How to Use an Optical Comparator

How to use an optical comparator

Optical inspection is one of the top methods of verifying parts in the modern manufacturing industry. Comparing parts to their specs is essential for ensuring that parts are completed as requested and within acceptable tolerances. No tool has been more critical to these optical inspection procedures than the optical comparator.

Table of Contents

  1. How to Use a Traditional Optical Comparator
  2. Using Traditional Optical Comparators vs. Using Digital Optical Comparators
  3. How to Use a Digital Optical Comparator
  4. Learn More Today About Digital Optical Comparators

First developed in the 1920s, the optical comparator has remained a staple of the manufacturing industry, used in metrology and quality control operations. While the principles behind the operation of the optical comparator have remained largely the same, models have been updated with technological enhancements to simplify ease of use. Today, inspection professionals use two primary types of optical comparators: traditional and digital models. This article discusses how to use each type of optical comparator and how they compare.

How to Use a Traditional Optical Comparator

Traditional optical comparators operate using many of the same principles and functions as the first optical comparator models. In practice, they work similarly to traditional overhead projectors — a light is directed through a stage area and toward a combination of lenses and mirrors, which project a silhouette from the stage onto a screen. This silhouette is often magnified for detailed observation and measurement.


As for using an optical comparator with a traditional setup, the steps are similarly simple:


  1. Placement: First, the operator must turn on the optical comparator and place the part to be observed on the staging area.
  2. Alignment: When the part’s image is projected on the comparator’s screen, the operator must place an overlay on the screen. The overlay, also known as a template or Mylar, is a part drawing printed on a transparent overlay that is scaled to match the magnification of the comparator. The operator must align this overlay with the part’s image.
  3. Comparison: Once the overlay is placed, the operator compares the drawing to the image and identifies any discrepancies. From these discrepancies, the operator will determine if the part is within tolerance.


This is the process used for most traditional optical comparators, though there are various methods available for this technology. For example, instead of an overlay with a part plan, an overlay may feature a grid or concentric circles to allow for more precise measurements of a part. Alternatively, a point comparison method may be used, where the image’s silhouette is centered on the screen compared to an overlay. The user then moves the stage to hit prescribed points on the overlay, measuring how much the stage had to move to match the part to each point.

Using Traditional Optical Comparators vs. Using Digital Optical Comparators

While traditional optical comparators are generally straightforward to use, they present significant disadvantages to both users and clients. Some of the most significant drawbacks include:


  • Little quantifiable data: Measurements using traditional comparator methods can be subjective and difficult to quantify. When comparing a part to an overlay with a plan, the goal is simply to pass or fail the part. However, clients are increasingly asking for quantifiable data about each deviation, which is difficult to achieve with this method.
  • Limited flexibility: Traditional optical comparators only project 2D images onto a screen. This presents an issue in an industry where parts are becoming increasingly complex and require analysis from multiple angles. With a traditional optical comparator, an operator analyzing a complex part must physically move the part and utilize multiple overlays for analysis, which can be difficult depending on the part’s geometry and requires a significant input of time and labor.
  • Reduced accuracy: Today’s parts require tighter tolerances and more quantifiable data. Although traditional optical comparators are capable of gathering accurate measurements, this requires a highly trained operator. Additionally, the manual nature of a traditional optical comparator always leaves room for human error.
  • High costs: Traditional optical comparators incur significant costs over time. Overlays are expensive to produce, and the labor input required for traditional optical comparators is significant, especially for complex parts.


In short, traditional optical comparators require extensive training to use properly and need significant labor input. While these limitations may be a non-issue for small operations that work with simple parts, the manufacturing industry as a whole is quickly growing in scale and complexity. It needs optical comparator tools that can keep up.


Manufacturing companies need tools that are quick and easy to use to handle large quantities of complex parts. This is where digital optical comparators come into play. Digital optical comparators take the concept of traditional optical comparators and apply new technology to key areas. The result is an automated technology that is faster and easier to use, reducing the labor input of operators.


Digital optical comparators directly reference CAD drawings of parts for comparison

How to Use a Digital Optical Comparator

Digital optical comparators pick up the slack where traditional models fall short. Digital optical comparators, like traditional models, utilize optics for comparing a part to its plans. However, digital optical comparators do so by augmenting the existing comparator technology with a combination of measurement and analysis tools. Digital optical comparators directly reference CAD drawings of parts for comparison, along with laser measurement tools and advanced comparison software.


So what does this mean for operators? Essentially, digital optical comparator instructions for operators are pared down from three steps to one: place the part on the staging area. From there, the digital optical comparator does the rest. The system will automatically handle alignment and comparison, providing a pass/fail result along with analytical data supporting the decision.


