An image display device having an RGB illuminating module that includes a diffractive beam combiner. The diffractive beam combiner includes a first light source arranged to provide a first light beam having red color and a second light source arranged to provide a second light beam having green color. A centerline of the first light beam and a centerline of the second light beam are arranged to intersect at an intersection point. A diffractive output grating is located in the vicinity of the intersection point. The diffractive output grating is arranged to form an output light beam by diffracting light of the first light beam and light of the second light beam substantially in the same direction.