In order to perform radiometric calibration, the appropriate metadata such as gains and offsets must be present in the file metadata so that the image pixel values can be converted to the physical units of radiance. Your data provider or sensor manufacturer should be able to provide you with the instructions on how to obtain this metadata.
However, UAV data is usually not calibrated in the same way as images taken from space platforms. UAV data is far less influenced by atmospheric effects and so a full atmospheric correction is unnecessary. In this case, the data collector will often use calibration panels in the field to help calibrate the data to surface reflectance. You should contact the camera manufacturer for the specifications on how to get calibrated reflectance..

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