Visual and photo-electric

Calibration of an autocollimator is made using the NPL-designed small angle generator. In the case of visual- and photoelectric-setting type autocollimators, small angles are generated to check the periodic and progressive errors of the micrometer screw which enables the displacement of the reflected image of the target cross-lines to be measured.

Uncertainties

The best measurement capability expressed as an expanded uncertainty (k=2) is given by:

• Visual and photoelectric: Up to 10 minutes of arc, 0.06 seconds of arc.

Notes on the use of autocollimators

An autocollimator is used to detect and measure small angular tilts of a reflecting surface placed in front of the objective lens of the autocollimator. Ideally, the area of the reflecting surface should be at least equal to the area of the objective lens. However, this is not generally the case when the autocollimator is used in conjunction with angle gauges or a polygon. Therefore, since the objective lenses fitted to most commercial instruments have small but significant wavefront errors, it is important to position the autocollimator so that its optical axis passes through the centre of the reflecting face of the angle gauge or polygon, reducing the effect of wavefront errors to a minimum.

An autocollimator should ideally be used in an environment where air currents in the optical path between the autocollimator and the reflecting surface are minimal. Such air currents, by introducing changes in density and therefore of refractive index, produce random movements of the observed image, impairing the accuracy of autocollimator setting. For this reason, the distance between the objective lens and the reflecting surface should be kept to a minimum and, where practicable, the light path should be shielded from the surrounding air.

The autocollimator should be rotated about its optical axis, if such provision exists, until a movement of the reflected image perpendicular to the direction of measurement produces no change of reading. For photoelectric autocollimators, this condition should be achieved using the photoelectric detector.

Contact

Customer Service tel: +44 20 8943 8631
E-mail: dimensional_enquiries@npl.co.uk