The Sparrow criterion is a threshold for optical resolution that states two point sources are just resolved when the second derivative of the combined illuminance profile is zero at the center point. Unlike the more conservative Rayleigh limit, which requires a visible dip in intensity between the peaks, the Sparrow limit marks the exact moment the profile becomes flat-topped. It is often used in astronomy and microscopy to define the absolute theoretical limit of resolving closely spaced objects.
The two emission lines are too close for the Rayleigh limit, but we can just barely distinguish them if we push to the Sparrow criterion.
Do not mix it up with
- Rayleigh's resolution limit: Rayleigh requires a 26 percent intensity dip between peaks, while Sparrow only requires a flat top, making Sparrow a tighter limit.
Related terms
Sparrow's resolution limit
The theoretical minimum angular separation required to resolve two point sources.
Rayleigh's resolution limit
The minimum separation required between two point sources for an optical system to distinguish them as separate.
Airy disk
The bright central spot of a diffraction pattern produced by a circular aperture.