neardiffractionlimited

Near diffraction-limited describes an optical system whose resolving power and image quality come close to the fundamental diffraction limit, but does not necessarily reach it. In such systems the point-spread function is dominated by diffraction with only small residual aberrations.

The diffraction limit is a consequence of light's wave nature. For a circular aperture, the smallest resolvable angular separation is about 1.22 λ/D, and in terms of numerical aperture, the lateral resolution is approximately 0.61 λ/NA for incoherent imaging. This sets a theoretical floor for resolving power independent of detector quality.

Closeness to the limit is often quantified by the Strehl ratio, the peak intensity of the system's point-spread function relative to the ideal. The Marechal criterion states that a Strehl ratio of about 0.8 or higher is diffraction-limited. Near-diffraction-limited performance often corresponds to Strehl ratios in the range roughly 0.7–0.95, depending on application.

In practice, achieving near-diffraction-limited performance requires controlling aberrations, alignment, and environmental factors. Sources of deviation include residual lens or mirror aberrations, misfocus, thermal changes, chromatic effects from finite bandwidth, atmospheric seeing for ground-based systems, vibration, and detector sampling.

Applications include astronomy with adaptive optics to approach diffraction limit under real-world seeing, and high-NA optical microscopy where objectives strive to approximate the diffraction limit for visible light. In both fields, performance is described as near-diffraction-limited when the PSF and resolution are close to ideal, but imperfections prevent exact diffraction-limited imaging.