Deflector Shields (often referred to as just shields or less often as screens) are forcefields that surround a starship, starbase, planet, or other installation for protection against natural hazards and enemy attacks. Weapons fire, energy or projectile-based, cannot directly penetrate the shields unless the shield frequency is known and matched exactly and may weaken the shields by draining energy from them as well as their respective energy sources. Transporters cannot be used when the shields are operative.
It is possible for a starship to have only certain areas of the vessel shielded, leaving other areas unprotected. This would be most useful in times when power reserves were low and only certain areas could be shielded. Lowering portions of the shields also allows usage of transporters without completely sacrificing the protection that shields provide. It takes time, however, to raise and lower a deflector shield. Constitution-Refit-class starships of the late twenty-third century required approximately fifteen-and-a-half seconds to lower and raise their shields when taking a shuttlecraft onboard via tractor beam.
A starship may also choose to extend their shields around another vessel by matching shield emitter frequencies.
Classification by Configuration
Ellipsoidal configurations project an ellipsoidal shield bubble around the ship and regions directly adjacent to the vessel. This shield is presumably stronger than contour-conforming but creates a larger target profile.
Contour-conforming configurations create a shield layer mere meters away from the hull, allowing for a slim target profile, but is generally weaker than ellipsoidal configurations because of the complexity of the configuration's geometry.
Classification by Type (Federation)
Light Shielding System
The Light Shielding System is the most basic defensive shielding system typically found on smaller auxiliary craft such as shuttles, starfighters, and runabouts. While effective, it lacks the power and protective abilities of the Standard and advanced shielding employed by larger starships and installations.
Standard Shielding System
The Standard Shielding System is the most basic defensive shielding system used by both Starfleet and Federation non-military craft. It is more powerful than Light Shielding Systems but not as effective or durable as the more advanced Regenerative, Auto-Modulating, or Metaphasic adaptations.
Metaphasic Shielding System
The Metaphasic Shielding System was developed in the late 2360s as a way to protect vessels from the volatile and increadibly dangerous environments found within the corona of most stars by fending off intense levels of radiation as well as extremely high temperatures. It has been found to operate best when maintained as a distinct subset of the Standard Shielding System, although Standard Shielding Systems can be modified to assume some related properties.
Regenerative Shielding System
The Regenerative Shielding System makes use of redundant shield generators which alternate coverage on a specific area when integrity drops below a pre-determined percentage. This allows the active shield generator to bear the brunt of incoming weapons fire while the redundant generator remains on hot standby. As the primary generator drops in integrity, power is then increased to the redundant generator, which seamlessly takes over the burden of shielding that portion of the ship and allows the other generator to once again recharge on standby.
Shielding systems of this type were originally field tested aboard the Sovereign-class starships with the purpose being to provide effective protection for a starship while at the same time preserving the shield generators in question. The tried and tested tactic of targeting shields could allow an enemy to disable or destroy a Starfleet vessel's protection with a disproportionately small attack. Regenerative shielding, on the other hand, actively applies all incoming fire over the entire shield grid network. While this actually lowers overall shielding capacity, it protects the active generator and allows it to recharge unhindered while the redundant generator system continues providing the ship with shielding.
Advances in this type of shielding have allowed Starfleet to incorporate the entire active-redundant set-up in single generators without any shortfall in effectiveness. General increases in shield power has also meant incoming fire does not need to be spread over the entire shield grid, only relevant sections.
Auto-Modulating Shielding System
The Auto-Modulating Shielding System represents an attempt to counter successful attacks through Starfleet shielding by advanced races such as the Borg. It is designed to immediately adapt to the characteristics of incoming fire by relying upon advanced computational algorithms spread across the entire shield generator network. Working together, the generators can monitor and analyze the smallest of variances across the entire grid. That information is then used to hypothesize a new shield frequency modulation which will be more successful in preventing incoming weapons fire from penetrating the shield grid. This process takes approximately three-tenths of a microsecond.
High Capacity Shielding System
The High Capacity Shielding System is the heaviest shielding system used aboard Federation starbases and other large installations, providing them with a high degree of protection against even the most determined attacks or disasterous occurrences. The significant power and technological requirements of High Capacity Shielding Systems prevent them from being deployed aboard even the largest Federation starships.
EM Dispersal Field
While technically not a shielding system in the strictest sense, the EM Dispersal Field actively nullifies any electro-magnetic energy or artificial particle emissions from starships and installations in order to reduce the effect of their presence on any ecosystem or environment they may be studying. It should be noted this system is not a Cloaking Device.