Optical Comparators

Optical comparators are a type of optical measuring instrument. The measurement principle is similar to that of optical microscopes. The target is placed on the stage, and a light is shined on the target from underneath. This causes the target's profile, or shadow, to be projected on the screen. A telecentric optical system is used to enable accurate measurements.
Optical comparators were originally developed to inspect the outlines of targets. Models equipped with measurement functions appeared later. Some large optical comparators have screen diameters that exceed 1 m.
Optical comparators are also commonly known as a profile projector or shadowgraph.

A typical optical comparator illuminates from below and projects the shadow of the measuring object placed on the stage through a projection lens onto a projection screen. For this reason, it’s also known as a profile projector or shadowgraph.
At this time, the size of the projected image is an image magnified with a correct magnification from the measuring object, and the dimension of the measuring object is measured by measuring this image.
An optical system called a "telecentric optical system" is used in the optical comparator so that it can be projected with accurate magnification from any position on the stage. With a general lens, near objects look large and distant objects appear small, which allows you to judge perspective. Telecentric lenses, on the contrary, project the same size for near and far objects.
This telecentric lens allows you to cast the shadow of an object at the correct magnification without warping the image.

Conventional optical comparators require regular maintenance in order to continuously and accurately perform measurements. Maintenance is often performed onsite by a technician as the tool is generally to large to send and receive regularly.
In addition, regular calibration is required to confirm that the accuracy is as specified. An optical comparator’s calibration cycle is 6 months to 3 years. As with maintenance, calibration is generally performed locally.

Place the object on the stage.
A scale is applied to the image projected enlarged on the screen to measure the dimensions. Alternatively, you can use the XY stage together and measure the dimensions from the amount of movement.
An optical comparator with a computerized calculation function, you can obtain various measurement results such as width, diameter, and angle by taking measurement points while moving the stage.

Place the object to be measured on the screen and adjust the height of the table to focus.
Next, align the orientation of the side you want to measure on the projected image with the orientation of the screen reference line, and adjust the value of the XY stage to 0.
Next, move the stage using the stage movement handle and align the other side of the projected image to be measured with the screen reference line.
At this time, the movement amount of the stage is displayed in each of the X and Y directions, so this value becomes the measured value. In the case of simple measurement in only one direction, the amount of movement in only the X or Y direction is used.

Place the object to be measured on the screen and adjust the height of the table to focus.
Next, align the center point of the circle on the projected image with the point where the screen reference line is public.
In the case of radius, take 0 points here and move the stage to check the movement amount at the point where the edge of the circle was at the center of the stage. In case of diameter, move the stage once from here to the edge of the circle, take 0 point, and move to the opposite edge to check the amount of movement. In either case, it is common to measure in four directions in a cross shape.
It is also possible to measure by applying a concentrically graduated sheet called a "chart" to the screen.
In the case of an optical comparator with a calculation function, the diameter and radius are automatically calculated by taking three measurement points at the edge of the circle.

There are several ways to measure the angle.
A method of checking the amount of rotation of the stage by rotating the stage in the θ direction by aligning the straight line of the projected image with the reference line of the screen.
There is a method of checking by putting a sheet called "chart" with fine scale like a protractor on the screen.
On an optical comparator with a calculation function, the angle is calculated by specifying two straight lines.

There are several types of charts.
For measuring the diameter and radius, there are concentric circle marks, for measuring angles, there are radial marks, and for both. In addition, some of the scales are written in a grid to see the XY coordinate values. Both are placed on the screen and measured by matching them with the projected image.

Fixtures used for optical comparators are used to fix the object in place to be measured in the correct orientation. For example, a round object can be fixtured horizontally by clamping it down, or for fixing an object whose bottom surface is not flat in an orientation suitable for measurement.
There are various types of fixtures including clips, clamps and magnets.

The overlay chart is used by matching it with the measurement image projected on the screen. There are various types of charts. For example, those with a grid or concentric scale are generally used.
In addition, by overlaying the diagram chart in which the design value of the measurement target is magnified at the same magnification, it is possible to see how the contour of the design value differs from the actual measurement target by superimposing it on the projected image.

The optical comparator can not only illuminate from below and transmit light to create a shadow, but can also illuminate from above (lens side) to project outlines.
Even if the measurement target is difficult to measure with only the transmission (backlit) image, it is possible to measure it by using the epi-illumination.

Blackout curtains are used to block light coming from outside. It is used for the purpose of projecting a shape more accurately by blocking ambient light.