Hi, I am using ENVI 5.1, I want to conduct image pre-processing before calculating the NDVI. However, I encounter some anomalies as below: If calculate the NDVI before pre-processing, the values of the forest are around 0.4 - 0.5. After radiometric calibration, I choose reflectance, the values are around 0.5- 0.7, I think this result is more accurate as it matches the findings from other literature. I continue to use the radiometric calibrated image to conduct atmospheric correction using Flaash, Abnormal result occurred. The values of NDVI for all pixels are negative values. I have tried many different settings, such as calibration type, output interleave, or apply Flaash settings in the Radiometric Calibration; for the settings of Flaash, most of them are just default values, the KT Upper Channel is defined as SWIR2 (Landsat 8), Maximum Upper Channel Reflectance as 0.08, Reflectance Ratio as 0.5 and Index to first band: 0. Different parameters lead to different strange results and none of them are normal. I have also tried QUAC and other dataset such as images from Landsat 5, and the results are also strange. Settings of radiometric calibration and Flaash can be found as below: http://www.fotop.net/windyfung/windyfung Anyone have idea about this problem? I want to conduct study about vegetation recovery after wildfire, is it acceptable if I only use radiometric calibrated images without atmospheric correction?