Direct measurement of the formation height difference of the 630 nm Fe I solar lines M. Faurobert, C. Aime, C. Périni, H. Uitenbroek, C. Grec, J. Arnaud and G. Ricort A&A, 507 2 (2009) L29-L32 Published online: 04 November 2009 DOI: 10.1051/0004-6361/200913330