What Is an Optical Comparator (Profile Projector)?

What is an optical comparator

The optical comparator (profile projector) 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. Uses of Digital Optical Comparators
  6. VisionGauge® Digital Optical Comparators




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 inspect, measure, and compare the dimensions of manufactured parts

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. Optical profile projectors 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

Profile projectors are generally straightforward and require little training to use. Traditional optical 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 profile projectors 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.

VisionGauge digital optical comparators offer a wide variety of benefits

Uses of Digital Optical Comparators

Companies across a range of industries use digital optical comparators to solve various applications. Below are some common digital optical comparator uses and applications

  • Aerospace: Aerospace manufacturers use optical comparators to inspect and measure turbine disks and slots, cooling holes, turbine blade fir trees, seal slots, and slots on five-axis parts. Digital optical comparators improve accuracy and repeatability, increase throughput, and collect electronic documentation – all essential functions in the aerospace industry.
  • Automotive: In the automotive industry, digital optical comparators inspect flexible auto seals and trim, measure, and inspect various components and parts. Digital optical comparators are ideal for comparing seals against CAD data, especially when the components are difficult to inspect.
  • Bearings: Accuracy in manufacturing is critical for precision bearings. A digital optical comparator checks bearings on the shop floor and works much faster and more efficiently than traditional methods.
  • Machining: Digital optical comparators include tools designed to check machining parts. The patent pending VisionGauge® Tooth Checker inspects screw teeth.
  • Medical devices: Multiple tools are available for inspecting and measuring medical devices and implants with digital optical comparators. For example, the VisionGauge® Tooth Checker tool can examine implant screws. Optical comparators can also inspect stents, medical rasps, orthopedic implants, and more.
  • Defense/military: Equipment used for military and defense purposes also benefits from inspection by a digital optical comparator. VisionGauge® can efficiently automate inspecting parts with large numbers of holes or other features and works much faster than other systems.
  • Power and energy: Micro-hole inspection also has applications in the power and energy industry. Additionally, cooling hole inspection and measurement tools measure the location of laser-drilled and electrical discharge matching (EDM) holes.
  • Tool and dye: Digital optical comparators are ideal for use when you need to inspect thread rolling dies. You can set the system to automatically identify and check features based on your specifications.


Digital optical comparators have many applications in addition to those listed above. The following are just a small sample of industries that benefit from using these systems for their measurement and inspection needs:

  • Biomedical
  • Electronics
  • Semiconductor
  • Telecommunications

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.