The possibility of using an interferometer with plates of spherical form was outlined by Pierre Connes in seminal papers. An important point of comparison is the insensitivity of the confocal interferometer to a small change of tilt or orientation of the plates that would be quite ruinous if it occurred with plane etalon plates. The spherical interferometer acts as an annular wedge etalon for which the angle is considerable in relation to the number of interfering beams. Wavelength scanning over the central paraxial fringe may be achieved by pressure or piezoelectric scanning. In this method the Fabry–Perot operates as an angle-independent filter whose transmission depends only on the plate separation. During the 1960s the spherical interferometer became widely used, in different modes of operation, as a diagnostic tool for laser studies. If the spherical interferometer is illuminated over a large aperture with a well collimated beam or a small, distant, on-axis source, a pattern of sharp concentric fringes is obtained.