Infrared and ultraviolet photography
The frequencies of infrared waves are less than the frequencies of visible light. It also has a long wavelength. When a painting is being exposed, the infrared radiation is reflected from or absorbed by the base layer after penetrating the paint layers. This allows conservators to find hidden sketches or artist’s signature that cannot be identifiable with eyes. For example, infrared imaging of Oh Ji-ho shows the name “藤四 吳.”
Ultraviolet light has shorter wavelengths than visible light. With high energy, the ultraviolet waves are used to analyze the surface of paintings. When exposed to ultraviolet radiation, some materials used in paintings become fluorescent or absorb the rays, depending on their chemical properties. This allows conservators to find any use of varnish, color retouching, overpainting, and various pollutants. The ultraviolet imaging of Oh Ji-ho shows color retouching and overpainting.
