Hi, The Radiometric Calibration tool in ENVI 5.1 calibrates Landsat-8 OLI imagery to top-of-atmosphere reflectance, when you select the "Reflectance" option from the drop-down menu. To further correct the imagery for atmospheric effects and create an apparent surface reflectance image, you have various options: 1. Dark Subtraction or IARR methods (available from the Toolbox) are simple techniques for atmospheric correction and are usually sufficient for subsequent image processing of multispectral imagery. Example: start the Dark Subtraction tool from the Toolbox and select the reflectance image that you created from the Radiometric Calibration tool. You do not need to use FLAASH in this case. Use the Dark Subtraction image for your image processing. 2. FLAASH and QUAC are more rigorous, model-based techniques for atmospheric correction. QUAC is simple to use and produces good results; however, it requires you to first create a mask to exclude the black background pixels from Landsat-8 imagery. I wrote a short tutorial that explains how to do this, which will be available in the next service pack release of ENVI. To use FLAASH: Select the "Radiance" option in the Radiometric Calibration tool. Click the "Apply FLAASH settings" button, then select an output filename. This creates a radiance image that can be input into FLAASH. Start FLAASH, then select the radiance image from Step 2. Important: when the scale factors dialog appears, keep the default selection of "1". You don't want to change the scale factor at this point. Fill in the remaining parameters to create a surface reflectance image. We are adding more tutorials on how to do this in the next service pack release. If you need further assistance, please contact Technical Support directly.

Thank you