Below is an example of a single pixel spectrum from AVIRIS. The x-axis is channel wavelength in micrometers, also known as microns (one micron = 1000 nm). The y-axis is radiance, usually expressed in units of microwatts per square
centimeter per nanometer per steradian, or uW / (cm^2 * nm * sr).
Click image to see larger graphic
The general shape of an AVIRIS spectrum is dominated by the light curve of the
Sun and the absorption features of the atmosphere. The Sun has a "blackbody" curve, which in the case of the Sun peaks in the green wavelengths and diminishes
at higher and lower wavelengths. The atmosphere absorbs light at wavelengths that correspond to the absorption wavelengths of the atmosphere's components: nitrogen, oxygen, carbon dioxide, water, and other elements. For example, the deep valleys that go down to near zero around 1.4 and 1.9 microns are due to water absorbing those wavelengths.
The peaks and valleys of a spectrum
not due to the Sun or the atmosphere reveal information about the chemical
composition of the pixel being examined. Every substance has its own spectrum,
and one can look for those features from those spectra in the AVIRIS pixel
spectra. Even living things have spectra. Green plants, for example, use
chlorophyll to absorb the visible light from the sun, but reflect the infrared
radiation. This manifests as a large jump in the spectra in the area where
the red light (0.7 microns) merges into the infrared. The spectra presented
above shows just such a "red edge", indicating that the pixel was showing
vegetation.
For a different representation of
AVIRIS spectra, along with another example of imaging living things, access
the AVIRIS Moffett Field Image Cube page. The image is 125KB, so you may want to delay the image loading so you can read the explanation while waiting for the image to load. |  |
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