Although the DIRSIG graphical user interface has tools to import data to define spectral channels, there are several steps involved in that process that can make it cumbersome in some situations. The option to import DIRSIG3-era spectral response files (usually named with a .rsp file extension) is supported, which can streamline the process of importing spectral channels. This approach can be quite useful in situations where a set of responses will be frequently incorporated into instrument descriptions.

A single RSP file can define either:

  • A tabulated description (response vs. wavelength) for a single channel

  • A parametric description (center and width) for a series of similarly shaped channels (e.g. a spectrometer).

Format Details

Bandpass

Each file must contain a BAND section that defines a default spectral window and sampling interval. The use of the _WAVELENGTH or _FREQUENCY variables allows the user to define this spectral bandpass in either microns or wavenumbers, respectively. The extents of the spectral window are defined by the MINIMUM_ and MAXIMUM_ variables and spectral resolution is defined by the DELTA_ variable.

Important When imported in the DIRSIG interface, the bandpass defined in the file will be used to update the Spectral Range and Sampling for the focal plane.

Gain and Bias (optional)

The optional GAIN and BIAS variables allow the user to supply a linear amplification to the sensor reaching radiance spectrum. A common use for the GAIN is to perform a unit conversion of the radiance units. If the GAIN or BIAS variables are not set, the default values are 1.0 and 0.0 respectively.

Data Type

The TYPE variable specifies whether a single, tabulated channel or series of parametric channels will be supplied in the RESPONSE section of the file. More detailed descriptions of these two types are provided in the following sections.

Single, Integrated Channel

The tabulated means that the RESPONSE section will define the response of a single channel as a series of wavelength and relative response pairs. This option is selected by setting the TYPE variable to INTEGRATED. The wavelength half of each response pair is assumed to be in the same spectral units defined in the BAND section. The following example is the spot_pan.rsp file that is included in the DIRSIG distribution, which contains the response for the PAN visible band in the SPOT, space-based pushbroom imaging satellite:

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.

A Set of Spectrometer Channels

To make it easier to model imaging spectrometers, the user can input the center and full-width at half max (FWHM) of each spectral channel in the instrument. The bandpass for this type of response file should span the extents of the spectral channels (end to end, not center to center). 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 and 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 following example is the aviris.rsp file that is included in the DIRSIG distribution, which contains the channels for the AVIRIS airborne, V/NIR/SWIR spectrometer:

DIRSIG_RSP
#
# NAME:     AVIRIS
# PURPOSE:  Channel response functions for the AVIRIS imaging spectrometer
#
BAND {
    NAME = AVIRIS Imaging Spectrometer
    MINIMUM_WAVELENGTH = 0.36
    MAXIMUM_WAVELENGTH = 2.55
    DELTA_WAVELENGTH = 0.001
}

TYPE = SPECTROMETER
SHAPE = GAUSSIAN

RESPONSE {
    0.3734 0.0099
    0.3829 0.0098
    0.3925 0.0098
    .
    . [middle wavelengths deleted for documentation purposes]
    .
    2.4836 0.0118
    2.4934 0.0118
    2.5033 0.0117
}

The channel center/width pairs must be in the same units as the bandpass defined in the BAND 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 about 5 spectral points will be utilized in each channel computation.