The invention relates to a mouthpiece (100) for illumination of teeth, e.g. for teeth whitening. The mouthpiece comprises a slab shaped light guide (203), a light source arranged for injecting light into the light guide, and an optically transparent element (101) arranged to contact the light guide. The optically transparent element has a lower refractive index than the light guide to enable total internal refection of the injected light. Preferably, light is coupled out of the light guide by an out-coupling structure (210) and through a part of the optically transparent element located between the light guide and the buccal side of the teeth to illuminated the teeth with uniform light.
