We wish to construct a double-pinhole setup illuminated by a uniform, quasimonochromatic, thermal slit source of incoherent light of mean wavelength 500 nm and width b, a distance of 1.5 m from the aperture screen. If the pinholes are 0.50 mm apart, how wide can the source be if the visibility of the fringes on the plane of observation is not to be less than 85%?