Apparent Reflectance function

This function adjusts image brightness values (DN) 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 images are adjusted to a theoretically common illumination condition so there should be less variation between scenes from different dates and from different sensors. This can be useful for image classification, color balancing, and mosaicking.

This function can only be used with specific imagery and may be automatically applied when adding data to a mosaic dataset using the appropriate raster type. The applicable sensors are Landsat, IKONOS, and QuickBird.

All of 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
• Gain, bias, and sun irradiance for each band

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 brightening the scenes captured with less sunlight energy available. In general, the output values are lower than the input values.

This function modifies the image values, so previous statistics and histograms are no longer valid.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 a dataset that is invalid it will slow the function chain but make no adjustments.