Batsignal

"Please inform the citizens of Gotham that Gotham City has earned a rest from crime. But if the forces of evil should rise again, to cast a shadow on the heart of the city, call me."

- Batman's letter to GCPD. The Bat-Signal is a distress signal device appearing in the various interpretations of the Batman mythos. It is a specially modified Klieg searchlight with a stylized symbol of a bat attached to the light so that it projects a large Bat emblem on the sky or buildings of Gotham City.

In the stories, the signal is used by the Gotham City Police Department as a method of contacting and summoning Batman to their assistance in the event of a serious crisis and as a weapon of psychological intimidation to the numerous villains of Gotham City.

Origins
The fictional origin of the signal varies between timeline and media. It made its first appearance in Detective Comics #60, February 1942. In the 1989 Batman film, Batman gave the signal to the police as a gift enabling them to call him when the city is in danger; in 2005's Batman Begins, then-detective James Gordon creates his own signal light, inspired by an incident when Batman strapped the defeated mobster Carmine Falcone to a large searchlight, which created a roughly bat-like image from the light's beam due to Carmine's tattered coat.

In the comic's post-Crisis continuity, the signal was introduced after the Batman's first encounter against the Joker (not unlike the first movie) in Batman: The Man Who Laughs; in Batman and the Mad Monk, Gordon initially used a pager, but during a meeting with Batman he threw it away, saying that he couldn't sneak around in the shadows like Batman and wanted a more above-board means of contacting him.

On Batman: The Animated Series, it was introduced in the episode "The Cape and Cowl Conspiracy". On The Batman, Gordon invented it to summon Batman in "Night in the City" (somewhat similar to the 2005 film).

Being used by others
Others have used the Bat-Signal for their own purposes.

In the "Lovers and Madmen" story arc from Batman Confidential, which retells the origin of the Joker and his first encounter with Batman, Batman sees the Bat-Signal for the first time and assumes that Gordon created it to ask for his help in battling the Joker. When he reaches the rooftop, however, he finds that the Joker actually created it, and used it to force a confrontation with Batman.

In Detective Comics #466 (1976), the villainous Signalman manages to trap the Batman inside the Bat-Signal device.

In Legends of the Dark Knight #6, a cadre of crime bosses projects the signal upside down in order to summon Batman to help them fight a killer they can't defeat. In the Halloween special comic series, Haunted Knight, Scarecrow alters the Bat-Signal to notify Batman that he has kidnapped then-Captain Jim Gordon. By adding an orange bulb and painting "eyes" on the signal, he turns the beam into a stylized Jack-o'-lantern image (with the bat symbol forming the mouth beneath two eyes).

In Batman: Dark Victory, Hangman sneaks onto the roof of Police Headquarters and turns the Bat-Signal on to lure then-recently appointed Commissioner James Gordon to the roof and try to kill him, but is thwarted when Two-Face cuts Gordon down.

Near the beginning of the Batman: No Man's Land story arc, a junior officer creates an improvised Bat-Signal out of spare parts. Gordon smashes it to pieces as he is angry that Batman hasn't shown up. Oracle also builds a small Bat-Signal to summon Batman to talk to her after having seen Huntress wearing a Batgirl costume.

In the comic book series Gotham Central, the purpose of the Bat-Signal is further expanded upon; as Batman's existence is not officially recognized by the Gotham City authorities, the Bat-Signal is explained as a method of using the "urban legend" around Batman to terrify Gotham's criminal underworld.

As official proven police interaction with the Bat-Signal and Batman himself can lead to cases against criminals arrested by Batman being dismissed, it is up to the civilian employees of the Gotham police department (including the Major Case Squad's civilian attache, Stacy) to operate the signal officially. Owing to the events in the "War Crimes" storyline, relations between Batman and the Gotham City Police Department under Commissioner Michael Akins are officially severed; the Bat-Signal is removed from the roof of Gotham Central. Needing Batman's help on an extraordinary case, Akins brings out a spare bat signal for a single use.

This signal is a more sophisticated laser which paints a green bat symbol in the clouds and is apparently more visible. This version of the signal is donated by Kord Industries (see the Blue Beetle). The laser signal is said to have been unused because the city council deems it an "inappropriate gift." (The characters are notably unimpressed by the more high tech version.)

In the 52 series, The Question alters the traditional Bat-Signal to project a spray-painted question mark. In the One Year Later series, however, with the re-installation of James Gordon as commissioner, relations with Batman appear to have thawed; upon Batman's return from one year of self-imposed exile, the Bat-Signal (restored to the roof of police headquarters) is activated once again. The familiar sight of the Bat symbol in the sky prompts cheers from most of the citizens of Gotham.

