more than three wavebands are simultaneously imaged and that the wavebands are
generally chosen specifically for their utility to discern the identity or biophysical
status of the objects being imaged. A technological difference is that, whereas a
digital camera instantaneously acquires a two-dimensional image, spectral imagers
typically scan a scene to build an image pixel-by-pixel or line-by-line. A
‘‘whiskbroom’’ imager uses a mirror to scan side-to-side along the sensor’s path,
reflecting light into a one-dimensional array of photosensitive elements representing the image’s spectral dimension, thus recording the digital data one pixel at
a time. A ‘‘push-broom’’ imager uses a two-dimensional array of photosensitive
elements; the side-to-side elements correspond to the image’s spatial dimension,
while the top-to-bottom elements correspond to the image’s spectral dimension.
The push-broom sensor thereby scans a scene one line at a time.
The terms multispectral and hyperspectral describe the spectral characteristics
of the imaging system. Multispectral sensors typically have few (3–10) wavebands
that are each relatively broad (*20–100 nm). The wavebands are not necessarily
contiguous, but are placed in regions of the spectrum that are deemed important for
a particular science measurement. In contrast, hyperspectral sensors image relatively narrow (*10 nm or less) wavebands across a continuous spectral range,
typically including the visible, near-infrared, and often shortwave-infrared
(1,000–2,500 nm). The key difference is that multispectral sensors measure in
discrete wavebands for each pixel, while hyperspectral sensors measure a continuous spectrum for each pixel.
Photography, multispectral and hyperspectral imaging are passive remote
sensing techniques, in that they rely on the reflection of ambient sunlight to
illuminate objects for measurement (Fig. 1.2). Passive sensors are therefore only
useful if there is a clear, well-lit view of the object of interest. Passive sensors
cannot be used through cloud cover or at night. Table 1.1 outlines the attributes of
several commonly used remote sensing photographic, multispectral and hyperspectral imaging systems.
All of these technologies have been used successfully for remote sensing of
coral reef ecosystems. The properties and processes of coral reefs that have been
Fig. 1.1 The electromagnetic spectrum as shown by wavelength units and corresponding
portions of the spectrum measured by remote sensing instruments (modified from Lillesand et al.
2008)
1 Visible and Infrared Overview
5
generally chosen specifically for their utility to discern the identity or biophysical
status of the objects being imaged. A technological difference is that, whereas a
digital camera instantaneously acquires a two-dimensional image, spectral imagers
typically scan a scene to build an image pixel-by-pixel or line-by-line. A
‘‘whiskbroom’’ imager uses a mirror to scan side-to-side along the sensor’s path,
reflecting light into a one-dimensional array of photosensitive elements representing the image’s spectral dimension, thus recording the digital data one pixel at
a time. A ‘‘push-broom’’ imager uses a two-dimensional array of photosensitive
elements; the side-to-side elements correspond to the image’s spatial dimension,
while the top-to-bottom elements correspond to the image’s spectral dimension.
The push-broom sensor thereby scans a scene one line at a time.
The terms multispectral and hyperspectral describe the spectral characteristics
of the imaging system. Multispectral sensors typically have few (3–10) wavebands
that are each relatively broad (*20–100 nm). The wavebands are not necessarily
contiguous, but are placed in regions of the spectrum that are deemed important for
a particular science measurement. In contrast, hyperspectral sensors image relatively narrow (*10 nm or less) wavebands across a continuous spectral range,
typically including the visible, near-infrared, and often shortwave-infrared
(1,000–2,500 nm). The key difference is that multispectral sensors measure in
discrete wavebands for each pixel, while hyperspectral sensors measure a continuous spectrum for each pixel.
Photography, multispectral and hyperspectral imaging are passive remote
sensing techniques, in that they rely on the reflection of ambient sunlight to
illuminate objects for measurement (Fig. 1.2). Passive sensors are therefore only
useful if there is a clear, well-lit view of the object of interest. Passive sensors
cannot be used through cloud cover or at night. Table 1.1 outlines the attributes of
several commonly used remote sensing photographic, multispectral and hyperspectral imaging systems.
All of these technologies have been used successfully for remote sensing of
coral reef ecosystems. The properties and processes of coral reefs that have been
Fig. 1.1 The electromagnetic spectrum as shown by wavelength units and corresponding
portions of the spectrum measured by remote sensing instruments (modified from Lillesand et al.
2008)
1 Visible and Infrared Overview
5