The lightguide assisted by diffraction gratings is used to implement colour-separating backlight for Liquid Crystal Display (LCD). In the configuration, the light incidence angle is required to be a precise designed one. However, the light incidence is conical always. In this paper, by rigorous coupled wave theory, the influence of conical input on the colourseparating backlight is investigated. It shows that, when the azimuthal angle of the light increases, the diffraction efficiency could be enhanced by a factor of 2, which means that the conical input will benefit the diffraction efficiency of the lightguide. However, it also leads to a larger output spot area at the same time, which could result in the mixing of RGB pix. When the azimuthal angle is smaller than 12°, the maximum diffraction angles of RGB light can be completely separated, which means that, with a designed distance or with an assisted micro-lens layer between the grating and the pix layer, the RGB light can reach their corresponding pix. The influence of the conical input on the polarized light is also investigated. The conical input will result in the output with composite polarization, which, in principle, disables the use of polarized light source in LCD.
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