Patent ID: 8676005

Claim:
A light guide for a light source device comprising: a core layer which is formed from a transparent resin and has a refractive index n 1 ; and cladding layers which are provided on both surfaces thereof and have a refractive index n 2 which is lower than the refractive index n 1 of the core layer; wherein a light reflecting layer which scatters and reflects light is provided in the front surface of one cladding layer, the front surface of the other cladding layer is set as a light emitting face, and a recess is provided which reaches from at least one of either of the front surface of the one cladding layer or the front surface of the other cladding layer to the core layer passing through the cladding layer, wherein at least one of the side edge surfaces of the light guide for a light source device, which is substantially perpendicular to a surface where the light emitting face and the light reflecting layer are provided, is set as a light incidence face, and the recess is provided in the light emitting face and the inclination angle θ of the light incidence face side of the recess satisfies equation (1) below θ≦90+cos −1 ( n 2 /n 1 )−sin −1 (1 /n 1 ) equation (1).