A hand-held, ergonomic instrument with an LED that emits actinic light in visible wavelengths mounts a replaceable applicator cup with a reservoir for a tooth whitening composition held in place against a user's tooth during a whitening procedure. Actinic light is directed onto the tooth surface through a window in the cup and the tooth whitening composition substantially without attenuation, delivering at least about 100 milliwatts per square centimeter of light energy to the tooth surface. The cup includes a rigid body for mounting the cup to the instrument and an opaque flexible sleeve molded in place over the outside of the body to form a seal with the tooth surface and prevent exposure of soft mouth surfaces to the actinic radiation during a whitening procedure. A safety switch disables the LED when the applicator cup is not in contact with a tooth surface.