6 Harsh Truths About an Optical Comparator That You Don't Know About

by Andrew Scarella on July 27, 2016 From Technology, News And Comment

Why you should choose an optical system for vision measurement.

So you’re looking for an optical comparator? Before you send in that purchase order, there are a few things you will need to consider. Read below and learn why optical comparators may not be the best choice for your company, and what alternatives are available. But first, let’s start with the basics.

What is an optical comparator?
The optical comparator is a shop floor device used for optical measurement to provide visual inspection of a component. Conventionally, the optical comparator was used in conjunction with an overlay of polyester film, known as mylar. In Engineering, the Design or CAD department would use an inkjet printer to print a large scaled image of the product and its tolerances onto the mylar. The operator would then place the mylar on the machine to compare the product with the mylar enlarged image.

Since its release, many modifications have been made to this technology, including adding basic edge detection, and microprocessors that enable users to measure simple geometry two dimensionally automatically in order to increase their capability. Optical comparators are valued because of the simplicity of the system and its operation.

Simplicity can be compelling. However, with simplicity comes major drawbacks in functionality that could be costing you. Here are some reasons why you should reconsider your optical comparator.

Lack of Quantitative Data 
As organizations leverage manufacturing information, quantitative data is becoming increasingly more important to your operation. Today, quantitative manufacturing data for businesses in need of manufactured parts is essential for business operations. Optical comparators are being put to the test as companies will no longer accept attribute gauge inspection as a validated data point. In fact, companies using optical comparators in the medical field are facing an even bigger challenge, as the FDA is starting to standardize quantifiable data of an inspected part, rather than accepting visual inspection data to ensure quality of product.

Low Resolution
One advantage of optical comparators is the large field of view, allowing you to see the entire component at one time. However, the wide field of view poses a problem of lower resolution — resulting in lower accuracy.  The optical comparator is great for applications that have a backlit component profile, as it is very clearly defined. On the contrary, for surface information, it is much more challenging. Take for example a square block with a blind hole. If a hole is not able to be lit by backlight, or a surface measurement is required where no backlight can pass through the object to give a strong contrast, the optical comparator invariably relies upon manual measurement to determine the edge. The optical comparator cannot see through it completely due to lower resolution, so top-lit features are not clearly defined. Lower resolution not only affects top-lit features, it also lessens your capability as tolerances become tighter. Comparators try to overcome this by increasing the screen size or increasing magnification. The trouble is that these “fixes” to the problem come at a cost, which brings us to our next point.

Cost
As tolerances become tighter, it may not be ideal to use an optical comparator as they become expensive to produce. Optical comparator prices can range from as low as $6K to as high as $138K. And depending on the size you will need to adjust for specific tolerances, you could be spending an upwards of 65K on size alone.

Temperature Fluctuations
In the manufacturing environment, your component may be subjected to various temperature fluctuations, causing it to expand and contract. Since your optical comparator overlay will be directly affected by temperature, your measurement, while easy to take, may not be as accurate or as repeatable. This is due to the different expansion coefficients of the mylar, growing and contracting as the temperature changes.

Environmental Dependency
Your measurements are only as good as the equipment you are using. When working on the shop floor, those measurements can be affected by dirt and oil. With optical comparators, the mylar used is subject to contamination and temperature as it is only a thin sheet, causing difficulty in tracing the component to the current product design.

No Traceability

Having quantifiable data is not only important for companies to verify that the measurement they took was in fact accurate, it is also important for verification and the history of every single project in a production situation. With an overlay, careful maintenance and database management is required to ensure revisions are created for production. This is especially true for companies associated with ISO (International Organization for Standardization). Standards for ISO and other organizations like the FDA and FAA require documented measurement. Having quantifiable, traceable data from a system rather than subjective data from an operator comparing and making a judgement call from a thin piece of plastic film is becoming more and more common. Measurement to CAD ensures the design intent is met with “measurable information” to aid the process to a verified and measured conclusion.

Results in 3-2-1

The Optiv321 from Hexagon Manufacturing Intelligence is an all-in-one vision system that encompasses all the benefits of a tool maker’s microscope, an optical comparator, and a CMM.

The Optiv321, much smaller than the optical comparator, performs at a much higher resolution. The zoom capability allows you to change magnification at any position for higher resolution, collecting not only profile but top lit data with speed and accuracy. With the Optiv Classic321, the two available surface illuminations provide a crisp edge for automatic detection, allowing the surface information to be easily seen. Combining automated inspection capabilities with PC-DMIS CAD for vision allows programming of the machine directly from the CAD model. You can continue to run a program automatically simply by creating a part program once. And if you’re already using PC-DMIS, then you already know how to program this vision system.

In larger production environments when multiple Optiv machines are required, transferring programs from one machine to another with no loss of accuracy is easy. PC-DMIS uses a full model ensuring 3 dimensional inspection to a full GD&T. Plus, with PC-DMIS reporting, you will always be able to pull up records of previous measurement, ensuring your customers of part validity and traceability.

The Optiv321 has a smaller footprint allowing you to place it on a benchtop. In a shop floor environment, this system excels due to its design. Hexagon provides users with recessed optic lighting to avoid contamination allowing you the confidence you need to obtain accurate results. With added calibrated light and camera, ambient light is a thing of the past as the Optiv321 is not susceptible to any ambient light.

Still thinking about submitting that PO? We didn’t think so. Get the benefits of a 3-in-1 solution at a relatively low cost. 

Andrew Scarella

Andrew Scarella is the Digital Marketing Manager for Hexagon Manufacturing Intelligence North America. He contributes to this blog as well as updates the North American website and provides content for the company social media channels including Facebook, Twitter and LinkedIn.