1. Field of the Invention
This invention relates generally to an imaging spectrometer and, more particularly, to a technique for quickly and accurately calibrating a hyperspectral imaging spectrometer in the field using a PTFE panel having a known reflectance spectra.
2. Discussion of the Related Art
Spectral reflectance from a scene contains information that provides for the discrimination of objects or areas in the scene, and can allow detection of certain features that are not possible with other methods of remote sensing. To provide spectral reflectance imaging, hyperspectral imaging spectrometers are known that record the reflected electromagnetic spectrum from the objects in the scene as a function of the spatial position of the objects over several hundred discreet and contiguous wavelengths. The spectral dispersed images formed by the spectrometer are recorded electronically using a focal plane array (FPA). These spectrometers typically operate in the 0.4-2.5 micron wavelength range of the electromagnetic spectrum because they rely on reflected solar illumination. The imaging spectrometers are typically flown aboard aircraft to image terrestrial scenes. This technology is currently being proposed for deployment on satellites.
One known imaging spectrometer of this type is the TRW imaging spectrometer III (TRWIS III) airborne hyperspectral imager. The TRW ISIII imager includes a two-dimensional FPA having a plurality of pixels arranged in a two-dimensional array. Each pixel is a separate detector, and can be, for example, a charge-coupled device. The pixels arranged in one direction, referred to as the cross-track direction, provide spatial imaging, and the pixels arranged in the opposite direction, referred to as the spectral direction, provide spectral imaging. Each pixel in the spectral direction provides detection over a separate, contiguous wavelength band so that the entire frequency band of interest is covered by the combination of all of the pixels in that direction. An aperture slit covers a row of pixels in the cross-track direction, and as the airborne platform moves, the slit images the scene in a push-broom type manner. A grating is used to separate the light into its various wavelengths to fall on the pixels. In one design, 256 pixels are provided in the cross-track direction and 384 pixels are provided in the spectral direction. Employing several hundred pixels in the spectral direction, where each pixel images a different range of wavelengths, provides significant information from the scene.
To provide for increased device performance, two types of calibrations are performed on the spectrometer at different times during operation. The first type of calibration determines the responsivity of the pixel detectors. The responsivity calibration typically includes an in-flight calibration by recording frames of data with no light, and then recording the spectrometer's response to a reflectance standard. This procedure is generally performed once before and after each image is generated.
The second type of calibration includes providing spectral calibration of the range of wavelengths that falls on each pixel. The discrimination capability between objects in the scene provided by the image from the hyperspectral imaging spectrometers relies heavily on accurate spectral calibration as a function of field angle in the spectrometer's field-of-view (FOV). Part of the spectral calibration includes removing the effects of atmospheric absorption across the frequency spectrum. Particularly, the absorption of a water band can be rapidly varying in frequency. In order to remove the effects of the water band from the resulting data, it is necessary to know which pixels the absorption spectrum falls on so that absorption is not attributed to objects in the scene.
Spectral calibration is currently performed in the laboratory prior to data collection using spectral lamps or other illumination sources operating at discrete wavelengths. Also, spectral calibration is known to be performed in the field using a monochrometer that separates the light into a series of separate wavelength bands. The spectrometer then images the light from the monochrometer, and the center wavelength detected by each pixel in the FPA is measured. Ideally, this calibration would be performed just before each data collection in the field to account for any changes in the spectrometer's calibration over time. However, routine spectral calibrations in the field are difficult and time consuming, and the calibration apparatus can be bulky and awkward. Additionally, improvements in calibration accuracy can be made.
What is needed is an apparatus and method for the quick, accurate and repeatable spectral calibration of a hyperspectral imaging spectrometer. It is therefore an object of the present invention to provide such an apparatus and method.