A method and apparatus comprising a housing, an excitation unit, and a detector. The housing is portable and capable of being positioned relative to a location on an object. Quantum dots are present in the location. The excitation unit is mounted to the housing. The excitation unit is capable of sending energy into the location in which the energy is capable of causing a response from the quantum dots. The detector is mounted to the housing. The detector is capable of detecting the response from the quantum dots in the location.