In addition to the simplified use, digital optical comparators also offer the following advantages:


  • Fast automation: Digital comparators use software and cameras to analyze and measure parts automatically. The system automatically aligns and compares parts with their CAD drawings, doing so within seconds. This minimizes operator input and allows for higher throughput.
  • 3D capability: Digital optical comparators use multiple lighting techniques, additional positioning stages (e.g. rotary stages), and lasers to analyze parts in all dimensions, allowing for quick, one-shot measurements with minimal operator interference.
  • Improved accuracy: Digital optical comparators are extremely accurate, accomplishing highly detailed measurements automatically and eliminating the potential for human error. Additionally, the digital comparator software provides detailed documentation backed with thorough data, including measurements, statistics, and pass/fail results for parts and batches.


While automation is one of the many significant benefits to using the VisionGauge® Digital Optical Comparator, the system is also accurate and efficient when used manually by an operator wanting to perform direct measurements on a part or make manual comparisons of a part to its CAD file. The VisionGauge® software interface is intuitive and easy-to-use even for manual operation: Operators can quickly load part CAD files and pre-programmed inspection routines with the system’s barcode reader, and the stages and overlay can be manually controlled with the system’s industrial-grade joysticks. Both automated and manual operation modes produce highly accurate results and complete documentation.

Learn more today about digital optical comparators

Learn More Today About Digital Optical Comparators

Using an optical comparator doesn’t need to require extensive training or massive labor inputs. Simplify your optical inspection with VISIONx, Inc’s VisionGauge® Digital Optical Comparator.


The VisionGauge® Digital Optical Comparator allows you to carry out fast and accurate inspections and measurements of parts, completely operator-independent. VISIONx, Inc. offers several digital optical comparator models, which all include VisionGauge® software with a wide range of applicability.


In addition to our optical comparators, VISIONx, Inc. develops, sells, and supports a range of software, systems, and hardware solutions for your imaging, inspection, and measurement needs. Our powerful and easy-to-use products add value to any operation in practically any industry, including aerospace, automotive, electronic, and medical device manufacturing. VISIONx, Inc. is ready to work with you on your specific applications and provide you with a standard or custom solution you can rely on.


Learn more about VISIONx, Inc.’s products, browse our list of offerings, or contact us online today.

What Is an Optical Comparator?

What is an optical comparator


The optical comparator has been used in quality control in the manufacturing industry since it was first patented in 1925. The overall design has changed little in that time, with the exception of some digital and software enhancements. The continuing popularity of this device is a statement of how useful it is for optically checking parts for conformance and deformities.


Here we will discuss what comparators are, as well as the following questions: what is an optical comparator used for, how does it work, and how do traditional models compare to digital ones?

Table of Contents

  1. What is an Optical Comparator?
  2. How Does an Optical Comparator Work?
  3. Flaws of Traditional Optical Comparators
  4. Digital Optical Comparators vs. Traditional Optical Comparators
  5. VisionGauge® Digital Optical Comparators

Comparators inspect, measure, and compare the dimensions of manufactured parts

What Is an Optical Comparator?

Optical comparators, also called comparators or profile projectors, are measurement tools used in the manufacturing industry. Comparators inspect, measure, and compare the dimensions of manufactured parts. These measurement tools function using the principles of optics by utilizing illumination, lenses, and mirrors to project a magnified silhouette of a part upon a screen. Doing this compares the part to its prescribed limits.


Optical comparators are used to check for both dimensional accuracy and surface defects, such as scratches and indentations. In short, they allow for non-contact measurement and observation, minimizing handling while still allowing for close inspection.


There are two primary configurations of optical comparators: horizontal and vertical. Here’s how they work:


  • Horizontal comparators: In a horizontal model, the optical comparator’s light travels horizontally so the viewer is looking at a silhouette taken from the side of a part. This model works best for parts held in a fixed location — some examples include screws that are fixed in place or castings that must be held in a vise.
  • Vertical comparators: In a vertical model, the optical comparator’s light travels vertically so the viewer is looking down on the part. This works best for flat components that can lie on the work stage, such as gaskets. They also work well on flexible or soft elements that need to lie on a flat surface to provide an accurate measurement.


Optical comparators of both types can be found in manufacturing shops and lab environments related to quality control. They are most popular in industrial sectors, including the scientific, automotive, medical manufacturing, aerospace, and defense industries.

How Does an Optical Comparator Work?

Optical comparators have changed little since they were invented in the 1920s — the reason for this is that the technology uses optics, which have only changed in quality, not function. Comparators work similarly to overhead projectors commonly used in classrooms. A light is directed through a stage to a series of lenses and mirrors, which then project the silhouette of whatever is on the stage onto a screen.


Optical comparators use this same principle. A part is affixed to a stage and a light source shines on it, resulting in a shadow image of the part. The shadow is magnified with lenses and bounced by mirrors onto the back of a screen. This screen is fixed at a known distance for measurement purposes.


Optical comparators can vary in the size and magnification of the projected image. Both of these metrics depend on the optics and screen size of the comparator. Screen sizes for optical comparators range from 12 to 36 inches, though models with larger screens are available. However, larger screen sizes are only possible with larger enclosures, as a greater distance is required to generate a bigger image without distortion.


