To measure slice-energy spread, momentum modulations are imposed on the beam in the accelerator. The electron beam (multiple pulses shown for clarity) is sampled by a spectrometer near the accelerator and by a phosphor screen at the end of its path. After the drive laser is deflected, electron pulses continue through vacuum, where their momentum modulations become microstructural density modulations. Encountering Mylar foils at 2.3 meters from the accelerator and again at 3.8 meters, the microstructure of the pulse strongly enhances coherent optical transition radiation (COTR). Electron and COTR photon patterns (insets, right) are compared with theoretical predictions to determine beam quality.