1.5 Types of Remote Sensing with Respect to Wavelength

1.5 Types of Remote Sensing with Respect to Wavelength Regions

Remote sensing is classified into three types with respect to the wavelength regions; (1)Visible and Reflective Infrared Remote Sensing, (2)Thermal Infrared Remote Sensing and (3)Microwave Remote Sensing, as shown in Figure 1.5.1.

The energy source used in the visible and reflective infrared remote sensing is the sun. The sun radiates electro-magnetic energy with a peak wavelength of 0.5 m (see 1.7 and 1.10). Remote sensing data obtained in the visible and reflective infrared regions mainly depends on the reflectance of objects on the ground surface (see 1.8). Therefore, information about objects can be obtained from the spectral reflectance. However laser radar is exceptional because it does not use the solar energy but the laser energy of the sensor.

The source of radiant energy used in thermal infrared remote sensing is the object itself, because any object with a normal temperature will emit electro-magnetic radiation with a peak at about 10 m (see 1.7), as illustrated in Figure 1.5.1.

One can compare the difference of spectral radiance between the sun (a) and an object with normal earth temperature (about 300K), as shown in Figure 1.5.1. However it should be noted that the figure neglects atmospheric absorption (see 1.11), for simplification, though the spectral curve varies with respect to the reflectance, emittance and temperature of the object.

The curves of (a) and (b) cross at about 3.0 m. Therefore in the wavelength region shorter than 3.0 m, spectral reflectance is mainly observed, while in the region longer than 3.0 m, thermal radiation is measured.

In the microwave region, there are two types of micro wave remote sensing, passive microwave remote sensing and active remote sensing. In passive microwave remote sensing, the microwave radiation emitted from an object is detected, while the back scattering coefficient is detected in active micro wave remote sensing. (see 3.4).

Remarks: the two curves (a) and (b) in Figure 1.5.1 show the black body's spectral radiances of the sun at a temperature of 6,000K and an object with a temperature of 300K, without atmospheric absorption.