William H. Mumler in the context of "Spirit photography"

Spirit photography (also called ghost photography) is a type of photography whose primary goal is to capture images of ghosts and other spiritual entities, especially in ghost hunting. It dates back to the late 19th century. The end of the American Civil War and the mid-19th Century Spiritualism movement contributed greatly to the popularity of spirit photography. The omnipresence of death in the Victorian period created a desire for evidence of the afterlife, and those who partook in spirit photography oftentimes hoped to receive images that depicted the likeness of a deceased relative or loved one. Photographers such as William Mumler and William Hope ran thriving businesses taking photos of people with their supposed dead relatives. Both were accused of fraud, but "true believers", such as Sir Arthur Conan Doyle, refused to accept the evidence as proof of a hoax. Though taken to court, Mumler was eventually acquitted of all charges. They were never able to prove his methods.

As cameras became available to the general public, ghost photographs became common due to natural camera artifacts such as flash reflecting off dust particles, a camera strap or hair close to the lens, lens flare, pareidolia, or in modern times, deceptions using smart phone applications that add ghost images to existing photographs.