Patent ID: 8854732

Claim:
A light guide substrate comprising a substrate main body formed to have a predetermined thickness, wherein a transmitting-type first sub-wavelength grating is formed on a surface of one side of the substrate main body in a thickness direction, wherein light emitted by a light emitting device is incident upon the first sub-wavelength grating in a direction substantially perpendicular to the surface of the one side of the substrate main body, and the first sub-wavelength grating passes the light and generates a 0th order light and ±1st order lights, a reflection-type second sub-wavelength grating is formed on a surface of the other side of the substrate main body in the thickness direction, wherein the second sub-wavelength grating reflects and diffracts the 0th order light having passed through the first sub-wavelength grating, thereby generating the ±1st order lights advancing with a predetermined diffraction angle, and the second sub-wavelength grating is formed to face the first sub-wavelength grating in the thickness direction, and the ±1st order lights generated by the first sub-wavelength grating and the ±1st order lights generated by the second sub-wavelength grating are totally reflected by the surface of the one side of the substrate main body in the thickness direction and the surface of the other side of the substrate main body in the thickness direction, and wherein conditional equation 1 below is satisfied, h ·tan [min(θ,θ′)]≧2 w (Equation 1), where h denotes a thickness of the light guide substrate, θ denotes a diffraction angle of the ±1st order light generated by the first sub-wavelength grating, θ′ denotes a diffraction angle of the ±1st order light generated by the second sub-wavelength grating, min(θ, θ′) denotes a smaller value of θ and θ′, and w denotes a beam diameter of the light emitted by the light emitting device and incident upon the first sub-wavelength grating.