An Introduction To Optical Comparators

In a world where precision is paramount, the need for accurate measurements has never been more crucial. From manufacturing to engineering, dimensional measurement plays a vital role in ensuring that products meet the highest standards of quality and safety.

However, achieving such precision is not always easy, and traditional measurement tools can often fall short.

Enter the optical comparator, a game-changing instrument that has revolutionized the way we measure and inspect parts.

With its ability to provide accurate and repeatable measurements in a fraction of the time, the optical comparator has become a must-have tool for anyone who values precision and efficiency.

In this article, I will explore the benefits and applications of this remarkable instrument and show you why it should be an essential part of your toolkit.

An optical comparator is a precision measurement tool that projects a magnified identical image of small parts or objects in order to collect dimension measurements and complete inspection protocols.

Optical comparators, also called comparators or profile projectors, are measurement tools used in the manufacturing industry.

They employ the principles of optics by utilizing illumination, lenses, and mirrors to project a magnified silhouette of a part upon a screen.

Comparators project an enlarged silhouette of the part onto a screen and measure the dimensions and shape of the part against specified limits.

Unlike micrometers and calipers, which measure one dimension at a time, comparators measure length and width simultaneously.

Optical comparators are used to check for both dimensional accuracy and surface defects, such as scratches and indentations.

They allow for non-contact measurement and observation, minimizing handling while still allowing for close inspection.

How Optical Comparators Work

An optical comparator works by following these main steps:

  1. A part is affixed to a stage.
  2. A light source shines on the part, resulting in a shadow image of the part.
  3. The shadow is magnified with lenses and bounced by mirrors onto the back of a screen.
  4. The screen is fixed at a known distance for measurement purposes.
  5. The magnified image is compared to a chart gage or standard parameter to determine if the part is within prescribed limits.

Types of Optical Comparators

There are several types of optical comparators used for dimensional measurement:

  • Simple optics: This is one of the three main types of optical systems used in optical comparators.
  • Corrected optics: This is another type of optical system used in optical comparators.
  • Fully corrected optics: This is the third type of optical system used in optical comparators.
  • Digital optical comparator: This type of optical comparator is fast, accurate, and easy to use. It eliminates the risk of operator error, has a higher throughput than other optical comparators, is capable of electronic documentation of data, is fully digital in functioning thus avoiding recording errors, is remarkably simple to use, and comes fully automated with state-of-the-art technology.
  • Zeiss ultra-optimeter: This is one type of optical comparator.
  • Laser scanning gauge method: This is another type of optical comparator.
  • Charge coupled device method (CCD): This is yet another type of optical comparator.
  • Laser diffraction method: This is one more type of optical comparator.

Advantages of Optical Comparators

Optical comparators offer several advantages for dimensional measurement:

  • Non-contact measurement: Optical comparators allow for non-contact measurement and observation, minimizing handling while still allowing for close inspection.
  • Two-dimensional measurement: Optical comparators can measure within a two-dimensional space, as opposed to other tools, like micrometers, that measure only one dimension at a time.
  • Accurate and repeatable measurement data: Optical comparators offer extremely accurate and repeatable measurement data.
  • Ability to measure in 2-D space: Unlike micrometers and calipers, which measure one dimension at a time, comparators can measure in 2-D space.
  • No wear and tear due to friction: Optical comparators don't have many mechanical parts, so there will be no wear and tear due to friction.
  • Can analyze tool wear: Optical comparators are very helpful in analyzing tool wear in many mechanical machine elements.
  • Versatile applications: Optical comparators are mostly used for inspection purposes in tool rooms and shop floors, quality control in production lines, and in most metrology laboratories for research and inspection purposes. They are also used by many product manufacturing industries to analyze the precision of their products.

Coordinate Measuring Machine: The High-Tech Companion of Optical Comparators

If you're looking for a high-tech way to measure dimensions, look no further than the coordinate measuring machine (CMM).

This machine uses a probe to measure the coordinates of a part, creating a precise 3D model that can be compared to the design specifications.

