Photography is understood as an optical-technical process that enables copying and keeping pictures permanently. The word "photography" comes from the Greek and means "to write light". The chemical foundation for photography, photochemistry, is based on the discovery of the light sensitivity of halogen silver salts. The physical and chemical effects of light are used on certain substances to create photographs. The idea of photography itself meanwhile can be traced back to the 15th century. The experiments of Brunelleschi, da Vinci and Dürer with a hole camera and those of Athanasius Kirchner with a projector are well known, and these created the technical basis for photography. The basic idea for photography as we know it today came from the siblings Claude and Joseph Niépce. Joseph Niépce created the first photograph (with an exposure time of 6 hours) that is still preserved today: a picture of his yard. However, M. Daguerre is considered the actual inventor of photography; his exposure time took only a few minutes and thus attracted more imitators and more interest among the public. The discovery of photography had in terms of art history immediate ramifications for impressionism, but also decades later for futurism and hyperrealism as well. Photography itself also became an art medium, for example in Bauhaus. Art photography however was for a long time not considered as such. Henri Cartier-Bresson, himself a photographer and a painter, saw photography not as a form of art but rather as a craft: "Photography is a handcraft. Many want to make art out of it, but we are simply craftsmen who must do their job well." At the same time, it was precisely Cartier-Bressons photographs that were displayed in museums and art exhibits very early on, for example in the MoMA retrospective (1947) and the Louvre exhibit (1955). Photography is now considered an artistic genre on equal footing with the others.