Optical Comparators

Optical comparators can be very complex machines. However every optical comparator has similar design features and follows the same working principles. Optical comparators are used to magnify inspected part and cast its shadow onto a screen with chart to check for imperfections. Every optical comparator has a light source, which emits light beam. That light beam travels through prism and projects the shadow of inspected part. The shadow is then reflected through series of mirrors and finally ends up on chart screen. The operator can then compare the part against preset chart. Optical comparators have multiple lenses available, with different magnification steps. Zoom options vary from 5X to 500X. Screen sizes are also available in different sizes, from 10” to astonishing 80”. Combining screen size with optical zoom operator can inspect really big parts for really small anomalies. Another features found in modern optical comparators are digital readouts. These can vary from very simple, 1 or 2 axes digital readouts to more complicated 3 axes, programmable readouts, with geometric displays. More sophisticated digital readouts feature serial or parallel interfaces to interact with personal computers. In the nearest future those readouts can be expected to be equipped with more modern ports, such as usb, firewire and possibly Ethernet or lightpeak. Digital readouts with computer connection interfaces allow to transfer inspection results and to further export to CAD applications to manipulate extracted data.

Last important element found in optical comparators is stage. Optical comparator stages can be fully motorized or manual. Stages allow adjusting the position of inspected part. Inspected elements mounted on stage are resistant to external vibration providing more accurate results.

To summarize, the principle of optical comparator is to magnify inspected part. Basic elements found in most, if not every optical comparator are:

• Light source
• Mirrors
• Screen with chart ring
• Stage, motorized or manual
• Digital readout

Optical comparators must receive proper care to ensure longer lifespan and retain measurement accuracy. There are few parts that require regular cleaning:

• lenses
• mirrors
• screens

Any accumulated dust must be removed from the surface of the lenses with a photographic lens brush. The surface of the inner optical comparator lens (projection or condenser) must not be touched. The element of the projection lens facing the mirror is most critical in this respect. A single finger mark on this surface will noticeably degrade the quality of the projected image. It should be noted that the projection lens element facing the work piece is far less critical.

Any grease or finger marks may be removed with a new photographic cleaning tissue. If necessary, the tissue may be moistened with methanol or lens cleaning fluid to assist cleaning.

Access to the optical comparator mirrors is gained by removing the screen. To remove grease or finger marks from the mirror surfaces use a surgical cotton wool swab soaked with methanol. Apply the swab very lightly to the mirror surface finally giving a light polish with another clean dry swab. Do not use force. All mirror surfaces should be cleaned as infrequently as possible. Never use polishes, window cleaning aerosols or liquids when cleaning the mirrors or lenses.

All optical comparator screens are made out of ground glass with printed cross lines. The cross lines can be removed if the screen is not cleaned properly. We recommend that the screens be cleaned only with soap (a mild non-lotion dish-washing solution) and plenty of water. Use a lint free soft cotton cloth to gently clean. Let the screen air dry.

If your environment is extremely dirty, please contact us about optional encapsulated image screens which cover the markings with a protective membrane.

In part 2 of this guide we will focus on selecting right lenses, digital readouts and additional options and accessories for optical comparators.

When selecting proper lens follow the chart below to decide what lens will match the tolerances required. A basic rule of thumb is that
a typical attentive operator can repeatedly discriminate .004″ on the comparator screen. Dividing the “discernible resolution” by the lens magnification determines the minimum resolution attainable for each lens.

Another thing to consider when selection optical comparator is digital readout. Select a basic XY digital readout if only positions and distances are required. If measurement of circles, angles, and parametric distance is required, then select a readout with geometric capability. Repetitive part measurement may encourage the selection of a CNC capable readout. Automatic edge sensing should be considered to eliminate operator subjectivity and increase repeatability and accuracy.

Additional options are available when choosing optical comparator. Repeatability and accuracy will suffer if the workpiece is not properly and securely held. Careful consideration should be given to tooling and to the surface on which you place your optical comparator.