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 our 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.