<p>In general, atmospheric correction to surface reflectance (using QUAC or FLAASH) is unnecessary when performing image difference or thematic change detection, however it really depends on the particular scenario. At the very least, I would recommend calibrating the Landsat imagery to radiance using ENVI's Radiometric Calibration tool.&nbsp;</p> <p>The following article provides more detailed guidelines:</p> <p>Song, C. et al. "Classification and Change Detection Using Landsat TM Data: When and How to Correct Atmospheric Effects?" <em>Remote Sensing of Environment</em>, Vol. 75 (2001).&nbsp;</p> <p>A web search will yield a PDF version of this article. Pages 232-233 offer a good explanation of atmospheric correction and change detection.</p>