  The generation of the ultra-bright beams required by modern accelerators and
drivers of free-electron lasers (FELs) has generally relied on chicane-based
bunch compressions that often result in the microbunching instability.
Following compression, spectral enhancements extend even into the visible
wavelengths through the longitudinal space charge (LSC) impedances. Optical
transition radiation (OTR) screens have been extensively used for transverse
electron beam size measurements for the bright beams, but the presence of such
longitudinal microstructures (microbunching) in the electron beam or the
leading edge spikes can result in strong, localized coherent enhancements
(COTR) that mask the actual beam profile. Generally, we have observed effects
in rf photocathode (PC) injected linacs with chicane compressions since an R56
term is needed. In the past COTR had been only reported in S-band and L-band
photoinjected based linacs with single or double bunch compression. Drive laser
modulations and charge shot noise have been suspected of contributing to the
cause. We now have evidence for the effects in both rf PC-gun injected linacs
and thermionic- cathode (TC)-gun injected linacs (the latter do not involve a
drive laser). Since the first observations, significant efforts have been made
to characterize, model, and mitigate COTR effects on beam diagnostics. An
update on the state-of-the-art for diagnosing these effects will be given as
illustrated by examples at the Linac Coherent Light Source (LCLS), Spring-8
Compact SASE Source (SCSS), Spring-8 Angstrom Compact Free Electron LAser
(SACLA), Advanced Photon Source (APS), and the Next Linear Collider Test
Accelerator (NLCTA). These observations continue to be of interest to the
accelerator community.
