Analysis I: RGB Images
For this study, two types of processing were performed. The first was the extraction of imaging spectrograph data at three different wavelengths, which was used to produce a red-green-blue (RGB) composite image. The second analysis procedure involved using images from the spectrographs at several wavelengths in an attempt to perorm a supervised spectral classification in order to unambiguously identify features within a scene. Before both of these could be attempted, some preprocessing must be done to extract images of a specific wavelength from the spectral imager data.
PreprocessingSince the imaging spectrometers use video-rate CCD detectors, the raw spectra are stored initially as either digital video files or on a casette tape. The BigSpex data, on digital video tape, were digitized via video capture software.
A demonstration of what data acquisition would look like appears in Figure 4. Assuming the webcam and spectrograph are co aligned, the video signal shown in the right-hand panel corresponds to a narrow slice through the center of the webcam image. Note that the vertical (i.e. colinear with the slit) field of view for the spectrograph is less than the webcam. Spatial structures colinear with the slit appear as vertical structure, while any relative spectral variation will show up as horizontal structure, which we expect to be fairly smoothly varying.
Figure 5. Keogram at 557.0-nm of the a section of the first 2000m pass during flight #1.
Red-Green-Blue (RGB) Composite ImagesOnce images of specific wavelengths have been created from the spectrograph video data, it is easy to select several images to create a red-green-blue (RGB) composite image. This is most simply done using an application called Image Calculator written by K. Heia at Fiskeriforskning division of Norut.
Figure 6. Image Calculator used to create a RGB composite image.
Figure 7. RGB composite image formed from three images of wavelengths 480.0-, 557.0- and 630.0-nm
Figure 8. Animation (SWF) showing the variation of magnification along the slit direction with wavelength.
Finally, it is also important to note that the magnification in the direction parallel to the slit is not uniform over the entire field of view of the detector. That is, the vertical magnification decreases with increasing wavelength, as illustrated in Figure 8. Therefore, as the spectral images are presented here, there is not a 1:1 correspondence between a pixel p(x,y,&lambda 1) and another pixel in a different spectral image p(x,y,&lambda 2). Although no attempt to quantify or correct for this aberration was attempted, it should be possible to crudely correct for this by performing a vertical scaling operation, the amount of which increases with increasing wavelength. Note that the resulting images would need to be cropped to the original size, as the Image Calculator software needs spectral images of identical size to perform a classification.
(Finished 10.05.05 -JMH)