WS/EO2 : Passive Optics Systems

Question 1

State the IR window wavelengths

Answer 1

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

Question 2

List the components of an IR system

Answer 2

• Optics
• Detector
• Processor
• Display

Question 3

Describe the function of the following IR system components - Optics

Answer 3

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.

Question 4

Describe the function of the following IR system components - Detector

Answer 4

Function
* To accept IR radiation and convert that radiation into a
useable electrical output for further processing.

Detector Types
* Thermal, and
* Photon.

Question 5

Describe the function of the following IR system components: Display.

Answer 5

Function
* The display system may consist of any display screen
capable of displaying video signals.

Question 6

Describe the function of the following IR system components Processor

Answer 6

Accepts the electrical signal from the detector, amplifies
it, then converts it into a composite video signal in order
to drive a display.

Question 7

List the types of IR system optics.

Answer 7

Types:
1. Scanning; used with a single detector.
2. Staring; used with a multi-element array.

Question 8

Describe Scanning Optics.

Answer 8

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.

Question 9

Describe Staring Optics.

Answer 9

• No moving parts.
• Each point in space is connected to its own individual detector.
• This provides continuous coverage of all points in the FOV.

Question 10

List the types of IR detectors.

Answer 10

Detector Types
* Thermal, and
* Photon.

Question 11

Describe Thermal Detectors.

Answer 11

• 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.

Question 12

Describe Photon Detectors.

Answer 12

• 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.

Question 13

DOTDR

List the Advantages of an IR System.

Answer 13

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.

Question 14

List the Disadvantages of an IR System.

Answer 14

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)

Question 15

Define Image Intensifier.

Answer 15

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 .

Question 16

List the components of an Image Intensifier

Answer 16

• Photo Cathode
• Photo Multiplier
• Phosphor Screen

Question 17

Describe the function of the following Image Intensifier
components. Photo Cathode

Answer 17

Dim light from a night scene enters the lens at the front. The light is made of photons (particles of light) of all colours.

Question 18

Describe the function of the following Image Intensifier
components. Photo Multiplier

Answer 18

The electrons are amplified by a photomultiplier, a kind of photoelectric cell. Each electron entering the photomultiplier results in many more electrons leaving it.

Question 19

Describe the function of the following Image Intensifier
components. Phosphor Screen

Answer 19

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.