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

Surface Illumination

Optical comparators illumination of surface features on solid objects is achieved through the use of high intensity fiber optic light guides. These guides are adjustable and can be extended by pulling on the cable ends protruding from either side of the lens mounts. They may be secured in place with the clamping screws on the side of each of the guide blocks.

For the brightest reflected image, place the light guides as close as possible to the face of the component to be inspected. Where components have a directional surface finish, e.g. ground faces, the surface texture should be placed at right angles to the table, this will give the brightest reflected image on the screen.

As the fiber optic cable is a “cold light” source, the heat radiated to the component under test is minimal.

Manual Table Adjustment

The X axis of the stage has a quick release for coarse motion. This feature should not be used to move the stage during a measuring routine. Also, when the quick release is engaged, the control knob of the X axis is disabled.

To engage the quick release, rotate the lever to the furthest position to the right. To re-engage the control knob rotate the lever to the furthest left position.

Stage Helix Adjustment

The top plate of the optical comparator stage has a helix adjustment, which allows the top plate of the stage to be rotated. To do this the 2 locking clamps located beneath the stage must first be loosened. The stage plate can now be rotated ±15 degrees. The helix indicator shows the amount of rotation in 5’ increments. Once the desired helix is adjusted tighten the locking clamps to hold this position.
To restore the helix to the zero (0) position, loosen the locking clamp rotate the helix adjustment to zero.

Vertical Stage Adjustment

Vertical adjustment of the optical comparator work stage is by means of the large hand wheel on the left below the work stage knee.

Screen Angular Measurement

The electronic venire protractor screen, consists of a frosted glass screen divided into four equal quadrants by precision cross lines. The screen is rotated by means of a small hand wheel on the right of the screen. The standard electronic venire protractor can then be used to determine angles with 1 minute resolution.

Electronic Rotary Protractor

16H optical comparators are equipped with a digital electronic rotary screen protractor. These compact units display the current rotary position of the screen and utilizes non-volatile memory to permanently store the calibration data regardless of power loss. Optical comparators display rotary position in either degrees and minutes or degrees and 1/100 of a degree.

The guide below applies to Dorsey Metrology 16H Benchmark optical comparator. It might not reflect actual features found on other products.

First step in optical comparator operation is of course switching the power on. The projector lamps are usually controlled by rocker switches on the lower front panel of the projector body. Separate switches control the profile and surface illumination lamps. The profile lamp uses a 2 position switch: down is low intensity, while up is high intensity.

Before using optical comparator to inspect part some pre-run checks might be required. First ensure that the projector is secure on its stand or on a substantial bench. Having selected a lens of the required magnification and locked it securely in position, locate the component to be inspected in a suitable work holding fixture which must be securely clamped to the table, the optical comparator is now ready for use.

Focusing

Focus is achieved by rotating the small hand wheel on the right hand side of the stage knee. The lens focal plane is approximately mid-way along the focus traverse, central to the dovetail slot in the table.

Profile Projection Intensity

There are two profile light intensity levels, high or low, selected from the rocker switch on the panel below the screen. Select the level that gives the most comfortable viewing, according to the object projected and the external lighting conditions prevailing. Profile lighting is provided by a 24V 150W tungsten halogen lamp. An iris diaphragm and or green filter is available as an optional extra which may be fitted direct onto the lamp house to reduce the level of reflection from brightly polished surfaces e.g. ground threads, etc.