ES 771 Lecture
James S. Aber

SCANNING AND SPECTRAL SIGNATURES

Introduction

Multispectral scanning operates in the optical spectrum, including ultraviolet, visible, and infrared energy. These wavelengths may be gathered, focused and separated using optical devices, such as mirrors, lenses, and prisms. The energy is measured with semiconductors, which convert the radiation into an electrical signal. These instruments are, thus, called electro-optical sensors. Multispectral scanning greatly extends the range of potential wavelengths beyond the limits of photographic systems. At least three kinds of multispectral scanners are in operation today.

  1. Whiskbroom scanner: Cross-track scanning motion of sensor, which builds up a dataset cell by cell, line by line. These are among the most common scanners in operation today.

  2. Pushbroom scanner: Linear detector array that moves forward along track. All cells within a line are measured simultaneously, and the dataset is built up line by line.

  3. Charged-couple scanner: A full matrix array of detectors that measures all cells within an image scene simultaneously, similar to the electronic still camera used on space shuttle missions.

Thermal Infrared Imagery

Multispectral scanning allows the possibility to acquire, display and interpret thermal properties of the Earth's surface. Thermal energy is generally emitted rather than reflected from the Earth's surface. The break in wavelength is at about 3 µm; shorter wavelengths are reflected solar energy, whereas longer are emitted from the Earth's surface. The Earth behaves overall as a blackbody with peak energy emission about 10 µm wavelength. However, the radiant temperature of a given object depends on many thermal factors, such as emissivity, conductivity, capacity, diffusivity and inertia. Because of these factors different materials warm and cool at different rates during the day and night. This gives rise to a diurnal cycle of temperature changes for features at the Earth's surface. On this basis, water bodies appear relatively cool compared to land features during the day, and relatively warm at night. Thermal shadows, caused by obstacles, clouds and wind, may also affect the appearance of thermal-IR images taken during the day.

Spectral Signatures

Objects at the Earth's surface emit and reflect many wavelengths of radiation. In principle, each object has a unique spectral signature, which could be used for identification much like a finger print. Remotely sensed spectral signatures could be utilized for recognizing and mapping for all manner of features. This can be done to a limited extent with conventional multispectral scanners that operate in a few broad bands of visible and infrared energy. However, broad spectral bands often do not distinguish between similar features.

To overcome this limitation, a more sophisticated approach toward spectral signatures has arisen during the past few years. This approach is called image spectroscopy or hyperspectral analysis, which is based on scanning hundreds of closely spaced and very narrow spectral bands. With this technique, it is possible to create continuous spectral response curves, which may be used to identify many objects positively. Some typical hyperspectral signatures are presented in the table below.

Representative spectral signatures
Vegetation Mineral
Maple leaves Hematite
Sagebrush Malachite
Lawn grass Galena

From the U.S. Geological Survey's
Spectroscopy Lab--spectral library.

NASA has developed and operates a hyperspectral scanner called the Airborne Visible InfraRed Imaging Spectrometer or AVIRIS. It is a whiskbroom scanner that collects data in 224 contiguous spectral bands, each of which is 10 nanometers (10 nm or 0.01 µm) wide. The overall spectral range is 0.38 µm to 2.5 µm. AVIRIS is flown at high altitude in a modified U-2 airplane, and the ground resolution is about 20 m. An AVIRIS scene covers a ground area about 10x11 km and contains approximately 140 megabytes of digital data (see RSE plate 7-6). The result is laboratory quality spectral data for each cell on the ground--see concept illustration.

More Information

USGS Spectroscopy Lab
AVIRIS

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