Door security systems are any device or system designed to prevent unauthorized access through a doorway, such as occurs during a break-in or burglary. They can take many forms: locks, chains, reinforcements, and alarms. Door construction itself is an important factor for door security as inferior materials or assembly methods can compromise a door's ability to maintain structural integrity during a break in.
The first consideration in any comprehensive door security plan is the construction of the door and its frame. Strongly constructed metal or solid wood doors with reinforced frames resist forced entry much better than cheaper laminated or hollow core doors. Laminated doors can splinter and de-laminate when struck forcefully. Solid wood or steel doors are much harder to damage, although sometimes a compromise is reached by reinforcing a laminated door with a metal plate that wraps around the door from front to back in the area where the lock is located. Reinforced door frames and door jambs are also important for structural integrity and can help prevent a door from being compromised by brute force.
Locks are the single most common type of door security and can take many forms. Standard doorknobs have locks which lock the door bolt in place, but these are not especially strong in most cases. A typical exterior door on many homes will have a separate keyed lock that operates a deadbolt, which is a metal bar that extends from the lock into the door frame to hold the door in place. These locks, when properly installed are reasonably effective. Some deadbolt locks are designed to be resistant to picking and tampering and may be mounted on the door with a large metal plate for reinforcement and added strength.
Other types of door locks are possible as well, including sliding chain locks which allow the door to be opened slightly when engaged, but prevent removal of the chain. Hook and eye latches, manual sliding bars, and other specialty locks and latches are also sometimes used. Metal or wooden blocks or bars placed across the door, extending over the frame in either direction are another very simple type of lock, although this may also be considered a type of reinforcement.
Door reinforcement can take many forms besides built-in or structural improvements like reinforced jambs and frames. Bars, locking doorstops, and braces can be used to reinforce a door, preventing it from being opened even if the lock is compromised. These kinds of devices, such as a bracing bar, typically mount on the floor and extend to the door. A sturdy floor mount anchors one end of the bar, which extends towards the door at an angle, where it locks in place in another fixture. These types of braces are very strong, making it nearly impossible to force open a door.
Alarm systems are another important form of door security. Electronic systems sound an alarm, either on-site, at a remote location that is monitored, or both. Local alarms can frighten off potential intruders, and remote monitoring can notify authorities automatically or alert a human monitor that an intrusion has occurred so that they can take appropriate action.
Some door security devices and systems don't really fit into any of these categories. Door viewing ports, or spy-holes, allow for direct visual inspection of visitors or potential intruders. Cameras serve the same purpose but allow for remote visual monitoring and long term recording of the door and surroundings for future reference if necessary. Intercom devices allow for secure communication from one side of the door to the other, and hinge screws are special devices that hold a door in place even if the hinge pins have been removed.