PURPOSE: To lower light energy loss, by eliminating use of a semitransparent mirror, dividing a single flux into two mutually interfereable parallel fluxes and generating an interference fringe by allowing both fluxes to be intersected with a small inclination angle.

CONSTITUTION: A diffusing flux flashed from a light source Q is reflected by a Fresnel mirror into two fluxed inclined mutually. The bisected flxes are built up into parallel fluxes respectivey by a collimating lens or a collimating mirror 3. these two parallel fluxed incline to the light axis 2 with an angle of respectively. These two parallel fluxes intersect with an angle of 2 generating two- beam plane wave interference fringe located in the X-X plane. The interference fringe thus formed is developed as an image and light intensity distribution of the interference fringe is measured by a photometer 6 per interference phase.