Rehn first appears in Philadelphia city directories in 1845 as a painter, but in 1849 he turned to the profession of photographer and traveled to Boston to become the partner of the daguerreotypist James A. Cutting. With Cutting, Rehn became part owner of patents for a variety of photographic processes, including one for a wet collodion negative process that became known as the ambrotype. Returning to Philadelphia in 1853, he travled for several years to promote this invention.
With the declining popularity of the ambrotype, Rehn continued his technical experiments in photographic processes. He became involved in photolithography and the production of paper prints ranging in size from the very small stereoscopic printe to lifesize enlargements made with a solar camera. His unique photogram of a fern suggests British photography pioneer William Henry Fox Talbot's first experiments in the mid-1830's. Like Rehn's later trials, Talbot had placed plant leaves directly onto sensitized paper, which slowly turned dark when exposed to sunlight, leaving the paper beneath the leaves white.
Merry A. Foresta American Photographs: The First Century (Washington, D.C.: National Museum of American Art with the Smithsonian Institution Press, 1996)