DIRSIG allows the user to supply a sensor response function that is to be applied to the sensor reaching radiance. The radiance or radiance vector resulting from the application of the response function dictates what type of image is generated for the bandpass defined for each band. The sensor responses and resulting output images can be generalized into three (3) forms:
Integrated broad-band Image
Spectrometer Image
Spectral radiance Image
Table 34-6. List of the variables that can be set in file.
| Variable Name | Usage | Description |
| DIRSIG_RSP | Required | File identification marker (must be first non-comment line) |
| BAND | Required | section to define the extents of the response function |
| TYPE | Required | Type of response function (INTEGRATED or SPECTROMETER ) |
| SHAPE | Spectrometers Only | Shape of the channel response (GAUSSIAN or TRIANGULAR ) |
| GAIN | Optional | A gain to be applied to the radiance spectrum |
| BIAS | Optional | A bias to be applied to the radiance spectrum |
| RESPONSE | Required | Response data (spectral function or channel widths) |
Although ``broad-band'' might imply that there is a width constraint on this output option there is not. The ``broad-band'' image option simply implies that the spectral range specified for the band given is to be output as a single broad-band integrated radiance value for each pixel. The spectral response file in this case contains the spectral response of the sensor for the bandpass region modeled. The sensor reaching radiance spectrum is multiplied by this spectral response before the spectral radiance is integrated. The units for a broad-band radiance image is therefore [W/(cm^2~sr)]. This type of sensor response is specified by setting the TYPE variable to the value INTEGRATED . For this case, the RESPONSE section will contain a series of wavelength/response or frequency/response pairs depending on what spectral basis was used in the section. The following example illustrates the RSP file for the SPOT Pan-Visible band used in our example CFG file:
DIRSIG_RSP
#
# NAME: SPOT Pan-Visible Channel
# PURPOSE: Spectral response function for SPOT Pan Channel
#
BAND {
NAME = SPOT Pan-Visible
MINIMUM_WAVELENGTH = 0.470
MAXIMUM_WAVELENGTH = 0.800
DELTA_WAVELENGTH = 0.010
}
TYPE = INTEGRATED
RESPONSE {
0.4780 0.0000
0.4800 0.0050
0.4900 0.1140
.
. [middle wavelengths deleted for documentation purposes]
.
0.7800 0.0040
0.7900 0.0020
0.8000 0.0000
}
There are no assumptions about the spectral spacing or number
of response pairs in the RESPONSE
section except that the wavelengths or frequencies be in
increasing magnitude. If the wavelength or
frequencies are found to be out of order, an error will be issued.
To make it easier to model imaging spectrometers, the user can input the center wavelength/frequency and wavelength/frequency full width at half max (FWHM) of each spectral channel in the instrument. The bandpass for this type of response file must cover the extents of the spectral channels. This type of sensor response is specified by setting the TYPE variable to the value SPECTROMETER . For this case, the RESPONSE section will contain a series of channel center wavelength/frequency and wavelength/frequency FWHM pairs. In addition, the SHAPE variable must also be set to define the shape of the channel response function (either GAUSSIAN or TRIANGULAR ). The example below is a condensed version of the HYDICE response file that can be found in the default sensor directory:
DIRSIG_RSP
#
# NAME: HYDICE
# PURPOSE: Channel response functions for the HYDICE imaging spectrometer
#
BAND {
NAME = HYDICE Spectrometer
MINIMUM_WAVELENGTH = 0.470
MAXIMUM_WAVELENGTH = 0.800
DELTA_WAVELENGTH = 0.010
}
TYPE = SPECTROMETER
SHAPE = GAUSSIAN
RESPONSE {
0.394214 0.002225
0.397510 0.004450
0.400806 0.004451
.
. [middle wavelengths deleted for documentation purposes]
.
2.471950 0.011165
2.480210 0.011124
2.488430 0.005485
}
The channel center/width pairs must be in the same units as the
bandpass defined in the section. The extents of the bandpass
in this example were chosen to extend well past the extents of
the first and last channel in the spectrometer. The spectral
resolution (see DELTA_WAVELENGTH ) for this
example assures that at least 5 spectral points will be utilized
in each channel computation.
The computed radiance spectrum for each pixel is then convolved and
integrated for each channel specified to produce a pixel spectra
with radiometric units [W/(cm^2~sr~ m)]. The pixel spectral
are written to the corresponding output image file using a band
interleaved-by-pixel (BIP) format. Each pixel spectra consists
of N single-precision floating point (float
) numbers (where N is the number of channels in the spectrometer).
The dimensions of the output image is therefore X width x Y width
x Number of channels.
In addition, an compatible header file is created that contains
an description of the image format for those attempting to read
the imagery into other analysis tools.
In some cases the user may wish to avoid any spectral integration effects. Therefore, DIRSIG provides a way to output the high spectral resolution radiance spectrum directly to an output file. This feature is used by setting the response filename to SPECTRAL in the CFG file. The pixel spectrum are written to the corresponding output image file using a band interleaved-by-pixel (BIP) format. Each pixel spectra consists of N single-precision floating point (float ) numbers (where N is the number of points in the spectral region). The bands in the output image are ordered in increasing wavelength, or decreasing frequency. The units for each point in the radiance spectrum is W/(cm^2 sr um) (for both wavelength and frequency based bands). The dimensions of the output image is therefore X width x Y width x Number of spectral points.