A defensive shield, commonly referred to as simply a shield and sometimes as a Holtzman shield, was a protective energy field that could surround the person who wore it. It was almost like a personal Pentashield.
A shield was produced by a Holtzman generator, the field deriving from Phase One of the suspensor-nullification effect. A shield would permit entry only to objects that moved at slow speeds. Depending on the the shield's setting, the object's speed while passing through the shield would range from six to nine centimeters per second. A shield could also be set to cover either the left or right side of a person if the specific need for it arose.
Use and History
The effect was first employed immediately prior to the Butlerian Jihad) for defensive force fields capable of scrambling the gel circuitry of thinking machines. Networks of towers generating the field from the surface, thus protecting entire planets from machine attacks. However, the machines soon realized that their cymeks, human-machine hybrids, could slip though the field to destroy the transmitters because they possessed human brains which were unaffected by the scrambler fields. Norma Cenva then had the idea to use the field as an offensive weapon, projecting it with portable transmitters to knock out machines and their installations. Holtzman later calculated that the field could be modified to prevent penetration from physical projectiles; Cenva agreed, correcting the flaws in his concept but noting that objects could still pass through the shield at a slow enough speed.
By the time of Muad'dib, when thinking machines had long ceased to be a threat, the shield had been adopted for use in personal defense. These shields were form-fitting energy fields which permited penetration only by objects that moved below a pre-set velocity. As one would be unable to breathe within a shield that did not permit atmospheric gases to penetrate it, man-portable shields had a relatively high penetration velocity, approximately six to ten centimeters per second. However, shields for ships and planetary installations could and often did have extremely low penetration velocities, as artificial life support technologies could be utilized while the shield was active.
Cenva realized early on that if a lasgun beam hit a Holtzman field, it would result in sub-atomic fusion and a nuclear explosion. The center of this blast was determined by random chance; sometimes it would originate within the shield, sometimes within the laser weapon, sometimes both. Thus, using lasguns in a shielded environment resulted in military and environmental catastrophe, though at least one commander (Duncan Idaho) has used this phenomenon deliberately as a discouragement to his enemies.
Behind the Scenes
The Holtzman Shield is a potent literary device: it makes directed-energy weaponry impossible against any worthwhile opponent, and also proves traditional projectile-based firearms and missiles ineffective, adding to the feudal atmosphere, and enforces the usage of mêlée weaponry despite other more advanced technology.