An optical system comprising: an image projection system (8, 9, 10, 11; 200, 202, 204, 206, 208), the image projection system configured to emit a coherent beam of light at a plurality of wavelengths in the visible spectral range; and a waveguide (182, 184, 186, 188; 710; 1010) comprising a first edge, a second edge and a pair of reflective surfaces disposed between the first and the second edges, the pair of reflective surfaces separated by a gap having a gap height d, the waveguide comprising a material having a refractive index n, the pair of reflective surfaces having a reflectivity r, the beam emitted from the image projection system (8, 9, 10, 11; 200, 202, 204, 206, 208) being coupled into the waveguide at an input angle θ; wherein the waveguide (182, 184, 186, 188; 710; 1010) is configured as an exit pupil expander that expands and multiplies the emitted beam; characterized in that the optical system further comprises: a control system (12; 210) configured to vary at least one parameter selected from the group consisting of: a wavelength from the plurality of wavelengths, the gap height d, and the refractive index n, wherein the variation of the at least one parameter is correlated with variation in the input angle θ; wherein the control system (12; 210) is configured to modulate the at least one parameter, the modulation rate configured such that the equation 2 nd cosθ = m λ is satisfied for all values of the input angle θ, wherein m is an integer and λ is wavelength of the beam. an image projection system (8, 9, 10, 11; 200, 202, 204, 206, 208), the image projection system configured to emit a coherent beam of light at a plurality of wavelengths in the visible spectral range; and a waveguide (182, 184, 186, 188; 710; 1010) comprising a first edge, a second edge and a pair of reflective surfaces disposed between the first and the second edges, the pair of reflective surfaces separated by a gap having a gap height d, the waveguide comprising a material having a refractive index n, the pair of reflective surfaces having a reflectivity r, the beam emitted from the image projection system (8, 9, 10, 11; 200, 202, 204, 206, 208) being coupled into the waveguide at an input angle θ; wherein the waveguide (182, 184, 186, 188; 710; 1010) is configured as an exit pupil expander that expands and multiplies the emitted beam; characterized in that the optical system further comprises: a control system (12; 210) configured to vary at least one parameter selected from the group consisting of: a wavelength from the plurality of wavelengths, the gap height d, and the refractive index n, wherein the variation of the at least one parameter is correlated with variation in the input angle θ; wherein the control system (12; 210) is configured to modulate the at least one parameter, the modulation rate configured such that the equation 2 nd cosθ = m λ is satisfied for all values of the input angle θ, wherein m is an integer and λ is wavelength of the beam. a control system (12; 210) configured to vary at least one parameter selected from the group consisting of: a wavelength from the plurality of wavelengths, the gap height d, and the refractive index n, wherein the variation of the at least one parameter is correlated with variation in the input angle θ; wherein the control system (12; 210) is configured to modulate the at least one parameter, the modulation rate configured such that the equation 2 nd cosθ = m λ is satisfied for all values of the input angle θ, wherein m is an integer and λ is wavelength of the beam.