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Operation
MEASUREMENT PRINCIPLE
The usual procedure to identify the centering errors is to rotate the sample in transmitted or reflected light. For the measurement, an autocollimator with additional optics is focussed either to the center of curvature of the surface (Reflection Mode) or to the focal plane of the lens (Transmission Mode).
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| For the measurement in transmission an additional collimator is needed. The parallel beam emerging from the collimator is focussed into the focal plane of the sample to be measured. The images reflected from the lens surfaces (Reflection method) as well as the images projected into the focus of the lens (Transmission method) are observed through the eyepiece of an autocollimator, a telescope or a microscope. In the Transmission method it is not possible to determine which one of the two surfaces of a lens is producing the measured centration error. In some cases, a lens tested in transmission can reveal no centering error, although the lens is tilted in the mount. The images reflected from the lens surfaces, however, represent an undoubtable criterion for the surface tilt and the individual centering errors.
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| The Reflection method is the only total and true method of measurement of centration errors. However, the Reflection method is in many cases difficult to manage. On the other hand, the Transmission method with some mechanical constraints gives in many cases satisfactory results. For a good economy of the optical manufacturing both methods should be considered. When a centration error is present, the observed image describes a circle, while the sample is rotated around a reference or datum axis. The diameter of this circle is proportional with the size of the centering errors. The result of the measurement can be given as a radius of the run out circle (in µm) or as a tilt of the surface or of the lens axis (in arcsec). |
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