Lasers emit light with a very limited wavelength so it is focused to a tight spot. This enables it to stay narrow over long distances. Lasers can also have high temporal coherence which allows them to have a very narrow spectrum, i.e., they only emit a single colour of light, compared here to other light souces:

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Lasers have been used to clean the six Caryatid maiden sisters who have supported the Erechtheion at Athens’ Acropolis since 600BC. Lord Elgin had one removed to his mansion in Scotland and now the others have been cleaned by a specially developed laser system. Caryatid_Erechtheion_BM_Sc407   Porch_of_Maidens

They are thought to have been sculpted by Alkamenes, a student of ancient Greece’s greatest artist, Phidias. Their initial function was to prop up a part of the Erechtheion, the sacred temple near the Parthenon that paid homage to the first kings of Athens and the Greek gods Athena and Poseidon.

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To coincide with the museum’s fifth anniversary, the women — minus one — went on full display in June, gleaming from their modern makeover. The other Caryatid is still at the British Museum along with adornments from the Parthenon.

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The dirt is vaporised by a laser beam and the residue blown away without harm to the substrate. The laser’s power density can be adjusted to a safe yet effective level. Limestone walls inside the Glasgow Museum and Art Gallery have also been cleaned by the same technique, removing the grimy blackness to reveal the honey coloured stone. The museum is close to the former Lord Elgin’s home.