Heliography


Heliography from helios , meaning "sun", and graphein is the photographic process invented by Joseph Nicéphore Niépce around 1822, which he used to make the earliest known surviving photograph from nature, View from the Window at Le Gras, and the first realisation of photoresist as means to reproduce artworks through inventions of photolithography and photogravure.

Invention

Niépce prepared a synopsis of his experiments in November 1829: On Heliography, or a method of automatically fixing by the action of light the image formed in the camera obscura which outlines his intention to use his “Heliographic” method of photogravure or photolithography as a means of making lithographic, intaglio or relief master plates for multiple printed reproductions.
He knew that the acid-resistant Bitumen of Judea used in etching hardened with exposure to light. In experiments he coated it on plates of glass, zinc, copper and silver-surfaced copper, pewter and lithographic stone, and found it resisted dissolution in oil of lavender and petroleum, so that the uncoated shadow areas could be traditionally treated through acid etching and aquatint to print black ink.
The exposed and solvent-treated plate itself, as in the case of View from the Window at Le Gras, presents a negative or positive image dependent upon ambient reflection, not unlike the daguerreotype which was based on Niépce's discoveries.

Chemistry

Bitumen has a complex and varied structure of polycyclic aromatic hydrocarbons, containing a small proportion of nitrogen and sulphur; its hardening in proportion to its exposure to light is understood to be due to further cross-linking of the rings, as is the hardening of tree resins by light, first noted by Jean Senebier in 1782. The photochemistry of these processes, which has been studied by Jean-Louis Marignier of Université Paris-Sud since the 1990s, is still to be fully understood.

Alternative meanings

The word has also been used to refer to other phenomena: for description of the sun, for photography in general, for signalling by heliograph, and for photography of the sun.
The abbreviations héliog. or héliogr., found on old reproductions, may stand for the French word héliogravure, and can then refer to any form of photogravure.