Imaging with frequencies
Waves in general can be exploited to detect things in space. Basically, they are broadcasted from from a transducer. A detector records the reflections and measures the time between the broadcast and the detection. Knowing the speed of the used wave in a medium, it is possible to calculate if there is an obstacle, a change in density, etc. The exploited principle to achieve this is called Doppler Effect - the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source.
The used waves could be audible sounds, but they would be very disturbing while running a session. Therefore, ultrasounds are preferred, which are mostly inaudible. Ultrasounds are just above audible sounds. Somebody can perceive them, but this is more frequent with animals, such as dogs.
With the refinement of imagine techniques, ultrasounds could be used for echographies. As the waves hit the internal tissues and bounce back from them, they are not deemed completely safe, but mostly they are not harmful, if used wisely.
A typical echography is shown below.
Another example is the fishfinder, which can image fishes through ultrasounds.
If a wide area needs to be covered or penetration below the gound level is desired, high frequency radio waves up to microwaves are generally used. Radars are the workhorse in traffic control rooms in airports.
Below, the frequencies reserved to radars are shown.
Coastal radar systems, over-the-horizon radar (OTH) radars; 'high frequency'
Very long range, ground penetrating; 'very high frequency'
< 300 MHz
> 1 m
'P' for 'previous', applied retrospectively to early radar systems; essentially HF + VHF
Very long range (e.g. ballistic missile early warning), ground penetrating, foliage penetrating; 'ultra high frequency'
Long range air traffic control and surveillance; 'L' for 'long'
Moderate range surveillance, Terminal air traffic control, long-range weather, marine radar; 'S' for 'short'
Satellite transponders; a compromise (hence 'C') between X and S bands; weather; long range tracking
Missile guidance, marine radar, weather, medium-resolution mapping and ground surveillance; in the United States the narrow range 10.525 GHz ±25 MHz is used for airport radar; short range tracking. Named X band because the frequency was a secret during WW2.
High-resolution, also used for satellite transponders, frequency under K band (hence 'u')
From German kurz, meaning 'short'; limited use due to absorption by water vapour, so Ku and Ka were used instead for surveillance. K-band is used for detecting clouds by meteorologists, and by police for detecting speeding motorists. K-band radar guns operate at 24.150 ± 0.100 GHz.
Mapping, short range, airport surveillance; frequency just above K band (hence 'a') Photo radar, used to trigger cameras which take pictures of license plates of cars running red lights, operates at 34.300 ± 0.100 GHz.
Millimetre band, subdivided as below. The frequency ranges depend on waveguide size. Multiple letters are assigned to these bands by different groups. These are from Baytron, a now defunct company that made test equipment.
Very strongly absorbed by atmospheric oxygen, which resonates at 60 GHz.
Used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.
Other uses of frequencies for imaging purposes include the following.
CT (CAT) scan: Also known as computed tomography scan, it makes use of computer-processed combinations of many X-ray measurements taken from different angles to produce cross-sectional images of specific areas of a scanned object, allowing the user to see inside the object without cutting. Other terms include computed axial tomography and computer aided tomography.
MRI scan: Magnetic resonance imaging is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body in both health and disease. MRI scanners use strong magnetic fields, electric field gradients, and radio waves to generate images of the organs in the body.