From the handbook (Page 10):
Built by Mechanic
Fixed Structure - Defensive - Weapon: Beam of light.
Exaggerated stories of Greek inventor Archimedes describe something akin to this. A parabolic mirror and lens array that rotates quickly to fire a hefty beam of searing light. This is a useful anti-air unit, although it's somewhat frail when subjected directly to the weapons of an angry foe.
Note: Prismatic Mirrors have similar use with the aramonian Trebuchets. However, a Prismatic Mirror has shorter range than a Trebuchet and it can also strike aerial targets. It is truly one of the most powerful and useful structures in Total Annihilation: Kingdoms videogame.