{{Unreferenced, date=March 2018 In general, detection is the action of accessing information without specific cooperation from with the sender. In the history of

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...

communications, the term " detector" was first used for a device that detected the simple presence or absence of a radio signal, since all communications were in

Morse code Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...

. The term is still in use today to describe a component that extracts a particular signal from all of the

electromagnetic waves In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) lig ...

present. Detection is usually based on the

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

of the carrier signal, as in the familiar frequencies of

radio broadcasting Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radio ...

, but it may also involve filtering a faint signal from

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...

, as in

radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming f ...

, or reconstructing a hidden signal, as in steganography. In optoelectronics, "detection" means converting a received optical input to an electrical output. For example, the light signal received through an

optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...

is converted to an electrical signal in a detector such as a

photodiode A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons. The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...

. In steganography, attempts to detect hidden signals in suspected carrier material is referred to as steganalysis. Steganalysis has an interesting difference from most other types of detection, in that it can often only determine the

probability Probability is the branch of mathematics concerning numerical descriptions of how likely an Event (probability theory), event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and ...

that a hidden message exists; this is in contrast to the detection of signals which are simply encrypted, as the

ciphertext In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext ...

can often be identified with certainty, even if it cannot be decoded. In the

military A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare. It is typically authorized and maintained by a sovereign state, with its members identifiable by their distinct ...

, detection refers to the special discipline of

reconnaissance In military operations, reconnaissance or scouting is the exploration of an area by military forces to obtain information about enemy forces, terrain, and other activities. Examples of reconnaissance include patrolling by troops (skirmisher ...

with the aim to recognize the presence of an object in a location or ambiance. Finally, the art of detection, also known as ''following clues'', is the work of a

detective A detective is an investigator, usually a member of a law enforcement agency. They often collect information to solve crimes by talking to witnesses and informants, collecting physical evidence, or searching records in databases. This leads th ...

in attempting to reconstruct a sequence of events by identifying the relevant information in a situation.

See also