The Importance of a Good Autocollimator
Written by: Al Collins 10/2006
The importance of a good autocollimator can not be fully appreciated without some knowledge of its purpose and an understanding of how it functions. An autocollimator is a device that helps determine if a lens or viewing system of camera is calibrated correctly. An autocollimator is a combination these six parts; a light source, a reticle (a glass target that light is projected through), an objective (a lens that is set for infinity), a beamsplitter, an ocular and perpendicular flat mirror. These parts are assembled in the following manner. The light source projects the reticle’s image (i.e. a Seaman’s star) through the beamsplitter, through the objective, through the lens being tested, to the perpendicular flat mirror. At this point the image is reflected and it makes a return trip through the system until it is diverted by the beamspliter to the ocular and into a human eye for evaluation of the image’s resolution.
A good autocollimator is one that takes advantage of new technologies and incorporates the best components insuring the most objective, thorough, and repeatable result. Newer more sophisticated autocollimators replace the ocular and incorporate video cameras to provide multiple, simultaneous, and independent evaluation. They also offer both one & two meter objectives and a cold light source. The one & two meter objectives permit lenses to be tested at not only infinity but also close focus. A cold light source, with light wire bundles, totally eliminates changes caused by expansion and contraction related to heat associated with incandescent light sources. Perhaps the most significant innovation is that of the addition of a secondary reticle projected at an oblique angle to the primary reticle. Traditional styled and older autocollimators were and are dependant on the subjective evaluation of the reticle’s test target sharpness. The addition of a secondary reticle changes the criterion of confirmation from the interpretation of sharpness to the identification of symmetrical position of three distinct bars. A simple illustration of the two collimation methods can be seen through an example. If you are standing, at 12:00, 25’ in front of a second person standing between two posts its difficult, if not impossible, to tell if that person is slightly in front of the posts, behind or at the same point. If you change your position however, from12:00 to 10:30 and remain at 25’ from the subject , you will be able to easily ascertain if the person is between the posts, in front of one or behind the other. The 12:00 position is synonymous to the subjective problem inherent in traditional autocollimator operation, while the second 10:30 position illustrates the objective certainty of the dual reticle or new style operation.
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