Using Optical Comparators In Precision
The optical comparator, or shadow graph, as it is sometimes called, is
one of the most useful precision measurement tools in the shop. With proper use, it can be highly accurate and
save a tremendous amount of time.
This precision measurement tool is quite capable of accuracies of .0002 in./.005 mm.
Time spent practicing with known gages is time well spent. There are some subtle aspects of the optical comparator
that require careful attention, but not any more than any other tool, such as a precision micrometer.
A classic mistake is to have the workpiece out of square with the table of the
comparator. This gives an obviously false reading, yet it happens more often than you might imagine. This is why it
is necessary to spend some time practicing with gages.
Injection mold makers, tool and die makers and precision machining
operations all use the comparator on a daily basis. It can be used to check angles, radii, distances between
centers, intersection points, thread depths, thread pitch and many more details.
Some of these details, such as thread pitch, are actually quite difficult to inspect
any other way. This is especially true in the case of tiny threads, where it is impractical to physically hold
wires and a micrometer to measure.
Time proven design
The basic design has not changed since the introduction in 1920. What has changed is
improved optics, user friendly interfaces, the addition of digital readouts, and now the use of computers to
In the past these precision measurement tools were large and had a huge footprint.
The newer generation of machines can easily fit on a table top and do everything that the huge tool of the past
The cost has also come down considerably. Because the basic design and technology is
unchanged, many competing companies have emerged with improvements and refinements, without increasing the cost
Now operators can compare their results with the CAD file and download the
information. This is a great improvement over the machines of the past.
Optical comparators vs. video measuring systems
In many respects, the shadow graph is being replaced by video measuring systems. This
is due primarily because of the following features the video system has over the comparator:
Fully corrected (i.e., not inverted) image
Image processing capabilities
Variable zoom lens, offering multiple magnification levels
Variable illumination with a cool fiber-optic light source, plus three light sources
Color and profile images
Capability to capture images of work pieces or part features
Capability to archive and document measurements
Nevertheless, many companies will continue to use the tried and proven comparator because people
are used to it, it works and it is easy to understand.
The choice between an optical comparator and a video measuring system is not an easy one. Depending
on your needs and budget, either one could do a superb job. One option is to purchase a used optical