A wavelength device includes a substrate, a photoluminescence layer, a light spot adjusting layer, and a reflecting layer. The photoluminescence layer is disposed over the substrate, and is configured to receive incident light and convert the incident light to excitation light. The light spot adjusting layer is disposed between the substrate and the photoluminescence layer, and is configured to receive the excitation light and the unconverted incident light and to adjust the light path of the excitation light and the unconverted incident light, in which a refractive index of the photoluminescence layer is different from a refractive index of the light spot adjusting layer. The reflecting layer is disposed between the light spot adjusting layer, and is configured to reflect the incident light and the excitation light.