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·fLIn this equation, fL, 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.

Resources

Top Categories

Product

Company