This function adjusts image brightness digital number (DN) values for some satellite sensors, similar to an atmospheric correction. The adjustments are based on sun elevation, acquisition date, and sensor properties (gain/bias settings for each band).
The Apparent Reflectance function is used to adjust reflectance, or brightness, values of some satellite imagery based on the scene illumination and sensor-gain settings. The images are adjusted to a theoretically common illumination condition, so there should be less variation between scenes from different dates and different sensors. This can be useful for image classification, color balancing, and mosaicking.
This function can only be used with specific imagery. The applicable sensors are Landsat MSS, Landsat TM, Landsat ETM+, Landsat 8, IKONOS, Quickbird, GeoEye-1, RapidEye, DMCii, WorldView-1, WorldView-2, SPOT 6 and Pleiades.
All the information required for the correction is extracted from the key metadata properties for each image when the function is initialized. In order to be eligible for this function, an image must have the following properties:
- Acquisition date and sun elevation for the dataset
- Radiance gain, radiance bias, and sun irradiance for each band
- Reflectance gain and reflectance bias (for Landsat 8)
The function performs two corrections. The first is based on the gain settings. The original brightness values are re-created from the image values by reversing the gain equations. The second correction has to do with differences in sun angle and brightness. The original brightness values are adjusted to a common lighting condition by normalizing scenes captured under variable illumination conditions. In general, while the output image data type is the same as the input image data type, the output values are lower than the input values, and are clipped to the valid data range.
The results of the Apparent Reflectance function can also be expressed as Albedo, which is the percentage of the available energy reflected by the planetary surface. Albedo data is used by scientific users for complex modeling and technical remote-sensing applications.
- Unchecked—The function returns Apparent Reflectance values. This is the default.
- Checked—The function returns 32-bit floating-point values, which most commonly are in the range of 0.0 to 1.0. No data clipping is performed if this option is selected.
This function modifies the image values, so previous statistics and histograms are no longer valid. This should be applied early in the function chain, after band extraction (reordering), and prior to any stretching or other radiometric function.
If this function is applied to an invalid dataset, it will slow the function chain but make no adjustments.