A shield is an energy-based defensive barrier which completely surrounds the vessel on which it is equipped, and serves to absorb the damage dealt to that vessel from hostile fire. Shields also deflect the force of explosions and shockwaves, making them an invaluable tool for bomber pilots. FreeSpace 2-era fighter and bomber shields are incapable of repelling energy from beam cannons, and are easily pierced by these weapons. Shields can not be used whilst in subspace.
Shield integrity drops when it absorbs damage, and increases again over time as it draws power from the ships reactor to recharge itself. The speed of this recharge can be increased if energy is diverted to shields, away from the engines and weapons, with the Energy Transfer System. Each shield is split into four quadrants (left, right, forward and rear) and specific quadrants can be recharged by diverting power from other quadrants.
All GTVA fighters and bombers are equipped with shield generators. The strength of the shield depends on the quality of the shield generator and on the type of the fighter. Interceptors, for example, typically lack strong shields, but compensate through high speed and manoeuvrability. Bombers, by contrast, are primarily designed to be tough and resilient, making shield strength a higher priority in their design.
Acquiring the Shield Technology
In 2335, the Shivans appeared for the first time, using advanced technology which included the fighter shield system, which lent them a huge advantage over the GTA and PVN at the time.
To The 14-Year War-era weapons that were in use at the time, Shivan shields were almost completely impenetrable. This advantage allowed the Shivans to win many critical, early battles, such as the destruction of the GTD Amadeus with only one wing of bombers.
It was immediately apparent that the only way to defeat the Shivans would be to develop weapons which could penetrate their shields. A series of daring raids by Terran pilots eventually succeeded in acquiring both scan data and actual Shivan cargo containers containing shield systems.
This allowed for a joint Terran Vasudan venture to reverse engineer the shielding system and begin mass production, eventually in Ribos. The technology was soon fitted to every Terran and Vasudan fighter in service, and the ready availability of shielded fighters to test them on allowed Terran and Vasudan scientists to develop weapons capable of penetrating them.
The Lucifer's Shields
The SD Lucifer is the only vessel above bomber size ever encountered by Terrans or Vasudans to be equipped with an energy shield. The shielding system it used was known as 'sheath shielding', and was utterly impenetrable to any weapons at the time. The only weakness of the technology was one shared with typical fighter shields - they were inoperable in subspace, and this left the Lucifer vulnerable to attack by GTA and PVN pilots while en route to Earth. So far, the GTVA has not been able to replicate this technology.
There is no canonical information about whether FreeSpace 2-era beams could penetrate the Lucifer's shields as they do fighter-mounted shields.
Notes for modders
If you want your figher/bomber to have shields, you must make a triangulated sphere and name it as 'shield'. Also, you must apply a single colour on it.