The painting, with photographic film on its paint surface, is put in a vacuum bag and irradiated with penetrating X-rays. The X-rays scarcely affect the kind of film used, and the photographic image is formed by the electrons emitted by the pigments in the paint layer on bombardment by the X-rays. Thus, whereas a conventional X-radiograph penetrates right through the picture, in this technique the image formed concerns only the paint layer, or conceivably something directly underneath. Illustrations show how this technique may clearly reveal areas of repaint.