While optical comparators are great for measuring 2D features like length and width, CMMs take it to the next level by measuring in 3D and providing detailed reports on the part's accuracy.

With a CMM, you can measure complex geometries and ensure that your parts meet the highest standards of quality.

So, if you're serious about dimensional measurement, consider adding a coordinate measuring machine to your arsenal.

For more information:

Intro to Coordinate Measuring Machines

Disadvantages of Optical Comparators

While optical comparators offer many advantages, they also have some limitations:

  • **Limited complexity required**: Production parts are becoming more complex, and observing them at more than one angle is becoming increasingly necessary. However, traditional comparators don't accommodate this well.
  • **Less accurate**: Although traditional optical comparators can obtain very accurate measurements, today's modern parts require tighter tolerances, reducing the room for error that is allowed with any manual measurement method.
  • **2D limitations**: Traditional optical comparators can only project 2D images onto a screen, which can present issues for analyzing multiple dimensions at once.
  • **Overlaying**: It is required to overlay a drawing that has been ten times magnified onto the projected image and visually compare the two images.
  • **Not suitable for complex parts**: As production parts become more complex, with more features to inspect to greater tolerances, the advantages of the traditional comparator diminish significantly.

Applications of Optical Comparators

Optical comparators have a wide range of applications:

  • Simple length and width measurements
  • Detecting imperfections
  • Inspection of gears, screws, thread, cams, (along with others)
  • Location of pitch circle diameter
  • Analyzing tool wear
  • Contour comparisons
  • Aerospace industry
  • Electronic manufacturing
  • Watch or clock making or repair

Choosing the Right Optical Comparator

When choosing an optical comparator, consider the following factors:

  1. Consider your measurement and inspection needs
  2. Types of measurement
  3. Magnification
  4. Accuracy
  5. Operator training
  6. Versatility

Best Practices for Using an Optical Comparator

  1. Placement
  2. Alignment
  3. Magnification
  4. Illumination
  5. Measurement
  6. Considerations
  7. Advantages
  8. Types

Maintenance and Calibration

Maintaining and calibrating an optical comparator is crucial to ensuring measurement accuracy. Follow these steps:


  • Keep the comparator clean and free of debris.
  • Lubricate moving parts as needed.
  • Check the light source regularly to ensure it is functioning properly.
  • Inspect the lenses for scratches or damage.


  • Length accuracy
  • Magnification accuracy of the transmitted/contour illumination
  • Magnification accuracy of the reflected/surface illumination
  • Squareness accuracy

Please note that there is no industry standard outlining acceptance criteria for comparator calibration. Therefore, it is recommended to follow the calibration procedures provided by the manufacturer or seek calibration services from a metrology manufacturing company.

Regular calibration is necessary to maintain measurement accuracy.

The last word on the matter

So, we've covered a lot about optical comparators - what they are, the different types, their limitations and accuracy, applications and selection, and even best practices and maintenance. But let me ask you this: have you ever stopped to think about the sheer precision and complexity that goes into dimensional measurement?

I mean, think about it. We're talking about measuring the tiniest of details, down to the micrometer. And yet, these instruments are able to do so with incredible accuracy and consistency. It's truly mind-boggling when you stop to think about it.

But what's even more fascinating to me is the fact that these instruments are constantly evolving and improving. With advancements in technology, we're able to achieve even greater levels of precision and accuracy than ever before. And who knows what the future holds? Perhaps one day we'll have instruments that can measure down to the nanometer or even the atomic level.

In the meantime, it's up to us to continue pushing the boundaries and exploring the possibilities of dimensional measurement. Whether you're a scientist, engineer, or just someone with a passion for precision, there's always something new to discover and learn.

So, the next time you're using an optical comparator or any other precision instrument, take a moment to appreciate the incredible feat of engineering that it represents. And who knows - maybe you'll be inspired to push the boundaries even further yourself.

Understanding Metrology Measurement Units

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My article on the topic:

Exploring Precision Instruments for Dimensional Measurement

Recording for myself: (Article status: plan)

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