The Airy disk diameter to the first dark ring is given by d = 1.22λ/NA, where λ is wavelength and NA is numerical aperture. The central maximum contains approximately 84% of the total light energy. Two point sources are considered resolved (by the Rayleigh criterion) when the center of one Airy disk falls on the first dark ring of the other.
Diffraction Pattern
Airy Disk
The fundamental unit of optical resolution
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Definition
The diffraction pattern formed when light from a point source passes through a circular aperture. Even a perfect lens cannot focus light to an infinitely small point—instead, it creates this characteristic pattern of concentric rings surrounding a central bright spot.
Technical Details
Simplified
Think of it like dropping a pebble in water—the ripples spread out from the center. Light does the same thing when focused through a lens, creating a central bright spot with fainter rings around it. The size of this pattern determines how fine the detail you can see.
Why It Matters
The Airy disk sets the fundamental limit on optical resolution. No matter how good your camera or how many pixels you have, you cannot resolve details smaller than the Airy disk. Understanding this helps explain why higher NA objectives achieve better resolution.
Practical Example
With a 1.2 NA water immersion objective at 525nm (green light), the Airy disk diameter is approximately 534nm. This means structures separated by less than ~270nm cannot be distinguished as separate objects.