Apparent Reflectance function

Overview

This function calibrates the digital number (DN) values of imagery from some satellite sensors. The calibration uses sun elevation, acquisition date, sensor gain and bias for each band to derive Top of Atmosphere reflectance, plus sun angle correction.

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. 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.

Notes

This function modifies the image values, so previous statistics and histograms are no longer valid. This function should be applied early in the function chain, after band extraction, and prior to any stretching or other radiometric function.

The output values can be expressed as an integer, using the Scale Factor and parameters.

You have the option to produce a brightness layer based on the proportion of radiation that is reflected by the surface, called albedo. Albedo is expressed as a dimensionless floating point number between 0 and 1. Zero corresponds to a black body that absorbs all incident radiation, such as wet coal, and 1 corresponds to a body that reflects all incident radiation, similar to fresh snow.

Parameters

Parameter name	Description
	The raster product on which the apparent reflectance will be applied.
	This is a table that will list the gains and bias values. This table will be populated by the system if the appropriate information can be read from the metadata. You can manually set or edit these values.
Sun Elevation	This is sun elevation value, expressed in degrees. This will be populated by the system if the appropriate information can be read from the metadata. You can manually set or edit this value.
Albedo	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. Since the output data type for albedo is floating point, you must explicitly set the preferred Output Pixel Type to either 32-bit float or 64-bit double. The Output Pixel Type parameter is found on the General tab in the Apparent Reflectance raster function. If the Output Pixel Type is not set correctly for the Albedo option, the resulting values will be invalid and given a value of 0.
Scale Factor	Your apparent reflectance output value can be expressed as an integer. The scaling factor is multiplied by the albedo to convert all floating-point values into integer values. If the scale factor is either 0 or not specified, default scaling will be applied depending on the pixel type of the input data: For 16-bit unsigned data types, the default scale factor is 50,000. For 8-bit unsigned data types, default scale factor is 255. The scaling factor is always applied when the output is apparent reflectance. No scaling is applied when the output is albedo.
	Your scaled albedo value can optionally have an offset value: For 16-bit unsigned data types, the default scale offset is 5,000. For 8-bit unsigned data types, the default scale offset is 0. No scaling is applied when the output is albedo.