A general graphical convolution method for fringe analysis of testing methods that use a spatially incoherent light source is presented. Results are shown for the grating-slit test using optical narrow and cross slits. The convolution method shows that phase shifting grating-slit test can be done with the cross slit at the expense of a reduced fringe contrast ratio. A phase shifting measurement with a cross-slit modulator yields high measurement precision and validates our fringe contrast theory. (C) 2010 Optical Society of America
