Microscopy - instrumentation and limitations Flashcards
What is a stereoscopic microscope?
Most frequently used in FS
10-12x range
large working distance
good for bulky artefacts
wide field of view and great depth of focus - allowing for easier visualisation of evidence
great first step for looking at physical features of trace evidence
However…. we want to increase resolution
What is a compound microscope?
40-450x range - can go up to 1000x
Contains an X-Y stage to move around the sample
transmitted and reflected illumination - can be used to visualise glitter traces
Precise light and intensity control
What is a comparison microscope?
Two identical microscopes are connected to a single comparison eye piece or screen
Allows point by point and side by side comparison to determine if two samples are from the same source
What is a Fluorescence microscope?
The illuminating light in the ultraviolet wavelength range
the illumination causes materials to fluoresce so they can be observed, counted, sized and mapped
Fluorescence tagging is not as common in forensic trace evidence
what is polarised light microscopy?
Useful when applied to anisotropic substances
What is brightfield microscopy?
It uses light from lamp source under the microscope stage to illuminate the specimen.
It is gathered in the condenser, then shaped into a cone where the apex is focused on the specimen.
however, in order to view a specimen the light rays that pass through it must be changed enough to contrast.
if a specimen has a refractive index similar to the surrounding medium, then the image cannot be seen.
to visualise these materials, they must have a contrast with the medium or be stained. Staining can be destructive to specimens.
What is darkfield microscopy?
It uses special condenser which forms a hollow cone to collect only highly refracted light.
Hence, a sample placed on stage appears bright against a dark background as only the scattered light is collected. Thus providing contrast without staining.
What is the becke line test?
This is used to measure the refractive index
What is the refractive index in uniaxial materials?
A uniaxial material allows for the rays to vibrate in two axes, omega (blue) and epsilon (red) depending on it’s orientation
the refractive index will be determined by the light vibrating in the epsilon plane, seen by its difference to mounting media.
the refractive index will be determined by the light vibrating in the omega plane, seen by its difference to mounting media.
What is retardation?
The exact distance that the slow ray, which is said to be retarded falls behind the fast ray.
measured by rotating the analyser relative to polariser
What is birefringence?
a characteristic that allows us to identify a fibre and compare it to a known sample
