Defense screens are an integral part of any interstellar starship, as they provide protection for both a ship and its crew from dangerous environmental conditions ranging from ionizing radiation, natural energy discharges, physical collisions, as well as weapons fire. Although the tactical usefulness of shields is often quite obvious, they more often serve a role of simply controlling the immediate environment of a starship and keeping it within acceptable limits.

Federation starships achieve this level of control through a basic field configuration consisting of a subspace "shell" and a series of highly controlled gravity field pulses. Subspace field amplifiers generate a dispersive Cochrane field outside a vessel, and a linked graviton pulse generator produces a gravity field which stops the momentum of approaching projectiles in an area called the "interaction layer" (Figure 1.1). A type of StarFleet shield generator can be seen in Figure 1.2. It is important to realize that the subspace field component of StarFleet shields is a clearly defined hollow shell; objects beyond the inside edge of the shell are not affected by its proximity, and not subject to the dispersive effects of the shield. Objects that breach the field demonstrate the classic "field bridge effect," a phenomenon caused by the establishment of a temporary link between subspace and normal space and the subsequent passage of matter or energy across it.

StarFleet shields use a hyperbolic configuration in order to generate a subspace field shaped like a hollow shell. This setup is preferred over a solid sphere or ellipsoid, as it is less energy intensive and precludes direct contact between the field and the ship itself. Direct contact between the subspace field and the starship it protects would translate part (or all) of the vessel out of realspace, exposing it to weapon energy shunted into subspace as well, defeating the purpose of the shield. Figure 2.1 illustrates a basic setup of the hyperbolic subspace field configuration when generated in a perfect sphere around a central object. It is important to note that the subspace component of shields is not a perfect unlimited converter; although capable of translating large amounts of energy harmlessly into subspace, this ability is not infinite.

During normal shield operation, significant amounts of energy will not be entirely shunted into subspace, and although the hyperbolic nature of the subspace field prevents this energy from directly contacting the hull of the ship (since it occupies the transverse volume, where no subspace is found), physics demands that the energy be instantaneously transferred to the opposite vertex of the hyperbola. The opposite vertex is located inside of the shield generator itself, and although it is armed with capacitors to handle the bleed over, inevitably it causes surges in the plasma distribution manifold which feeds energy to the generator to begin with. It is these power surges which cause cascade overloads in computer panels and equipment, often making otherwise low-powered computer panels and plasma conduits explode (Figure 2.2). When the shield capacitors are overloaded, a preprogrammed safety protocol cuts in and drops the shields to prevent direct unbuffered energy conduction between the protective subspace field and the ship's internal systems. This is known as shield failure (Figure 2.3).

In addition to the subspace field component, StarFleet shields employ a limited graviton field which helps to cushion or deflect impacts and to spread threat weapon energies over a large area of the shield, thereby reducing a weapon's ability to pierce through the subspace field shell (Figure 3.1). Weapons which are immune to graviton dispersion (zero mass particles) would normally be considered more of a threat to starships, except that these particles are much more readily absorbed into subspace than particles with mass. Rarely, a type of weapon will be encountered which is immune to graviton dispersion and not readily absorbed into subspace, and these weapons are considered an extreme threat. Figure 3.2a depicts a typical StarFleet shield system status display, noting the status of the shields, the output of the graviton field, and the modulation of the subspace field. Figure 3.2b depicts an analysis of the graviton field configuration of a Galaxy-class starship.

Unlike with the subspace component of a shield, the gravity component permeates all throughout the transverse volume, although the gravitons are generated in frequency sync with the subspace field. This provides maximum protection from weapons across a variety of phase frequencies, scattering phased (and non-phased) weapons such as phasers or sometimes even deflecting projectile explosions from torpedoes (Figure 3.3). Significant physical impacts can also be deflected by the shields (Figure 3.4 and Figure 3.5), and if necessary, they can exert considerable constant force as well (Figure 3.6). However, phase-inverted (commonly called by the somewhat misleading term "phase matched") weapons and objects will "miss" the gravitons in the same manner as they bypass the subspace field, compromising it. Therefore, the phase modulation of a starship's shields is of extreme tactical importance, as the knowledge of such information can render the shields useless (Figure 3.7). The frequency of a starship's shields can not be externally scanned by any process currently known to powers of the Alpha Quadrant. The only known group capable of this are the Borg.

It was a lack of phase tuning altogether of Imperial deflectors which prevented Sher Khal'Saad's fleet from adequately defending against Federation photon and quantum torpedoes during the skirmish with the StarFleet task force at Imatia in 2372. This problem, however, was rectified by Imperial engineers by the time of the onset of open hostilities against the Federation in 2374. By this time, the Federation had made some of it's own alterations to their shield technology as well to make them more effective against powerful Imperial turbolasers.