Optical metrology

Optical comparators are optical inspection tools. They grant non-contact inspection of manufactured parts in final result being repellent to external variables, such as additional pressure, vibration or wear of contact points, which in final result could in malformed inspection results. Optical comparators are also recognized as profile projectors or shadow graphs. The major function of these devices is to amplify inspected material onto projection screen. Optical comparators are commonly used in thread review.
Optical comparator was invented in early 20th century. It was invented by James Hartness. First optical comparator, known then as shadow graph, projected a tail of an physical object onto a projection screen a few feet away. Then the physical object was compared with a graph showing allowance levels for that part. The optical comparator was invented to standardise screw thread sizes. Shortly it became one integrated machine that could be placed on top of a bench (hence the name benchtop comparator).
The optical comparators became widely utilized during World War II as they were accommodated by US military. They became frequent inspection instruments in artillery production. Virtually every factory-made part was inspected with optical comparator then. Shortly after military, aerospace and automotive industry began using optical comparators in their quality departments. Rapid increase of optical comparators use granted quicker evolution of new applied sciences. Their accuracy was improving due to better lens systems executions.
In the following years the maturation of optical comparators was concentrated on adding extra features and further improvements to accuracy. Additional attempts were made to fully automate optical comparators. By the end of 20th century features such as automatic edge detecting, digital readouts, programmable motorized stage control became general equipment of optical comparator.
In the future, optical comparators will likely be superseded by video measurement systems. These systems have a few advantages over optical comparators. Image processing capabilities, variable zoom lens systems, color and profile images, capturing image of inspected part are among others.

The demand and application of micron and sub-micron manufacturing requirements is growing, which offers unique challenges and immense opportunities to a wide group of tool shops and production parts manufacturers in the United States. The term micromachining generally refers to part details and holes lesser than the human hair that are measured only in microns-or one thousandth of a millimeter.

This focus on micromachining has caught the imagination of nearly every industrial segment. Technical and application engineers say that such industries as biomedical, medical appliance, personal electronics, fluid transfer, optical comparators, optics and fiber optics, RF electronics, communications, military, aerospace products, and the automotive world are centered on micromanufacturing. They all see the future in new and exciting consumer and industrial products emerging daily.

These smaller, lighter parts with higher grades of functionality have set new needs on original equipment manufacturers to reevaluate the purpose and concepts of various machining schemes and technologies. You may have already known a number of future uses in micromachined parts in your computer, heart monitor or pacemaker, car, cell phone, and many more applications.

The capacity to produce parts with such high accuracy and surface quality on a variety of newer materials, including metal alloys and ceramic, is in very high demand. Unique new machines can produce holes as tiny as 0.00078 inches in diameter, 100 times smaller than many past machining operations.

The application of micromanufacturing represents a “business reality” to machine manufacturers and suppliers. Learning to implement these high-tech designs, concepts and machine tools will permit U.S. manufacturers to offer a greater understanding and service capability to combat outside manufacturing challenger.

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Optical comparator was invented in early 20^th century. It was invented in order to standardize screw thread size. It uses the principles of optics to magnify an image of inspected object and cast its shadow on a screen.

Optical comparators became widely used during World War II by military. They became widely used in inspection of artillery parts. Shortly after the military, aerospace and automotive industry adopted optical comparators as standard equipment in quality control.

Automatic edge detection was probably the first additional feature introduced into optical comparators beyond standard magnification. That invention allows the machine to determine inspected part edge, rather than leaving it to the operator. It greatly improves accuracy by elimination operator subjectivity.

Introduction of digital readouts and motorized machines allows optical comparators to become almost fully automatic and independent from operator. Programmable functions are incorporated in most models.

Computer software becomes standard equipment of most optical comparators. They can interface with computers. Image processing and analysis can be run by software. Points form manual or automatic edge detection can now be transferred to a computer when they can be compared directly with original drafts in CAD files.

Modern optical comparators introduced reflective scale technology, which eliminated backlash from linear encoders. Also three axis touch probing was introduced. Touch probe or laser non-contact device allows for measurement of Z-axis.

Recent improvements in lens production allowed for surface illumination. Surface illumination introduced operators’ ability to view not only the profile of inspected object but also features on its surface.

Optical comparators have not changed much since their invention. Most improvements lead to completely eliminate subjectivity of an operator.

These machines are still in wide use by manufacturing companies. Comparing a part profile to a chart is easy to understand, easy to teach and easy to implement. Newer devices have more capabilities and features. Electronic geometric processors allow more accurate dimensional measurements.

Optical comparators are constantly being replaced by video measuring systems.

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