2.10 Optical Mechanical Scanner

2.10 Optical Mechanical Scanner

An optical mechanical scanner is a multispectral radiometer by which two dimensional imagery can be recorded using a combination of the motion of the platform and a rotating or oscillating mirror scanning perpendicular to the flight direction. Optical mechanical scanners are composed of an optical system, spectrographic system, scanning system, detector system and reference system.

Optical mechanical scanners can be carried on polar orbit satellites or aircraft. Multispectral scanner (MSS) and thematic mapper (TM) of LANDSAT, and Advanced Very High Resolution Radiometer (AVHRR) of NOAA are the examples of optical mechanical scanners. M2S made by Daedalus Company is an example of an airborne type optical mechanical scanner.

Figure 2.10.1 shows the concept of optical mechanical scanners, while Figure 2.10.2 shows a schematic diagram of the optical process of an optical mechanical scanner.

The function of the elements of an optical mechanical scanner are as follows.

a. Optical system: Reflective telescope system such as Newton, Cassegrain or Ritchey-Chretien is used to avoid color aberration.

b. Spectrographic system: Dichroic mirror, grating, prism or filter are utilized.

c. Scanning system: rotating mirror or oscillating mirror is used for scanning perpendicular to the flight direction.

d. Detector system: Electro magnetic energy is converted to an electric signal by the optical electronic detectors. Photomultiplier detectors utilized in the near ultra violet and visible region, silicon diode in the visible and near infrared, cooled ingium antimony (InSb) in the short wave infrared, and thermal barometer or cooled Hq Cd Te in the thermal infrared.

e. Reference system: The converted electric signal is influenced by a change of sensitivity of the detector. Therefore light sources or thermal sources with constant intensity or temperature should be installed as a reference for calibration of the electric signal.

Compared to the pushbroom scanner, the optical mechanical scanner has certain advantages. For examples, the view angle of the optical system can be very narrow, band to band registration error is small and resolution is higher, while it has the disadvantage that signal to noise ratio (S/N) is rather less because the integration time at the optical detector cannot be very long due to the scanner motion.