Another way that optical comparators differ is in the measuring processes they use. There are three different measuring processes for comparators:

  1. Silhouette measurement: The simplest measurement method is to project a silhouette of the image onto a screen for measuring. Because the magnification is known, the silhouette can be used to gain accurate measurements.
  2. Point comparison: The second measurement method is to compare the image’s silhouette to prescribed plan points on a screen. The part’s silhouette is centered on the screen and the user moves the stage to hit various points on the screen. This measures how much the stage had to move to match the part to the point.
  3. Software analysis: The last measuring process is digital, using software to analyze and measure the image generated by the optical comparator.


The first two methods are used by traditional optical comparators and are the most common in the industry. The third is employed by digital optical comparators, which handle the entire process electronically.

Flaws of Traditional Optical Comparators

Optical comparators are generally straightforward and require little training to use. Traditional comparators that use silhouette measurement or point comparison simply require the user to fix a part in place and observe the on-screen image.


While traditional optical comparators are easy to use and operate, they also present disadvantages because of their simplicity. Some primary flaws of traditional optical comparators include the following:


  • 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: How accurate is an optical comparator? 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.
  • Labor intensive: Traditional optical comparators can only measure one part at a time. This poses a problem when needing to inspect large quantities of parts, as is often needed in the manufacturing industry. This is particularly the case when inspecting large quantities of parts at once, since a vision system can allow you to place multiple parts for inspection on the stage at the same time.
  • 2D limitations: Traditional optical comparators can only project 2D images onto a screen, which can present issues for analyzing multiple dimensions at once.


Although these limitations present no issues for non-repetitive tasks used to analyze 2D parts, anything outside of this defined operating bubble is an obstacle for traditional optical comparators. For large-scale, complex analysis, a different model is necessary.

Digital Optical Comparators vs. Traditional Optical Comparators

Where traditional optical comparators fall short, digital models pick up the slack. Manual comparator technology is highly useful in small-quantity applications, but with the rise of more complex parts and large-scale manufacturing, automation is necessary. Digital optical comparators present the solution.


Digital optical comparators offer the following advantages:


  • Automation capabilities: These models use software and cameras instead of human eyes to analyze and measure parts. The software automates the measuring process and completes it more quickly than a human can.
  • 3D capabilities: Digital optical comparators can use multiple lighting techniques and 3D inspection methods to analyze parts in all three dimensions.
  • Quantity management: The automated nature of digital optical comparators means they can analyze multiple parts automatically without human intervention.
  • Accuracy: By removing the potential for human error, digital optical comparators are extremely accurate in their measurements, which is necessary for many modern industries and technologies.


These advantages effectively maximize the accuracy of optical comparator measurements while reducing labor.

Primary benefits of the VisionGauge line of digital optical comparators

VisionGauge® Digital Optical Comparators

If your company is interested in digital optical comparators that leverage the utility of optical inspection and the functionality of automation, look no further than the VisionGauge® Digital Optical Comparator.


The VisionGauge® Digital Optical Comparator from VISIONx, Inc. is an advanced optical analysis tool that maximizes the function of optical comparator technology. These digital profile projectors are extremely precise yet easy to use, delivering fast, high-quality results. Some primary benefits of the VisionGauge® line of digital optical comparators include the following:


  • Higher throughput: The VisionGauge® system is fully automated, completing measurements swiftly and reliably to maximize throughput and process as many parts as possible.
  • Work directly with CAD data (no Mylars required): The system captures images of manufactured parts and compares them directly to their CAD drawings. This means no more overlays or templates, which saves inspection time.
  • Improved accuracy: Using our sub-pixel edge detection technology and using a part’s CAD file, the digital optical comparator system maximizes accuracy in its measurements and comparisons.
  • Automated reporting: The software system obtains complete electronic documentation of measurements, along with multiple reporting options, including reports, charts, statistics, and more.
  • Easy to program, Easy to use: The VisionGauge® Digital Optical Comparator is quick to install and easy to use, meaning operators can get started quickly. Our intuitive Program Toolbox options make it easy to set up automated inspection programs.


VisionGauge® Digital Optical Comparators use the latest technology to deliver high-accuracy measurement and inspection results every time.


Learn More Today About Digital Optical Comparators

VISIONx, Inc. has multiple digital optical comparators for sale. Each model presents competitive advantages and includes VisionGauge® software with a wide range of applicability. On top of our optical comparators, VISIONx, Inc. develops, sells, and supports software, systems, and hardware for other automated imaging, visual inspection, and measurement solutions. With powerful and easy-to-use products, you can rely on VISIONx, Inc. for custom or standard solutions in various industries — from aerospace and automotive to electronic and medical device manufacturing.


To learn more about VISIONx, Inc.’s products, including our digital profile projectors, browse through our list of offerings or contact us online today.