What is the input‑output curve used to express the change in density with dose for radiographic film, also referred to as an H&D curve?

• A. Characteristic curve
• B. Density curve
• C. Exposure curve
• D. Optical density curve

Answer: (A)

The input-output curve for radiographic film is the characteristic curve, also known as the Hurter-Driffield (H&D) curve. It shows how the film’s density (darkness) changes with exposure to X-rays, plotting optical density on the vertical axis versus exposure (usually the log of exposure) on the horizontal axis. This curve typically has a toe, a straight-line portion, and a shoulder. The straight-line region is where density increases roughly linearly with log exposure, and the slope in that region (gamma) relates to image contrast. The position of the curve and the width of the straight-line portion describe film speed and exposure latitude, while processing can shift or bend the curve.

This terminology is standard because it captures the input-output relationship of the film-screen system, reflecting how the material responds to dose. Other terms like density curve, exposure curve, or optical density curve aren’t the conventional way to name this relationship.