Forensic Physics
Comparison Microscope: Principle, Features & Applications

Comparison Microscope: Principle, Features & Applications

The invention of the microscope has been a boon in the scientific field which allowed scientists to visualize things or matter or objects which are not visible to the naked eye. The first microscope came into existence with the invention of the early microscope, by a Dutch spectacle maker Zacharias Janssen in the 16th century.

With the passage of time and different needs for visualization, various kinds of microscopes were invented and one of them was the comparison microscope. 

The comparison microscope was invented for side-by-side analysis of objects. As the name suggests, it compares two objects simultaneously. It is an instrument that combines two light microscopes side-by-side connected via an optical bridge which facilitates a split view window. It is one of the important instruments used in forensics.

Development of Comparison Microscope

The comparison microscope was first launched as a prototype in Germany in 1913. However, it was not popularised in use. In 1929, the comparison microscope was introduced to the branch of Forensic Ballistics

Sir Calvin Goddard is considered the father of Forensic Ballistics because he was the first person who offered ballistic identification evidence in 1921 to help secure convictions of accused murderers and anarchists Nicola Sacco and Bartolomeo Vanzetti. 

Then in 1929, Sir Calvin Goddard along with his partner Phillip O. Gravelle, a chemist, developed it for comparing the fired bullets and cartridge cases, found in the St. Valentine’s Day Massacre case.

Goddard was the key forensic expert for this massacre, where seven gangsters were killed by rival Al Capone mobsters dressed as Chicago police officers. He used similar techniques of comparison microscope to absolve the Chicago Police Department of participation in the St. Valentine’s Day Massacre. 

There were proposals to computerize the comparison microscope technique in the 1960s, but due to a lack of software and hardware at that time, this theory was not implemented. 

However by the 1980s, with the development of high-capacity computers which adapted to the requirements of digital imaging, pattern recognition, image storage, and comparison algorithms, the earlier proposed theory was ready to be accepted.

Finally, with the cooperation of private industry, law enforcement, and forensic subject matter experts the Integrated Ballistics Identification System was evolved (IBIS). It is deployed worldwide by Forensic Technology, Incorporated (FTI) of Montreal. 

Working of Comparison Microscope

As earlier mentioned, a comparison microscope is a combination of two light microscopes that are connected by an optical bridge that provides a split view.  

The optical bridge is the main hero in this instrument which is a series of lenses and mirrors that brings the two images back together at the single eyepiece. The image formed is similar to that of the usual microscope with the only difference of a line in the middle separating the circular view field into two parts.

The left side image is produced by the left light microscope and the right side image is produced by the right light microscope. 

Some comparison microscopes facilitate the superimposing of the view fields generated by the two microscopes.

With the advancement of technology, now it is connected to a computer system that shows the microscopic images of the samples to be compared on the computer screen.

Features of Comparison Microscope

An effective comparison microscope must have the following characteristics:

  • The microscope should have a high-quality optical system, which can produce sharp, clear, and highly magnified images.
  • A comparison microscope should offer easily-adjustable height ranges and optical ergonomic angles, with room comfortable for elbow and wrist placement. 
  • The working distance, that is the distance between the objective lenses and the specimen being examined, should be able to accommodate larger objects like firearm components, etc. 
  • The microscope stage should be easy to manipulate on multiple axes, either separately or locked for synchronized movement. The stage should also provide centralized controls for viewing precision and ease to use.   

Advantages of Comparison Microscope

  • The image magnification ranges from 6x to 400x.
  • It has high-resolution power.
  • Simultaneous comparison is possible.
  • In some cases, superimposition is also possible for better comparison.

Disadvantages of Comparison Microscope

  • The instrument is highly expensive.
  • Requires a lot of maintenance.
  • Handling is difficult. Only experts can operate it.

Applications of Comparison Microscope

These microscopes have wide applications in forensic science.

  • Forensic Ballistics: It is used for the comparison of fired bullets and cartridges. It compares the rifling marks, ejector marks, extractor marks, firing pin marks, striation marks, etc. 
  • Questioned Documents: It is helpful for the comparison of inks, handwriting, and other marks present on the document.
  • Forensic Physics: Here it is used to compare the various types of tool marks, paint chips, soil particles, glass fragments, etc.
  • Forensic Biology: A comparison microscope is used to compare biological samples like-pollen grains, fibers, hair, diatoms, etc.
  • Counterfeit Currency: This microscope is helpful in the comparison of bank notes and currency notes.


A comparison microscope is an instrument that uses two light microscopes connected by an optical bridge which have a series of mirrors and lenses that combine the two images back to a single eyepiece. This facilitates the comparison of two different objects simultaneously.

It has a wide application in forensic science where comparison of various tool marks, glass fragments, question documents, inks, fired cartridges and bullets, soil, hair, diatoms, etc. is required. 

Now with the growth of technology computerized comparison microscopes have been developed, which are providing more accurate comparison results in two-dimensions and three-dimensions. 

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