Emissive armor is a type of armor which is able to "bleed off" energy from attacks, thus reducing their damage potential. In Space Empires III it was quite powerful, as it deflected a minimum of 1 damage per hit and all components had exactly 1 hitpoint, so basically every time your ship got hit, you would effectively "save" at least 1 component every time you got hit! In Space Empires IV, it deflects a minimum of 10 damage per hit, which is equivalent to 1 damage on the SE3 scale, but since components in SE4 can have more than 10 hitpoints, and weapons in SE4 can use mounts to multiply their damage capacity, and there exist several armor-piercing weapons which cannot have their damage bled off, the armor is much less useful in this game.
Emissive armor does not stack, so a ship in SE4 with ten Emissive Armor III's will only bleed off 30 damage per hit (the bleed-off capacity of one of the components), not 300. If you want to create a similar effect which does stack, please look at Leaky Shields; you can use the shield-regeneration-from-damage ability along with an equal or greater shield generation ability and the standard hit-first armor ability to create a self-regenerating forcefield of arbitrary strength around a ship which can only be penetrated by certain weapons.
Note that in Space Empires IV Classic, emissive armor did not reduce the damage of attacks that were more powerful than its bleed-off capacity; it only negated damage of attacks of equal or lesser value. This ability was reverted to its traditional SE3 behavior in Space Empires IV Gold (and thus in SE4 Deluxe as well); however the ability description "negates any damage of X amount or less" did not change, leading to some confusion.