The descriptor used to express the rate of density change on the H&D curve is which?

• A. Average gradient
• B. Subject contrast
• C. Gradient value
• D. Open-mouth LUT

Answer: (C)

The rate at which density changes with exposure on an H&D curve is captured by the slope of the curve, i.e., its gradient. Since the H&D curve plots optical density against exposure (often log exposure), the amount density changes for each unit of exposure is the gradient. This single number—the gradient value—describes how quickly density responds to changes in exposure, which directly relates to image contrast: a steeper gradient means greater density change for a small exposure change (higher contrast), while a gentler gradient means less change (lower contrast). The other terms don’t describe this rate: subject contrast refers to the perceptual difference between tissues, and average gradient is an overall slope over a region, but the standard descriptor for the rate of density change is the gradient value.