latent image

latent image, in photography, the invisible effect produced by light on a photosensitive surface, such as film or a photographic plate. It forms when light interacts with the silver halide crystals in the emulsion on a piece of film. Though not visible, the latent image is distinguishable from unexposed silver halide by its ability to be reduced to metallic silver by a developing agent. This process is unique to analog photography; digital cameras use sensors to capture and convert light into data.

History and formation

The term latent refers to the invisibility of the image before development, although this invisibility is relative. After prolonged exposure, faint traces of the brightest parts of the image may sometimes be seen. This effect is characteristic of silver bromide and silver iodide, while silver chloride darkens quickly upon exposure to light. In 1727 German professor of anatomy Johann Heinrich Schulze demonstrated that the darkening of silver compounds was because of light and not heat, helping to lay the foundation for the invention of photography.

The formation of the latent image begins when light strikes the silver halide crystals dispersed throughout the film’s gelatin emulsion. Each crystal contains mobile silver ions. Exposure begins when an incident photon is absorbed by a crystal, creating an electron-hole pair. The electron diffuses through the lattice until it is captured by a surface defect, or electron trap. If the electron remains trapped and draws in an interstitial silver ion, the pair forms a neutral silver atom. Additional photoelectrons and silver ions may join the same site; once two or more silver atoms are present, the cluster becomes stable and continues to grow. These submicroscopic aggregates of silver constitute the latent image: They remain invisible in the exposed film, but during chemical development, they catalyze the wholesale reduction of the host grain to metallic silver, producing the dark specks that compose the visible photograph.