Which unit is usually used to measure spatial resolution?

• A. Pixels per inch (PPI)
• B. Bits per pixel
• C. Frames per second
• D. Line pairs per millimeter (lp/mm)

Answer: (D)

Spatial resolution is about how finely a system can distinguish adjacent details in an image. The unit line pairs per millimeter measures exactly that by counting how many alternating dark and light line pairs can be resolved within one millimeter. A line pair is one dark line plus one light line, so more line pairs in the same length mean finer detail can be distinguished. This is why lp/mm is the standard measure in optics and imaging for resolving power.

Other units describe different aspects: pixels per inch is about how densely a digital image or display samples or shows pixels, not the optical ability to resolve fine features; bits per pixel relates to color or brightness depth, not spatial detail; frames per second concerns temporal changes, not how small a spatial detail is.