The optical comparator, or profile projector, has been a staple in quality control for the manufacturing industry ever since it was first patented back in 1925. Over the years, its overall design hasn’t changed much—aside from some upgrades with digital features and software. The fact that it’s still widely used today really shows how reliable it is for checking parts for accuracy and spotting any deformities.

In this write-up, we’ll go over what optical comparators are and answer some key questions: What are they used for? How do they work? And how do the traditional models hold up compared to the newer digital ones?

Optical Comparator

What’s an Optical Comparator?

Optical comparators—also known as profile projectors or just comparators—are measurement tools commonly used in manufacturing. Their job is to inspect, measure, and compare the dimensions of parts. They work based on optics, using light, lenses, and mirrors to project a magnified image (or silhouette) of a part onto a screen. This makes it easy to compare the part’s shape and size to its specified limits.

Optical comparators are great for checking both the dimensional accuracy and surface defects of parts—things like scratches or indentations. In a nutshell, they let you measure and inspect parts without actually touching them, which reduces handling but still gives you a close-up look.

There are two main types of optical comparators: horizontal and vertical. Here’s a quick breakdown of how each one works:

Horizontal Comparators
In a horizontal model, the light travels sideways, and you’re viewing a silhouette of the part from the side. This setup is ideal for parts that need to stay in a fixed position. Think of things like screws held in place or castings clamped in a vise.

Vertical Comparators
In a vertical model, the light travels straight down, so you’re looking at the part from above. These are perfect for flat components that can sit on the work stage, like gaskets. They’re also a good fit for soft or flexible parts that need a flat surface to get an accurate measurement.

Both types of optical comparators are commonly used in manufacturing and quality control labs. You’ll find them in industries like automotive, aerospace, medical manufacturing, defense, and even scientific research.

How Does an Optical Comparator Work

Optical comparators haven’t changed much since they were first invented in the 1920s—and for good reason. The core technology relies on optics, which have improved in quality over time but not in how they fundamentally work. In fact, optical comparators work a lot like those old-school overhead projectors you might remember from classrooms. Light passes through a stage, then travels through lenses and mirrors to project the shadow of whatever’s on the stage onto a screen.

Here’s the basic idea:
A part is placed on the stage, and a light source shines on it, creating a shadow image. That shadow gets magnified by lenses and reflected by mirrors onto a screen at the back. The screen is set at a fixed distance to ensure accurate measurements.

Customization and Variations
The size and magnification of the image can vary depending on the comparator’s design. For instance, screen sizes typically range from 12 to 36 inches, though larger screens are available. Keep in mind, larger screens need bigger enclosures since the image requires more distance to scale up without distortion.

How Optical comparators Measure

Optical comparators use three main measurement methods:

1. Silhouette MeasurementThis is the simplest method. The part’s shadow (silhouette) is projected onto the screen, and since the magnification is known, you can measure directly from the silhouette.
2. Point ComparisonIn this method, you compare the part’s silhouette to specific reference points on the screen. You center the image on the screen and move the stage to align the silhouette with the marked points. The distance the stage moves gives you precise measurements.
3. Software AnalysisThe third method uses software to analyze and measure the image automatically. This is exclusive to digital optical comparators, which handle the entire process electronically.

The first two methods are what you’ll find in traditional optical comparators—they’re still the go-to choice in most industries. The third method is more modern and used in digital models, offering a faster and more automated approach.

How to Use a Traditional Optical Comparator

Using a traditional optical comparator is pretty straightforward. Here’s the basic process:

1. Placement: Start by turning on the comparator and placing the part you want to inspect on the staging area.
2. Alignment: Once the part’s image is projected onto the screen, you’ll need to place an overlay (also called a template or Mylar) on the screen. This overlay is a transparent sheet with the part’s drawing scaled to match the comparator’s magnification. Align the overlay with the image of the part.
3. Comparison: After the overlay is in place, compare the drawing on the overlay to the part’s image. Look for any discrepancies and determine if the part meets the required tolerances.

That’s the typical process for most traditional optical comparators. However, there are variations depending on the setup. For instance, instead of using a part drawing, you might use an overlay with grids or concentric circles for precise measurements. Another option is the point comparison method, where you center the image on the screen and move the stage to match specific points on the overlay, measuring the distance moved to ensure accuracy.

Limitations of Traditional Optical Comparators

While traditional optical comparators are easy to use and require minimal training, their simplicity also brings some limitations:

• Limited Views: As production parts get more complex, the need to inspect them from multiple angles is increasing. Traditional comparators don’t handle this well.
• Lower Precision: Although accurate, traditional comparators may struggle to meet the tighter tolerances required for today’s modern parts, leaving less room for error.
• Labor-Intensive: They can only measure one part at a time, which can be a bottleneck when inspecting large batches of parts—especially when a vision system could handle multiple parts at once.
• 2D-Only: Traditional comparators can only display 2D images, which limits their ability to analyze parts with multiple dimensions simultaneously.

These limitations aren’t a big deal for simpler, one-off inspections of 2D parts. But for more complex, large-scale analyses, traditional optical comparators might not cut it. In those cases, a more advanced system is the way to go.

Digital Optical Comparators vs. Traditional Optical Comparators

Traditional optical comparators are great for smaller jobs, but as parts get more complex and production scales up, they start to show their limits. That’s where digital models step in. Digital optical comparators are designed to handle the challenges of modern manufacturing, offering automation and precision where manual methods fall short.

