In the case of forensic investigation where the details that characterized the structure of many physical pieces of evidence. These do not always require examination under very high magnification for such types of the specimen.
The stereomicroscope has proven out to be quiet and adequate instrumentation. The magnifying power of the stereo microscope ranges from 20X to 200 X.
The stereo microscope or dissecting microscope is an optical microscope variant design for low magnification and observation.
The average human eyes are separated by a distance of approximately 62 to 64mm and each eye perceives an object from somewhat different angles. This difference is of a few degrees and went transmitted to the brain the images are fused.
The stereo microscope takes the advantage of this ability to perceive the image data that are inclined at the small angle approximately 10ﾟ to 12ﾟ to yield the stereoscopic effect.
A stereo microscope is a crucial piece of equipment used in a forensic laboratory for the examination of a wide variety of evidence ranging from glass paint chips soil fiber hair document bones and its fragments etc. the primary motive of the stereo microscope is to produce a moderately magnified 3-D image.
Thereby allowing a close-up view examination of most of the physical pieces of evidence. The stereo microscope provides the view of the sample like the view of the human eyes.
In early 1890 biologist and instrument maker Horatio S. Greenough invented this microscope.
In the case of a stereo microscope, two optical parts transfer the image. These are as follows:
• Greenough/paired objective stereomicroscope
This design was developed by dentist Horatio Greenough in the year 1987. This design uses two separate monocular microscopes that are arranged in a convergent fashion and have paired objectives whose visual fields co-inside.
• CMO Design
These are modern design stereomicroscopes with a single objective lens instead of two lenses. Hence its name is CMO that is the common main objective. This single objective lens is shared between a pair of ocular tubes.
The CMO design has the dual advantage:
• It has a larger working distance.
• The magnification can be changed using the system of zooming in and zooming out.
Illumination with respect to a stereomicroscope.
There are two types of illumination when we talk in terms of stereomicroscope
• Reflected or incident light illumination and
• Transmitted illumination.
The reflected light is used for the objects that are opaque in nature whereas transmitted light is used for sample which is transparent in nature.
Path of Light in Stereomicroscope
The light which enters the objective lens is divergent in nature but as soon as it leaves the objective lens it gets converted into parallel light which is then re-directed by a series of prisms to the ocular lens.
The objective lens produces the image and the ocular lens receives this image to re-focus it onto the viewer’s eyes.
Parts of Stereomicroscopes
A stereo microscope has several components that gather the light and re-direct that light so that the magnified image of the object can be focused and formed.
The stereomicroscope is quite different in its constructions from the normal microscope because there is no presence of a condenser lens in a stereomicroscope.
The basic parts are as follows:
• The Base [also known as stage plate]
It is situated directly under the objective lens. It is the area where the specimen is placed for viewing.
• The Stage Clips
It helps to hold the slides or any objects on the stage.
• The Stereo Head
It is movable and top portion of the microscope. There are two eyepieces mounted on the stereo head. The stereo head holds the eyepiece and contains prisms that turn the image right side up.
• Objective Lens
It has two separate objective lens and each lens is connecting with eyepieces. It helps to determine the magnification of the microscope. It also allows to change the magnification levels depending on the applications.
• Diopter Setting
It creates the differences between the left and right eye.
• Focusing Knobs
It has only one focusing knob which helps to move the head of the microscope.
• Lighting Controls
Microscope has both top and bottom lighting. The top light shines down the object and reflects them. The bottom light transmits the light to show translucent specimens.
• Ocular Lens
These are the eyepieces through which we can see the specimen. The eyepieces are situated at 10x magnification and it will change to a higher magnification level.
• Its wide field of view and great depth focus makes it ideal for trace pieces of evidence debris garments and weapons.
• It has a large working distance [distance between lens and specimen] makes it applicable to bulky items.
• When coupled with vertical illumination it becomes the primary tool for characterizing physical evidence, paint, soil, gunpowder, drugs, etc.