Infrared and microwave bands

Both infrared and microwave bands are invisible to the human eye. Then how are IR and microwave images from remote sensing satellites printed? How do they substitute the wavelengths?

M. Atma Bharathi, Chennai

ANSWER: Both infrared and microwaves are part of the electromagnetic spectrum, which stretches from radio waves to gamma rays. Each band of this spectrum corresponds to a small range of frequencies. Each frequency is uniquely connected with an energy level.

The total spectrum is very wide. Our eyes respond only to a very small band called the visible band. However, different chemicals do react to certain higher or lower level of energy bands and hence can be used as photographic plates for detecting and measuring some of these radiations.

Satellites nowadays use sensors whose working principle is based on photoelectric effect, converting radiation reaching the sensor into electric charge, which can then be easily measured and processed.

Essentially, the energy detected over a given spectral band, whether it is in the visible, infrared or microwave band, is converted to an array of digits corresponding to the energy range and radioed to ground stations.

After receiving it, those digital values corresponding to the measured energy range (known as grey levels) are finally printed on a computer screen as a picture or as a conventional photograph.

Prof. A.V. Patki

Satyam Technology Centre Hyderabad

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