Thursday, August 11, 2016

Types of sensors

A sensor is a device that gathers energy (EMR) and converts it to a signal and presents it in a form suitable for obtaining information about the object under investigation.

Remote sensors can broadly be classified as passive sensors and active sensors.
Passive sensors measure the natural light emitted from the sun.
Active sensors have their own source of light and the sensors measure the reflected energy.
The Earth’s surface interacts with the incoming Electro-Magnetic Radiation (EMR) from the Sun.  This is known as incident energy (Ei). The three fundamental interactions with incident energy are:
  1. Reflected energy (Er)
  2. Absorbed energy (Ea) and
  3. Transmitted energy (Et)
 Incident energy formula:
Ei = Er + Ea + Et
Passive sensors measure this natural energy at specific frequencies or wavelengths. Wavelength is conventionally measured in ‘m’ or multiples thereof ‘nm’ etc. The frequencies radiations typically sensed are listed below:
Visible radiation – 390 to 700 nm
Infra-red radiation – 750 to 1 mm
Ultra-violet radiation – 100 to 400 nm
These wavelength ranges are known as “bands

Sensors can have multiple bands (3 to 10 bands) and this is known as MSS (Multi-Spectral Sensing).
Hundereds of finer bands are known as Hyper-spectral imaging.

The solar radiation that is incident on Earth, is reflected back to the passive sensors and this reflected energy is detected by the passive sensors.
Reflected energy formula
Er = Ei - Ea - Et

Different objects on Earth, reflect, transmit and absorb different amounts of energy and this implies that each feature on Earth a unique property called spectral reflectance (p).

Spectral reflectance formula:
p = E/ Ei

Examples of active sensors are:
  1. Radar
  2. Camera with flashlight
Examples of passive sensors are:
  1. Camera without flashlight
  2. ALL remote sensing sensors
Non-scanning or framing sensors: These sensors measure the radiation coming from the entire scene at once.
Examples of non scanning sensors are:

  1. Our eyes
  2. Photographic cameras
Imaging sensors: These sensors form image by collected radiation. They may be scanning sensors or non-imaging sensors. In scanning sensors, the image is sensed point-by-point. These scanners may be along track scanners in which the image is acquired line by line or across track scanners in which the image is acquired pixel by pixel.

Non imaging sensors: These type of sensors do not form the image. They are used to recors spectral quantity as afunction of time.
Examples are: Sensors for temperature measurement, study of the atmosphere, etc.

Image plane scanning: In this type of sensor, the lens is used after the scan mirror to focus the light on the detector

Object plane scanning: In this type of sensor, the lens is placed before the scan mirror to focus the light on the detector.

Most of the active sensors operate in the microwave portion of the electromagnetic spectrum. A few of the active sensors are listed below:

  1. Laser Altimeter
  2. Radar
  3. Lidar
  4. Ranging Instrument
  5. Scatterometer and
  6. Sounder
Passive sensors include different types of radiometers and spectrometers. Passive remote sensors in remote sensing operate in the visible, infrared, thermal infrared and microwave regions of the electromagnetic spectrum. Passive remote sensors used are listed below:
  1. Accelerometer
  2. Hyperspectral radiometer
  3. Imaging radiometer
  4. Radiometer
  5. Sounder
  6. Spectrometer and
  7. Spectroradiometer