PRE is the acronym indicating the series of International Workshops on Photoluminescence of Rare-Earth: Photonic Materials and Applications, begun in Trento, Italy, in May 2005.
All the Workshops were quite successful and provided excellent opportunities for discussions between researchers working on photoluminescence of rare-earth ions in various materials and devices, as well as on their applications.

Here there are some figures (the number of papers includes all types of presentation: invited-oral-poster):

PRE’05 (Trento, Italy)
        Co-chairs: Bernard Champagnon, Maurizio Ferrari, Giancarlo C. Righini
        Attendance: over 70 people, from 9 Countries - 47 papers

PRE’07 (Trento, Italy)
        Co-chairs: Maurizio Ferrari, Giancarlo C. Righini, Setsuhisa Tanabe
        Attendance: over 70 people, from 9 Countries - 57 papers

PRE’10 (Florence, Italy)
        Co-chairs: Maurizio Ferrari, Giancarlo C. Righini, Setsuhisa Tanabe
        Attendance: 126 people from 20 Countries - 96 papers

PRE’12 (Kyoto, Japan)
        Co-chairs: Setsuhisa Tanabe, Giancarlo C. Righini, Maurizio Ferrari
        Attendance: 167 participants from 21 Countries - 140 papers

PRE’14 (San Sebastian, Spain)
        Co-chairs: Rolindes Balda, Joaquìn Fernández, Maurizio Ferrari
        Attendance: 110 participants from 23 Countries - 122 papers

PRE’16 (Greenville, USA)
        Co-chairs: Luiz G. Jacobsohn, John Ballato
        Attendance: 65 participants from 11 Countries - 59 papers

After each Workshop a Special Issue of the journal 'Optical Materials' was published, that included a number of selected papers, peer-reviewed according to the standard journal’s procedure:

PRE’05: Optical Materials, Volume 28, Issue 11, August 2006, Page v
        Preface, by G. C. Righini, M. Ferrari, B. Champagnon

PRE’07: Optical Materials, Volume 31, Issue 9, July 2009, Page v
        Preface, by G. C. Righini, M. Ferrari, S. Tanabe

PRE’10: Optical Materials, Volume 33, Issue 2, December 2010, Page 117-118
        Preface, by G. C. Righini

PRE’12: Optical Materials, Volume 35, Issue 11, September 2013, Pages 1877-1878
        Preface, by S. Tanabe, G. C. Righini, M. Ferrari

PRE’14: Optical Materials, Volume 41, March 2015, Pages 1-2
        Preface, by R. Balda, J. Fernández, M. Ferrari

PRE’16: Optical Materials, Volume 68, June 2017
        Edited by Luiz G. Jacobsohn and John Ballato

The story continues: see you in Nice, in Spring 2019, at PRE’19 !!