A light emitting device, comprising: a coherent light source for emitting a coherent light; a diffusing element having a first surface and a second surface opposite each other, for diffusing the coherent light from the coherent light source to generate an incoherent light; and a light guide element disposed on a side of the first surface of the diffusing element, for guiding the coherent light emitted by the coherent light source to incident on the first surface of the diffusing element to form a first light path, for guiding a portion of the incoherent light from the first surface of the diffusing device to exit via the first light path, for guiding a remaining portion of the incoherent light from the first surface of the diffusing device to exit via a second light path, and for separating the first and second light paths; wherein a luminous flux of the incoherent light exit via the first light path is less than a luminous flux of the incoherent light exit via the second light path; the light emitting device further comprising a first reflective element disposed on a side of the second surface of the diffusing element; the light guide element includes a second reflective element having an aperture and a reflective surface surrounding the aperture, wherein the aperture guides the coherent light from the coherent light source by transmission onto the first surface of the diffusing element to form the first light path, and guides the portion of the incoherent light from the first surface of the diffusing element to exit via the first light path, and wherein the reflective surface of the second reflective element guide the remaining portion of the incoherent light from the first surface by reflection to exit via the second light path, wherein the second reflective element is an arc shaped reflective element having the aperture and the reflective surface, wherein the aperture is an offset aperture of the arc shaped reflective element, wherein the coherent light from the coherent light source is incident on the diffusing element perpendicularly wherein the second reflective element is saw-tooth shaped reflecting device having the aperture and the reflective surface. a coherent light source for emitting a coherent light; a diffusing element having a first surface and a second surface opposite each other, for diffusing the coherent light from the coherent light source to generate an incoherent light; and a light guide element disposed on a side of the first surface of the diffusing element, for guiding the coherent light emitted by the coherent light source to incident on the first surface of the diffusing element to form a first light path, for guiding a portion of the incoherent light from the first surface of the diffusing device to exit via the first light path, for guiding a remaining portion of the incoherent light from the first surface of the diffusing device to exit via a second light path, and for separating the first and second light paths; wherein a luminous flux of the incoherent light exit via the first light path is less than a luminous flux of the incoherent light exit via the second light path; the light emitting device further comprising a first reflective element disposed on a side of the second surface of the diffusing element; the light guide element includes a second reflective element having an aperture and a reflective surface surrounding the aperture, wherein the aperture guides the coherent light from the coherent light source by transmission onto the first surface of the diffusing element to form the first light path, and guides the portion of the incoherent light from the first surface of the diffusing element to exit via the first light path, and wherein the reflective surface of the second reflective element guide the remaining portion of the incoherent light from the first surface by reflection to exit via the second light path, wherein the second reflective element is an arc shaped reflective element having the aperture and the reflective surface, wherein the aperture is an offset aperture of the arc shaped reflective element, wherein the coherent light from the coherent light source is incident on the diffusing element perpendicularly wherein the second reflective element is saw-tooth shaped reflecting device having the aperture and the reflective surface.