A modified Ronchi test with an extended source is presented. By the use of
a matched pair of source grating and Ronchi grating, a pure shearogram betw
een two shifted wave fronts is obtained, and the extra interference with ot
her disturbing grating orders is largely suppressed by the use of a specifi
c grating layout. (C) 1999 Optical Society of America [S0740-3232(99)00501-
3].