The Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) sensors acquire Thermal InfraRed (TIR) data and store this information as a digital number (DN) with a range between 0 and 255. The Landsat 8 OLI sensor stores these data as DNS with a range from 0 to 65536. ENVI Standard can easily convert these DNs to degrees Kelvin.
You need to open the original USGS supplied file in USGS GeoTIFF with Metatdata format. Open the file that ends with “_MTL.TXT”. ENVI will automatically open the Landsat image as multiple files. For Landsat TM there will be a single TIR band, for ETM and OLI there will be two bands of TIR data.
Using ENVI Standard, select from the Toolbox | Radiometric Correction | Radiometric Calibration. Select the TIR data file and the Radiometric Calibration dialog opens. Under Calibration Type choose Brightness Temperature and save the new file.