Final answer:
Latent image formation occurs during the exposure phase of film processing, which starts the chemical change in the film's emulsion, leading to the development of a visible image later on.
Step-by-step explanation:
The latent image formation occurs during the exposure phase of film processing. When a film is exposed to light or radiation (such as radioactive DNA in autoradiographs), the energy of the exposure starts a chemical change in the film's emulsion.
The image, however, is not visible at this stage and is called a 'latent image.' It is during the development phase that the latent image becomes visible after being treated with various chemical solutions. Nonetheless, the invisible change produced during the exposure phase is crucial, as it is the blueprint for the subsequent visible image after development.