DAVIDSON OPTRONICS -- Principles of Autocollimation

PRINCIPLES OF AUTOCOLLIMATION

An optical schematic illustrating the basics of autocollimation is shown in Figure 1 above. The system begins with a light source followed by condensing lenses and a projection reticle. The light source is neither coherent nor especially monochromatic; however intensity is important as some applications may require a working distance of several or more meters. The condensing lenses are used to maximize both the intensity and uniformity of the light directed through the projection reticle in focal plane P₁. After passing through the beamsplitter, light enters the objective lens where it is collimated prior to exiting the instrument. Collimation simply means that the light rays exit the instrument parallel to one another. After being reflected by a mirror or other high-quality reflective surface, light re-enters the autocollimator and is focused by the objective lens. The return image appears in sharp focus on the measuring reticle in focal plane P₂ after being redirected 90° by the beamsplitter. An eyepiece is used to view the return image.

Deviation of the mirror by an amount A as shown in the figure will cause the return image to be laterally displaced in P₂ a distance X with respect to the measuring reticle. The amount of deviation of the reflective surface can then be determined from the relationship:
X = 2·A·f_L In this equation, f_L, is a constant equal to the focal length of the objective lens. From the equation it is apparent that X is independent of the distance between the instrument and the reflecting surface.

Deviations in azimuth and elevation can then be recorded in one of two ways: manually, by aligning the measuring reticle with the return image in the field of view and then reading the deviation from a micrometer dial; or electronically, using a position-sensing photodiode in focal plan P₂. The electronic method offers the advantage of complete objectivity in data recording as well as a computer interface.

Autocollimator Applications


Verify the accuracy of a rotary table using autocollimator and polygon	Determine flatness of surface plates

Comparison of work angle with standard angle block	Verify straightness of movement along machined rails

Determine parallelism of end surfaces	Verify the squareness of base surfaces with pentaprism and reference mirror

Align autocollimator to optical bench	Measure angle of optical cube

Measure Deviation of a Wedge	Measure Penta Prism's Deviation Determine Sign of Penta Prism Deviation Angle

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Davidson Optronics, Inc.
Phone: (626) 962-5181
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E-mail: sales@davidsonoptronics.com

PRINCIPLES OF AUTOCOLLIMATION

An optical schematic illustrating the basics of autocollimation is shown in Figure 1 above. The system begins with a light source followed by condensing lenses and a projection reticle. The light source is neither coherent nor especially monochromatic; however intensity is important as some applications may require a working distance of several or more meters. The condensing lenses are used to maximize both the intensity and uniformity of the light directed through the projection reticle in focal plane P₁. After passing through the beamsplitter, light enters the objective lens where it is collimated prior to exiting the instrument. Collimation simply means that the light rays exit the instrument parallel to one another.	After being reflected by a mirror or other high-quality reflective surface, light re-enters the autocollimator and is focused by the objective lens. The return image appears in sharp focus on the measuring reticle in focal plane P₂ after being redirected 90° by the beamsplitter. An eyepiece is used to view the return image. Deviation of the mirror by an amount A as shown in the figure will cause the return image to be laterally displaced in P₂ a distance X with respect to the measuring reticle. The amount of deviation of the reflective surface can then be determined from the relationship: X = 2·A·f_L	In this equation, f_L, is a constant equal to the focal length of the objective lens. From the equation it is apparent that X is independent of the distance between the instrument and the reflecting surface. Deviations in azimuth and elevation can then be recorded in one of two ways: manually, by aligning the measuring reticle with the return image in the field of view and then reading the deviation from a micrometer dial; or electronically, using a position-sensing photodiode in focal plan P₂. The electronic method offers the advantage of complete objectivity in data recording as well as a computer interface.