WS/EO2 : Passive Optics Systems Flashcards
State the IR window wavelengths
Near Infra-Red - NIR - 0.75 to 3 µm
Middle Infra-Red - MIR - 3 to 6 µm
Far Infra-Red - FIR - 6 to 15 µm
Extreme Infra-Red - XIR - 15 to 1000 µm
List the components of an IR system
- Optics
- Detector
- Processor
- Display
Describe the function of the following IR system components - Optics
Function
* To collect IR energy (within the system field of view) and
focus this energy onto the IR detector.
Types:
1. Scanning; used with a single detector.
2. Staring; used with a multi-element array.
Describe the function of the following IR system components - Detector
Function
* To accept IR radiation and convert that radiation into a
useable electrical output for further processing.
Detector Types
* Thermal, and
* Photon.
Describe the function of the following IR system components: Display.
Function
* The display system may consist of any display screen
capable of displaying video signals.
Describe the function of the following IR system components Processor
Accepts the electrical signal from the detector, amplifies
it, then converts it into a composite video signal in order
to drive a display.
List the types of IR system optics.
Types:
1. Scanning; used with a single detector.
2. Staring; used with a multi-element array.
Describe Scanning Optics.
The focused image of the scene is scanned line by line over a single detector element by the combined motion of two oscillating mirrors. The varying output of the detector is fed to the signal processor.
Describe Staring Optics.
- No moving parts.
- Each point in space is connected to its own individual detector.
- This provides continuous coverage of all points in the FOV.
List the types of IR detectors.
Detector Types
* Thermal, and
* Photon.
Describe Thermal Detectors.
- Incident IR radiation is detected and the change in temperature of the absorbing material is measured.
- Thermal detectors are slow in response, but sensitive to all wavelengths.
Describe Photon Detectors.
- A photon detector is one in which the incident radiation causes changes in the electronic state of the detector material.
There are two main types.
* Photo-voltaic; IR increase, voltage increase.
* Photo-conductive; IR increase, current flow increase.
DOTDR
List the Advantages of an IR System.
Advantage
* Detects IR radiation day or night.
* Operate well in low-light conditions
* Target classification.
* Detects use of vehicles/aircraft
* Requires only 0.1 °C temp difference.
List the Disadvantages of an IR System.
Disadvantage
* Unable to penetrate cloud/moisture.
* High humidity reduces range.
* Heavy rain can equalise temp and eliminate radiation
differences.
* Unusable at thermal cross-over (dusk and dawn)
Define Image Intensifier.
A device for increasing the intensity of available light in an optical system to allow use under low light conditions such as at night, to facilitate visual imaging .
List the components of an Image Intensifier
- Photo Cathode
- Photo Multiplier
- Phosphor Screen
Describe the function of the following Image Intensifier
components. Photo Cathode
Dim light from a night scene enters the lens at the front. The light is made of photons (particles of light) of all colours.
Describe the function of the following Image Intensifier
components. Photo Multiplier
The electrons are amplified by a photomultiplier, a kind of photoelectric cell. Each electron entering the photomultiplier results in many more electrons leaving it.
Describe the function of the following Image Intensifier
components. Phosphor Screen
The electrons leaving the photomultiplier hit a phosphor screen, similar to the screen in an old-fashioned television. As the electrons hit the phosphor, they create tiny flashes of light.
