A light guide device includes a triangular prism unit and a plurality of quadrate prism units. The triangular prism unit has a first light entrance side, a first light exit side angled relative to the first light entrance side, and a first light transition interfacial side angled relative to the first light entrance side and the first light exit side. The first light transition interfacial side has a coating applied thereon. The quadrate prism units are connected optically to the triangular prism unit. Each of the quadrate prism units has a second light entrance side, a second light transition interfacial side opposite to the second light entrance side, and a second light exit side interconnecting the second light entrance side and the second light transition interfacial side. The second light transition interfacial side has a coating applied thereon.