Abstract: This paper describes the reason of flare in holographic combiners,and discusses two different methods of ray trace in holographic gratings.It points out that through these two methods one can get the same result when the fringes in grating are parallel to the surface.It can be simplified as follows:a reflective volum holographic grating with a planar interface can be equivalent to a planar mirror and one with a spherical interface can be regarded as a spherical mirror.In this situation,the flare is avoided.