A backlight unit for illuminating a binocular holographic display comprising: a light source configured to provide a light beam; a first beam expander having a grating element comprising a plurality of grooves arranged along a third direction inclined with respect to a first direction, each of the plurality of grooves having a wedge shape, the first beam expander being configured to: mix the light beam provided from the light source, expand the light beam in the first direction, and output the mixed and expanded light beam as white light; and a second beam expander configured to: expand the white light emitted from the first beam expander in a second direction perpendicular to the first direction, and output the expanded white light as surface light, and characterised in that: the light source comprises a first light source configured to emit light having a first wavelength, a second light source configured to emit light having a second wavelength different from the first wavelength, and a third light source configured to emit light having a third wavelength different from the first wavelength and the second wavelength; the first light source, the second light source, and the third light source are arranged such that the incidence angle of light from the first light source, the incidence angle of light from the second light source, and the incidence angle of light from the third light source, respectively with respect to the grating element, are different from one another; and the plurality of grooves are adapted such that light of different wavelengths incident at different angles is mixed to form white light and diffracted in the same direction. a light source configured to provide a light beam; a first beam expander having a grating element comprising a plurality of grooves arranged along a third direction inclined with respect to a first direction, each of the plurality of grooves having a wedge shape, the first beam expander being configured to: mix the light beam provided from the light source, expand the light beam in the first direction, and output the mixed and expanded light beam as white light; and mix the light beam provided from the light source, expand the light beam in the first direction, and output the mixed and expanded light beam as white light; and a second beam expander configured to: expand the white light emitted from the first beam expander in a second direction perpendicular to the first direction, and output the expanded white light as surface light, expand the white light emitted from the first beam expander in a second direction perpendicular to the first direction, and output the expanded white light as surface light, and characterised in that: the light source comprises a first light source configured to emit light having a first wavelength, a second light source configured to emit light having a second wavelength different from the first wavelength, and a third light source configured to emit light having a third wavelength different from the first wavelength and the second wavelength; the first light source, the second light source, and the third light source are arranged such that the incidence angle of light from the first light source, the incidence angle of light from the second light source, and the incidence angle of light from the third light source, respectively with respect to the grating element, are different from one another; and the plurality of grooves are adapted such that light of different wavelengths incident at different angles is mixed to form white light and diffracted in the same direction. the light source comprises a first light source configured to emit light having a first wavelength, a second light source configured to emit light having a second wavelength different from the first wavelength, and a third light source configured to emit light having a third wavelength different from the first wavelength and the second wavelength; the first light source, the second light source, and the third light source are arranged such that the incidence angle of light from the first light source, the incidence angle of light from the second light source, and the incidence angle of light from the third light source, respectively with respect to the grating element, are different from one another; and the plurality of grooves are adapted such that light of different wavelengths incident at different angles is mixed to form white light and diffracted in the same direction.