Here are some key advantages of digital optical comparators:

• Automation: Digital models rely on software and cameras instead of the human eye for analyzing and measuring parts. The software handles the entire process automatically and much faster than a person could.
• 3D Capabilities: Unlike traditional models, digital comparators can inspect parts in all three dimensions by using advanced lighting techniques and 3D inspection methods.
• Batch Analysis: Because they’re automated, digital comparators can analyze multiple parts at once without requiring constant human input.
• Improved Accuracy: By removing human error from the equation, digital models are incredibly precise—something that’s essential for today’s high-tech industries.

These benefits make digital optical comparators a powerhouse for maximizing accuracy while cutting down on labor and boosting efficiency.

Using Traditional vs. Digital Optical Comparators

Traditional optical comparators are simple to use, but they come with some pretty big downsides for both operators and clients. Here’s a breakdown of the major issues:

• Lack of Quantifiable DataTraditional methods don’t provide much detailed data. When you’re comparing a part to an overlay, it’s mostly a pass-or-fail situation. But these days, clients often want detailed numbers about every little deviation, and that’s tough to get with old-school methods.
• Limited FlexibilitySince traditional comparators only show 2D images, they struggle with more complex parts that need multi-angle analysis. If you’re working with a tricky part, you’ll probably have to manually move it around and use multiple overlays—time-consuming and frustrating, especially for parts with odd shapes.
• Lower AccuracyModern parts need super-tight tolerances and precise data. Traditional comparators can be accurate, but it takes a skilled operator to get those results. Plus, since everything is done manually, there’s always a chance for human error.
• Higher CostsOverlays aren’t cheap to produce, and using traditional comparators eats up a lot of time and labor—especially when you’re dealing with complicated parts. Those costs can really add up.

In short, traditional optical comparators require lots of training and hands-on work. While they’re fine for small-scale jobs with simple parts, they’re not really built for the fast-paced, complex demands of modern manufacturing.

That’s where digital optical comparators shine. They take the basic idea of traditional comparators but supercharge it with modern tech. The result? A system that’s automated, faster, and easier to use—cutting down on operator workload while keeping up with the speed and complexity of today’s manufacturing world.

How to Use a Digital Optical Comparator

Digital optical comparators really step in where traditional models can’t keep up. Like the older models, they use optics to compare a part to its design, but they don’t stop there. Digital comparators take it further by adding tools like CAD integration, laser measurements, and smart analysis software.

What’s the big deal for operators? It means the process is way simpler. Instead of three steps—place, align, and compare—you only need to do one thing: place the part on the staging area. After that, the comparator takes over. It handles alignment, comparison, and even spits out a pass/fail result with detailed data to explain why.

Why Go Digital?

Here are some standout perks of digital optical comparators:

• Fast AutomationThese machines use software and cameras to measure and analyze parts automatically. They align and compare parts with CAD drawings in just seconds, cutting down on operator effort and speeding things up.
• 3D CapabilityWith features like advanced lighting, rotary stages, and lasers, digital comparators can measure parts in all three dimensions. It’s quick and accurate, requiring almost no input from the operator.
• Improved AccuracyAutomation takes human error out of the equation. These systems give you super precise measurements, detailed reports, and all the supporting data—stats, measurements, and pass/fail results—for single parts or whole batches.

Applications of Digital Optical Comparators

Digital optical comparators are used by companies in many industries to solve a wide range of problems. Here are some common applications:

• AerospaceAerospace manufacturers use digital optical comparators to inspect turbine disks and slots, cooling holes, turbine blade fir trees, seal slots, and even five-axis parts. They help improve accuracy and repeatability, speed up throughput, and collect electronic documentation—critical for the aerospace industry.
• AutomotiveIn the automotive industry, digital optical comparators inspect flexible auto seals and trim, and measure various components and parts. They’re especially useful for comparing seals against CAD data, especially when the parts are tricky to inspect.
• BearingsPrecision is key when making bearings. Digital optical comparators quickly and efficiently check bearings on the shop floor, saving time compared to traditional methods.
• MachiningDigital optical comparators come with special tools to inspect machining parts. For example, the VisionGauge® Tooth Checker tool inspects screw teeth with high precision.
• Medical DevicesA range of tools are available for inspecting medical devices and implants using digital optical comparators. The VisionGauge® Tooth Checker can even examine implant screws. These comparators also inspect stents, medical rasps, orthopedic implants, and more.
• Defense/MilitaryMilitary and defense equipment also benefits from inspection by digital optical comparators. They can quickly automate the inspection of parts with lots of holes or other features, making the process much faster than other methods.
• Power and EnergyMicro-hole inspection is important in the power and energy industry. Digital optical comparators are also used to inspect cooling holes and measure the location of laser-drilled and EDM (electrical discharge matching) holes.
• Tool and DieIf you need to inspect thread rolling dies, digital optical comparators are perfect for the job. You can set them up to automatically identify and check features based on your specifications.

Digital optical comparators have many more uses beyond just these examples! Let me know if you need more details on any specific application.

Learn More Today About Digital Optical Comparators

Using an optical comparator doesn’t have to be complicated or require a ton of training or labor. With JMBom, you can simplify your optical inspection process. Our digital optical comparators are advanced tools that really make the most of optical technology. They're super precise but easy to use, giving you fast, high-quality results. You can quickly and accurately inspect and measure parts, all without the need for operator intervention.

JMBom offers several models of optical comparators for sale. Each one has its own set of advantages and comes with software that works across many different applications. On top of our optical comparators, JMBom also develops, sells, and supports software, systems, and hardware for automated imaging, visual inspection, and measurement solutions. With our powerful and user-friendly products, you can count on JMBom for both custom and standard solutions in a variety of industries — from aerospace and automotive to electronics and medical device manufacturing.

Related Articles

Introduction to Mechanical Comparator

A Complete Guide to 5V Relay Module

What Is A Solid State Relay?