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Collimators and Telescopes

In optical metrology, collimators and telescopes are mostly used together to measure different optical and geometrical parameters.

Angle measurement

A telescope mounted in the front of a collimator enables the simultaneous observation of both collimator and telescope reticle. When a collimator is perfectly aligned to a telescope the reticles are superimposed and no displacement occurs.

The presence of an angle between the the collimator and telescope axes is shown by a linear displacement d between the two reticles. The displacement d gives the size of the angular disalignment of the two instruments:

= d / f


d = linear displacement measured in the reticle plane (focal plane)
= effective focal length (EFL) of the observing instrument (i.e.telescope)

[ Diagram ]

To ease the angular alignment of equipment with flat surfaces, a special type of instruments can be used: the square body collimators and telescopes. The reticles of these instruments are accurately aligned to the outer square body surfaces.

Measurement of Optical Parameters

Focal Length

A typical application is the measurement of the effective focal length EFL. A highly corrected collimator set to infinity (emerging beam is parallel) has a reticle with a pair of spaced lines located in its focal plane. The image of the reticle is projected over the lens under test and focused in its focal plane. By means of a microscope (or a telescope with a supplementary achromat) the size of spaced lines is determined and the EFL calculated:

f = fcol S' / S

[ Diagram ]

Centration Errors

A collimator set to infinity contains a reticle with a dark or bright cross. The lens under test is placed in a precision rotary holder. The image of a reticle projected over the lens under test is observed by means of a telescope with additional achromat having a graduated reticle.

While rotating the holder with the lens, the circle described by the reticle image is measured.

[ Diagram ]

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