A mouthpiece that adjusts manually to accommodate a broad range of different size sets of upper and lower teeth in the mouth and yet seals the treatment area from oxygen exposure. The mouthpiece includes light emitting diodes and heat generating resistors all arranged in an array. A series of parallel texture bands are provides to guide and direct the light from the LEDs to diffuse generally evenly onto teeth to be treated. The seal arises from an inner surface of the mouthpiece titling inwardly so that a seal bead seals in the vicinity of the gum above the teeth to be treated.