lp/mm
line pairs per millimeter
Wavenumberexact by definition
Line pairs per millimetre is a spatial frequency of 10³ m⁻¹, and it is the standard measure of optical resolution. A lens's modulation transfer function is plotted against it, film and sensor resolving power is quoted in it, and a resolution target is a set of bar patterns at known lp/mm.
Watch out: A line pair is one dark bar plus one light bar, so a figure in lines per millimetre is twice the same figure in line pairs per millimetre. Sensor specifications sometimes quote the Nyquist limit in lp/mm, which is half the pixels per millimetre, and comparing a lens MTF figure against a raw pixel count without that factor of two overstates the system by a factor of two.
| 1 lp/mm | 1,000 m⁻¹ |