Tutorial

A. An operator views a hot target through a small telescope containing a calibrated lamp filament in the image plane.

B.1. The lamp filament is initially cooler than the target and appears dark before it. The sighting index appears black below the filament.

B.2. The lamp current is raised until the filament becomes hotter than the target and it appears brighter than the target.

B.3. The lamp current is adjusted until the lamp temperature equals that of the target. The filament blends into the glowing background of the target.