Red Light, Green Light (2 of 2) (IMAGE) Optica Caption The opal-based alcohol sensor is green before use but turns redder in the presence of alcohol vapor. Credit Riccardo Pernice, Università degli Studi di Palermo Usage Restrictions Must include caption & credit. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.