* Corresponding author. Tel.: # 33 1 64468477; fax: #33 1 69071499; e-mail: chehab@lalcls.in2p3.fr. Nuclear Instruments and Methods in Physics Research A 410 (1998) 148—158 Experimental investigations on geometrical resolution of optical transition radiation (OTR) X. Artru, M. Castellano, L. Catani, R. Chehab*, D. Giove, K. Honkavaara, P. Patteri, M. Taurigna-Quere, A. Variola, L. Wartski  Institut de Physique Nucle & aire de Lyon IN2P3-CNRS, 69622 Villeurbanne Cedex, France  Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, 00044 Frascati, Italy  Laboratoire de l+Acce & le & rateur Line & aire IN2P3-CNRS, Universite & de Paris-Sud, 91405 Orsay Cedex, France  Istituto Nazionale di Fisica Nucleare, LASA, 20090 Segrate (Milano), Italy  Helsinki Institute of Physics, P.O.Box 9, 00014 University of Helsinki, Finland  Institut d+Electronique Fondamentale, CNRS, Bat.220, 91405 Orsay Cedex, France Received 2 June 1997; received in revised form 25 November 1997 Abstract Optical Transition Radiation (OTR) provides an attractive method for diagnostics on electron/positron beams of small dimensions at high energies (GeV). However, some limits on the geometrical resolution at very high energies have been often discussed in the literature and a minimum value given by ‘’ has been invoked. In order to bring an experimental contribution to the problem, systematic measurements of electron beam profiles, in the energy range of 1—2 GeV and at optical wavelength between 400 and 700 nm, have been carried out at the Orsay 2 GeV Linear Accelerator. OTR emitted from an aluminium foil at a 30° incidence angle was collected by a two-lens telescope and recorded by an intensified CCD camera. The OTR beam profiles were compared to the profiles obtained by a SEM Grid having a resolution better than 0.5 mm. After a theoretical introduction presenting the different resolution limits, which can be invoked with diffraction phenomenon, and a presentation of our calibration procedure, the experimental results are presented and compared to these limits. They show that the resolution in OTR measurements is definitely better than the already invoked -limit. For a small size electron beam an r.m.s. beam width about 170 m has been determined. This value can be compared to "2.5 mm (E"2 GeV and "650 nm). Our theoretical analysis also provides a more precise evaluation of the resolution power of OTR, which depends on the sensitivity of the detector; this could be of interest for much higher energies.  1998 Elsevier Science B.V. All rights reserved. Keywords: Optical transition radiation; Geometrical resolution; Diffraction 1. Introduction In order to ensure high luminosities in ee Linear Colliders (LC) or good brilliances in Free Electron Lasers (FEL), strong constraints are put 0168-9002/98/$19.00  1998 Elsevier Science B.V. All rights reserved PII S0168-9002(97)00203-4