Batman (1966)
In the 1966 Batman film, Robin asks Commissioner Gordon to get Chief O'Hara to activate it as a distraction so the criminals would think they were heading for Police Headquarters. The Batsignal is rarely used because the Commissioner usually contacts Batman in the day by the red phone.

Burton Films
In Tim Burton's Batman, Batman gives the signal to the police after the death of The Joker as a gift so that when he is needed they can summon him.

In Batman Returns, Bruce has miniature Bat-Signals stationed up around Wayne Manor as a beacon to call him. It is only used when Commissioner Gordon needs his help when the Red Triangle Circus Gang attack Shreck Plaza in the lead up to Christmas.

Schumacher Films
In Joel Schumacher's 1995 film Batman Forever, the criminal psychologist Dr. Chase Meridian uses the Bat-Signal to call Batman in order to seduce him. Batman is slightly peeved at her usage of the Bat-Signal as a mere "beeper". A music video for Kiss from a Rose, also from the film and directed by Schumacher, features singer Seal performing the song while standing beside the Bat-Signal.

Also in Batman Forever, The Riddler alters the Bat-Signal by projecting a question mark into the sky with the original bat symbol forming the dot at the base. Similarly, in Batman: Dark Victory, after brokering a tentative alliance with Batman, the Riddler changes the signal, projecting a question mark into the sky in order to let Batman know that he has an answer for him. In the 1997 film Batman & Robin, Poison Ivy acquires the location and keys to the Batsignal after seducing Commissioner Gordon who reveals information about it. She and Bane remove the signal from the top of police HQ and take it back to their lair.

She then altered the Bat-Signal by changing the Bat-symbol to a Robin-symbol to lure Robin into a trap. However, Robin was able to trick Poison Ivy into revealing Mr. Freeze's plot to freeze Gotham. At the end of the film, it is later destroyed by Bane, but repaired before the credits.

Batman: Arkham City
In the video game Batman: Arkham City, the Batsignal is used as an objective marker while roaming around Arkham City and can be found on the roof of the GCPD Building.

DC Animated Universe
In Batman: The Animated Series, Barbara Gordon uses it to contact Batman in "Heart of Steel" when she believes that an impostor has replaced her father. At this meeting, the signal is partially destroyed when Batman is attacked by a Harvey Bullock duplicate, and Barbara uses Batman's grapple gun to pull the robot into the signal, electrocuting it. Likewise, the real Harvey Bullock uses the signal for the first time when reluctantly asking for Batman's help in discovering who is trying to kill him in "A Bullet for Bullock".

In the movie based on the aforementioned series, Batman: Mask of the Phantasm, Batman is being hunted by the police on suspicions that he has murdered several gang lords, and Bullock, under orders from Councilman Arthur Reeves, tries to use the Bat-Signal to lure him in. Batman, knowing that it is a trap, does not respond.

In Batman Beyond, Terry McGinnis destroys the Bat-Signal after it is used for the first time in years by Paxton Powers, the son of Derek Powers (Blight), who uses the signal to contact Batman with the intention of using him to locate and deal with his father.

Nolan Films
In the 2005 film Batman Begins, Batman initially 'improvises' a Bat-Signal by draping crime boss Carmine Falcone over a spotlight at the docks after defeating him, his spread-out coat making the light look like a bat. At the conclusion of the movie, Lieutenant Gordon creates the actual Bat-Signal to call Batman to the roof for a talk, although he jokingly comments that he only uses it because they couldn't find any mob bosses to strap to it.

In the 2008 sequel, The Dark Knight, Gordon uses the Bat-Signal to remind Gotham of Batman's presence. The signal proves to be very effective, with drug dealers and criminals becoming apprehensive at its appearance. However, since the police are supposed to be tracking the vigilantee, they chalk it up to "light problems" when its used. At the end of the film, after agreeing to blame Batman for the murders committed by Harvey Dent in order to preserve the latter's image as Gotham's hero, Gordon destroys the signal in front of various members of the police force and the press.

In the 2012 sequel, The Dark Knight Rises, at the beginning, Gordon is seen contemplating Harvey Dent over the heavily rusted remains of the searchlight. While at the end, a new rebuilt signal is installed in its place, surprising Gordon, though honoring the Batman, but has no use, as the Batman has seemingly died.