The simple addition of a charge-coupled-device (CCD) camera to a theodolite makes it safe to measure the pointing direction of a laser beam. The present state of the art requires this to be a custom addition because theodolites are manufactured without CCD cameras as standard or even optional equipment.
A theodolite is an alignment telescope equipped with mechanisms to measure the azimuth and elevation angles to the sub-arc- second level. When measuring the angular pointing direction of a Class ll laser with a theodolite, one could place a calculated amount of neutral density (ND) filters in front of the theodolite's telescope. One could then safely view and measure the laser's boresight looking through the theodolite's telescope without great risk to one's eyes. This method for a Class ll visible wavelength laser is not acceptable to even consider tempting for a Class IV laser and not applicable for an infrared (IR) laser. If one chooses insufficient attenuation or forgets to use the filters, then looking at the laser beam through the theodolite could cause instant blindness.
The CCD camera is already commercially available. It is a small, inexpensive, black-and-white CCD circuit-board-level camera. An interface adaptor was designed and fabricated to mount the camera onto the eyepiece of the specific theodolite's viewing telescope.
Other equipment needed for operation of the camera are power supplies, cables, and a black-and-white television monitor. The picture displayed on the monitor is equivalent to what one would see when looking directly through the theodolite. Again, the additional advantage afforded by a cheap black-and-white CCD camera is that it is sensitive to infrared as well as to visible light. Hence, one can use the camera coupled to a theodolite to measure the pointing of an infrared as well as a visible laser.
Just as it is necessary to use filters to protect the eye when looking directly through the theodolite, it is necessary to use filters to protect the CCD camera and the theodolite's internal optics against damage by the laser beam. One should be aware of the metrology accuracy requirements and use high-quality filters for tighter accuracy metrology requirements. The main benefits of using the CCD camera are being able to view the IR laser and for high-powered lasers, that in the event that one chooses insufficient attenuation or forgets to use the filters, the equipment may be damaged, but there is no injury to the human eye.
This work was done by Julie A. Crooke ofGoddard Space Flight Center.