:

• Autofluorescence. Natural fluorophores in the specimen emit on their own. Semen, saliva, urine, certain inks, some fibres (especially those with optical brighteners) and minerals all autofluoresce.
• Induced fluorescence. A dye like acridine orange, DAPI or fluorescein is bound to the target (DNA, blood) and then excited.

Forensic uses to remember:

1. Body-fluid screening. Alternate-light-source plus fluorescence microscope localises and confirms semen and saliva stains that are invisible under white light.
2. Fibre discrimination. Two fibres that look identical under white light can fluoresce different colours under UV because of different optical brighteners or dyes.
3. Questioned documents. Ink discrimination, security-feature verification on passports, currency and stamps.
4. Latent fingerprints. Cyanoacrylate-fumed prints dyed with rhodamine 6G or basic yellow 40 are visualised under fluorescence microscopy.

The Indian anchor: CFSL Hyderabad's biology division uses fluorescence microscopy in routine semen-stain confirmation and in DNA-stained slide work. NICFS Delhi runs training modules on alternate-light fluorescence techniques for state police labs.

Light microscopes hit a hard physical wall at about 200 nm because of the wavelength of visible light. Electrons have a far shorter de Broglie wavelength, so an electron microscope breaks through that wall. Two designs matter for NET.

Scanning Electron Microscope (SEM). A focused beam of electrons (typically 1 to 30 kV) raster-scans the specimen surface. Secondary electrons knocked out of the surface form the image; backscattered electrons give compositional contrast (heavier elements look brighter). Resolution is a few nanometres, depth of field is huge, and the image is intuitively 3D-looking. The killer combination is SEM-EDX: the same beam ejects inner-shell electrons, atoms relax by emitting characteristic X-rays, the EDX detector reads those X-rays and identifies elements present.

Transmission Electron Microscope (TEM). Electrons pass through an ultra-thin section (under 100 nm). Resolution drops below 1 nm. TEM gives internal structure (think viruses, crystal lattices, nanoparticles) but requires demanding sample preparation. It is rare in routine Indian forensic casework and common in academic research.

Forensic uses (NET-relevant):

1. Gunshot residue (GSR). SEM-EDX on tape lifts from a suspect's hands identifies particles containing lead, barium and antimony together: the diagnostic GSR signature.
2. Paint and pigment analysis. Layer structure of automotive paint and elemental composition of pigments.
3. Soil and gemmology. Trace elemental mapping.
4. Counterfeit currency and document inks. Elemental signatures of inks and paper coatings.
5. Explosive residue. Morphology and composition of post-blast debris.

The Indian anchor:

For revision, lock the mapping. A typical case dataset moves through the lab in this order: a stereoscopic microscope on the trace-recovery bench picks up the relevant fibres, glass and hair; the polarising microscope on the next bench classifies the fibres and identifies any crystalline drug; if body fluids are suspected, the slides move to the fluorescence bench; if a firearm was involved, the comparison microscope in ballistics handles the bullets; if GSR is in question, tape lifts go to the SEM-EDX in the physics division. Five instruments, five disciplines, one case.