The autocollimator combines both optical tools, the collimator and the telescope into one instrument using a single objective lens. Both beam paths are seperated by using a beam splitter. The autocollimator is a very sensitive angle measuring device and is thus used for the precise angular adjustment of optical or machine components. Due to the collimated beam (infinity adjustment) the measurement results are independent from the distance to the object under test. The operating principle is explained in the following.
Operating principle of the autocollimator
Like in the collimator the image of the illuminated object reticle is projected by the objective lens to infinity. In some distance, the collimated beam is reflected back from a mirrored surface. If the mirror surface is tilted by an angle α with respect to the optical axis, the reflected beam will enter the objective lens with an angle 2α. This leads to a shift d of the image in the image plane which can be calculated with the objective focal length f giving d = 2α x f or α = d/( 2f ). Thus, the sample angle is directly proportional to the measured shift in the image plane (small angles assumed). The resolution of an autocollimator increases proportionally and the angular field of view reciprocally with the focal length of the objective lens.
In an electronic autocollimator the eyepiece is replaced by an electronic camera with discrete sensor pixels (e.g. CCD or CMOS sensor type). It can be of a 2D frame type allowing angular measurements in two directions, or a 1D line scan sensor for single axis measurements. The digital camera is usually connected to a PC which calculates the measured angle from the image by using image analysis software. The PC assisted measurement guarantees much higher resolution, accuracy and repeatability of the results compared to the visual inspection, since it does not depend on the operators experience and attention.
The high resolution of electronic autocollimators is due to the evaluation of gray scale levels in the image which allows for sub pixel interpolation of the image position. Depending on the focal length of the objective lens and the stability of the setup, angular resolutions of 1/100 up to 1/1000 arcsecs can be achieved.
Sub pixel resolution by evaluating image gray levels