Deflector shield

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A deflector shield is a fictional technology commonly found in science fiction. Typically, deflector shields (often referred to as simply "shields") are some form of energy field that is projected along the surface of, or into the space around, a starship, space station, planet, moon, or building.

The abilities and exact functionality of deflector shields vary; in some works (such as in the Star Trek universe), deflector shields can stop both energy beams (e.g. phasers) and physical projectiles, both natural and artificial; in others, such as the Star Wars universe, there are multiple types of deflector shields -- so-called ray shields which are designed to stop energy beams (such as laser and blaster weapons), and particle shields which are designed to stop kinetic projectiles, missiles, bombs, etc.

Deflector shields usually work by absorbing or dissipating the energy of the incoming attack; prolonged exposure to such attacks weakens the shield and eventually results in the shield's collapse, making the ship's hull (or building's walls, or planet's surface) vulnerable to attack. Larger deflector shield systems, or those powered by bigger energy sources, can absorb/dissipate more damage before failing -- so that larger starships, for example, can mount much stronger shields than a small, single-person starfighter, much in the way that a sea-going battleship has much thicker armor than a tiny patrol boat.

Deflector shields in Star Trek

This part is fictional, but within the scientific logic of Star Trek. Sources: The book by Rick Sternbach and Michael Okuda, as well as by Herman Zimmerman, Rick Sternbach, and Doug Drexler.

The Star Trek deflector shield creates areas of highly focused space distortion containing a field of gravitons. The shape of this shield resembles the ship's shape. If the shield is being hit - no matter if it's a subatomic particle or a massive object - the field energy is being focused on that particular point, creating a small but powerful spatial distortion. For an observer on board of the ship, it looks as like the object bounced off the shields; for an observer on the object itself it would look as if the trajectory never changed - just like it would be with a regular space distortion.

Shielding the ship from the whole spectrum of electromagnetic radiation is almost impossible because it would take too much energy and conflict with the sensors. In normal flight, shields are on 5% of the maximum possible output, the only exception being the frequencies of the EM spectrum from which the crew needs to be protected. In battle situations, the shields are brought to at least 85% of the maximum possible output.

The frequency modulation and spectrum of the shields is varied randomly, so it is possible to set weapons to pass the shields unaltered. On the other hand, knowledge of the modulation and spectrum of an enemy weapon can be used to increase the efficiency of the shields.

To use weapons, sensors, and shuttles through the shield, electromagnetic shield windows can be opened temporarily. This is usually done via EM-window transponders present in shuttles and photon torpedoes.