Numerical aperture (NA)

Numerical aperture is the parameter that defines an optical system’s ability to collect light and resolve fine surface details. It characterizes the range of angles over which light can enter or exit the optical system and directly influences the light-collection capability of an objective lens.

The numerical aperture (NA) is defined as:
NA = n sin(θ)
where n is the refractive index of the medium between the lens and the sample, and θ is half the angular aperture of the objective. This formula shows that both the optical medium and the objective geometry directly influence the optical system’s resolving power and light-collection capability.

  Resolution: Higher NA improves resolving power, enabling the detection of smaller, more closely spaced submicron or nanoscale features.

  Brightness: High-NA objectives collect more light, improving image brightness and contrast, particularly at high magnifications.

  Depth of field: Increasing NA reduces the depth of field, meaning fewer layers of a sample remain in focus simultaneously.

  Working distance: Objectives with high NA often have shorter working distances, which may limit their use with large samples.

In metrology, selecting the right NA is critical for balancing resolution, field of view, and measurement efficiency in applications such as surface roughness analysis or defect inspection.