Editor’s Note on the term ‘Latent Image’ as used in this article: Photographic paper is coated with a light-sensitive emulsion. When it is exposed, a latent image is formed. This image is invisible to the naked eye. When photographic paper is exposed to light for an extended period of time, you are actually able to view an image on the paper. This image that you are able to see is not a latent image – it is an image caused by a chemical reaction of the emulsion to light over an extended period of time. In this article, the image is referred to as a latent image, even though technically, it is not. If you are interested in reading more about the reaction of light sensitive emulsions, books such as Photographic Materials and Processes 3rd Edition (focalpress.com) or Imaging Processes and Materials: Neblette’s (wiley.com) are excellent sources.
For several years I have been interested in taking images that show the passage of the sun through the seasons. The big problem was how to capture it all in one image? I thought of pinhole photography to limit the light on the film, but I needed some sort of high neutral density filter to get normal film into the correct exposure range.
Note: I have to use some sort of film technique here due to noise limitations of digital sensors−besides who wants to tie up a digital camera for a few months for one exposure? I remembered an old article on using the latent image of paper to create an image, but it was very slow−this was exactly what I was looking for. An Internet search revealed that there is a technique to expose paper and use the latent image, but was it real? To test out the latent image technique I cut a 4×5 inch sheet of paper and exposed it in an old Graflex camera at 4.0 for one hour on a sunny day. I then tested out my scanner to see the fastest highest resolution that the scanner could do in one pass. If the scanner has to pause, then the light from the scanner will expose the paper and creates lines in the digital images as the paper becomes exposed. This technique works, but you can only scan the image once, and you had better get the image right on the first try
My first thought about the process was to determine if there is a way to fix the latent image. It turns out that to fix the image the rest of the silver would have to go though the development chemistry and this was not going to work. So this is what it is, a process free of wet chemistry. My paper turns out to have a sensitivity of about ISO 8.