What is a sign of exhausted developer solution?

A thin faded image on the radiograph is indeed a sign of an exhausted developer solution. The developer solution is responsible for converting the exposed silver halide crystals in the film emulsion into metallic silver, which creates the visible image. As the developer becomes exhausted, its chemical activity diminishes, leading to inadequate development of the film. This results in a film that appears thin and faded because not enough contrast and density have been produced during the development process.

In contrast, a yellow-brown film often indicates insufficient washing or fixing of the film rather than developer exhaustion. A milky coating can result from contamination or improper processing, leading to a hazy appearance. A green film usually refers to issues with the film itself or exposure rather than the action of the developer. Therefore, the observable effect of a thin faded image specifically aligns with the deficiency in the developer solution's ability to process the film properly.