The Kyrgyz Republic, a small, mountainous state of the former Soviet republic, is using invisible ink and ultraviolet readers in the country's elections as part of a drive to prevent multiple voting.
....The ink is sprayed on a person's left thumb. It dries and is not visible under normal light. However, the presence of ultraviolet light (of the kind used to verify money) causes the ink to glow with a neon yellow light.
At the entrance to each polling station, one election official will scan voter's fingers with UV lamp before allowing them to enter, and every voter will have his/her left thumb sprayed with ink before receiving the ballot. If the ink shows under the UV light the voter will not be allowed to enter the polling station. Likewise, any voter who refuses to be inked will not receive the ballot.
....see "Kirghiz Light" in Gravity's Rainbow@ Amazon.com index