Theodolite ring lights have been invented to ease a difficulty encountered in the well-established opticalmetrology practice of using highly reflective spherical tooling balls as position references. As described in more detail below, a theodolite ring light is attached to a theodolite or telescope and used to generate a visible target on a tooling ball.

A common technique for aiming an instrument (specifically, a theodolite or an alignment telescope) precisely at the center of a tooling ball involves the use of an autocollimating device. This technique is less than ideal because the reflection from the ball is demagnified by the spherical surface of the ball. The demagnification makes it extremely difficult to resolve the reflection and align it accurately within the cross hairs of the instrument. This difficulty increases with increasing distance and with decreasing size of the ball. Also, the autocollimating device adds greatly to the cost of the instrument.

ImageA theodolite ring light produces a more easily visible reflection and eliminates the need for an autocollimating device. A theodolite ring light is a very bright light source that is well centered on the optical axis of the instrument. It can be fabricated, easily and inexpensively, for use on a theodolite or telescope of any diameter. It can be quickly mounted on or removed from the instrument. No calibration or modification of the instrument is needed. Moreover, the theodolite ring light does not reduce the field of view of the instrument and does not scatter light into the objective-lens housing of the instrument.

A theodolite ring light consists of a number of light-emitting diodes (LEDs) placed at equal angular intervals in a ring sized to fit snugly over the objective lens of the instrument. The prototype theodolite ring light, shown in the figure, includes eight 5.5-candela blue light-emitting diodes (LEDs) mounted in a ring fabricated of black polyacetal. The LEDs are powered by two D-size alkaline cells. In general, the number, brightness, and color of the LEDs can be chosen according to the conditions prevailing in a given measurement environment.

The prototype has been used to locate tooling balls of 0.5- in. (1.27-cm) and 1-in. (2.54-cm) diameter at distances from 10 to 25 ft (about 3 to 7.6 m). Operators consistently achieved accuracy and repeatability at or near the limiting resolution of the instrument. These results were significantly better than those typically obtained with conventional targets. In addition, the extreme demagnification by the small spherical target surfaces eliminated the need for calibration and for fabricating the ring to close tolerances.

This work was done by David Clark of Goddard Space Flight Center. For further information, contact the Goddard Innovative Partnerships Office at (301) 286-5810. GSC-14898-1