Final answer:
Line pairs per millimeter (lp/mm) is a unit used to express resolution, which is relevant to physics and refers to the clarity of an image, while line-spread function relates to how a line is spread and captured but is not a direct unit of measurement.
Step-by-step explanation:
The units used to express resolution are critical for understanding how clear or detailed an image can be. Resolution in the context of physics, especially when discussing optical instruments like microscopes or cameras, refers to the ability to distinguish between two closely spaced objects.
One unit used to express resolution is line pairs per millimeter (lp/mm), which measures how many distinct pairs of lines (one white and one black) can be seen within a one millimeter space. Another concept related to resolution is the line-spread function, which describes how a single line is spread out and captured by the imaging system, although it's not a direct unit of measure like lp/mm.