An extensive visual investigation by the artist David Hockney, supported by optical evidence detailed in subsequent technical papers, shows that important artists began using optical devices as aids for creating their work early in the Renaissance, approximately 175 years before the time of Galileo. These discoveries show there has been a continuous use of optics for artistic purposes continuing until today, that started about 1425 with Jan van Eyck and Robert Campin in Flanders, followed by such well-known artists as Bartholomé Bermejo in Spain about 1474, Hans Holbein in England about 1530, and Caravaggio in Italy about 1600. Before the optical evidence in representative Renaissance paintings is described, the state of optical knowledge at the time will be discussed to establish the context for these discoveries.