5.1 Spectral
As indicated in the preceding sections, different materials respond in different,
and often distinctive, ways to EM radiation. This means that a specific
spectral response curve, or spectral signature, can be determined for each
material type. Basic categories of matter (such as specific minerals) can
be identified on the basis of their spectral signatures alone, but may require
that the spectra be sufficiently detailed in terms of wavelength intervals
and covers a wide spectral range. Composite categories of matter (such as
soil which contains several different minerals) however, may not be uniquely
identifiable on the basis of spectral data alone.
Remote sensing devices generally only sample the EM spectrum by detecting
the combined radiation over a range of wavelengths. For example, a sensor
which is receptive to wavelengths in the range 0.4p;0.5 µm would
be sensing 'blue' light. This range is referred to as a spectral band, or
channel, of data in an image. The spectral ranges which can be used for
remote sensing of the Earth's surface are limited to the atmospheric windows,
as described in Section 1.3.
As discussed in Section 2, photographic recording
devices only detect radiation within the ultra violet to near infrared regions
of the electromagnetic spectrum (depending on the type of film being used).
Within this range, spectral channels are defined by using filters to block
out wavelengths outside the required sub-region. Multi-spectral scanners,
however, may also detect radiation in the middle and thermal infrared regions
as discussed above. Microwave radiation may be recorded using radar and
passive microwave sensors.
Aircraft-borne scanners are typically used to provide increased spectral
resolution relative to satellite-borne scanners. Many individual minerals,
for example, have diagnostic narrow absorption bands at known wavelengths.
Spectrometers can be used in laboratory or field work to identify specific
types of mineral. Airborne Imaging Spectrometers (AIS) have recently been
developed to detect wavelength bands as narrow as 2 nm. The satellite-borne
High-Resolution Imaging Spectrometer (HIRIS) planned for the Earth Observation
System will have bandwidths of approximately 10 nm to allow identification
of most minerals.
However, in most satellite imagery, the channels are selected to differentiate
between the major cover types. One advantage of airborne imagery is that
the position, width and number of spectral channels being sensed can be
tailored to the specific cover type or range of covers of interest.
Spectral extent describes the range of wavelengths being sensed in all channels
of an image. Spectral resolution can be defined in terms of both the number
of spectral channels being imaged over a given spectral region and the range
of wavelengths incorporated into each single channel. An increase in spectral
resolution over a given spectral range will result in a greater number of
spectral channels. However, this additional resolution also 'costs' in terms
of increased data volume and the consequent increase in costs associated
with its processing. The theoretically optimal spectral range and resolution
for a particular cover type therefore may need to be modified with respect
to the practical considerations of data collection and processing.
