What is line pair per mm?

Line Pair per mm (lp/mm)

Line Pair per mm (lp/mm) is a unit of measurement used to describe the resolution of an optical imaging system, such as a camera lens, microscope, or telescope. It quantifies the ability of the system to distinguish fine details in the object being imaged.

One line pair consists of a dark line and an adjacent light line. The term 'per mm' indicates the number of such line pairs that can be resolved per millimeter across the image plane. A higher value of lp/mm signifies a higher resolving power, meaning the system can distinguish finer details.

Importance in Optical Engineering

In optical engineering, lp/mm is crucial for evaluating and designing imaging systems. It helps in determining the system's performance in terms of spatial resolution. High-resolution systems are essential for applications requiring detailed imaging, such as satellite imaging, microscopy, and high-quality photography.


Resolution in lp/mm is typically measured using a resolution test chart, which contains sets of line pairs with varying spacing. The highest number of line pairs per millimeter that can be distinctly resolved by the system indicates its resolution.

Factors Affecting Resolution

  • Lens quality: Aberrations and imperfections in the lens can reduce resolution.
  • Aperture size: Smaller apertures increase depth of field but can decrease resolution due to diffraction.
  • Sensor size and quality: Larger and higher-quality sensors generally capture more detail.
  • Atmospheric conditions: For telescopes and outdoor photography, atmospheric turbulence can reduce resolution.

Understanding and optimizing for lp/mm is essential in the design and selection of optical systems, ensuring they meet the specific needs of their intended application.

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