Radiance and Scale Factors Background

This topic provides guidelines on how to determine the correct scale factors to apply in FLAASH.

Note: Instead of multiplying the scale factor, FLAASH divides the scale factor into the radiance data.

FLAASH requires input data to be floating-point values in units of µW/(cm2 * sr * µm). If the input radiance image does not meet these criteria, you must know the scale factor(s) used to convert radiance data into these units.

  • If you used the Radiometric Calibration tool to create a radiance image suitable for FLAASH, leave the default value of 1 in the Single scale factor field of the Radiance Scale Factors dialog. ENVI has already applied the correct scale factor needed to get the image into units of µW/(cm2 * sr * nm). Otherwise, follow these steps:
  • If the radiance image is in units of W/(m2 * sr * µm), enter a value of 10 in the Single scale factor field of the Radiance Scale Factors dialog. This will divide the input data by 10.
  • If the scale factor is constant for all bands, select Use single scale factor for all bands. The Single scale factor text field appears. If the input image already has the correct units, then set the scale factors to 1 for all bands.
  • If the scale factor is not a constant for all bands, you must supply an ASCII file that specifies the scale factor for each band. In the Radiance Scale Factors dialog, select Read array of scale factors (1 per band) from ASCII file, and select an ASCII file.
  • Be sure to specify the correct column for the scale factor. For convenience, you can apply a multiplication factor and/or inverse. If you specify both, the multiplication factor is applied before the inverse. You can also re-define the scale factors later using the FLAASH Advanced Settings dialog.
  • ENVI automatically calibrates and applies scale factors to ASTER Level 1A and 1B files when you read them. You do not need to perform these steps separately.