Calculate the theoretical minimum spot size of a focused laser beam.
Optical Parameters
nm
mm
mm
Ratio
Focus Results
Enter parameters to calculate focal spot
About this calculator
Quick Reference
dFocused spot diameter
λWavelength
fFocal length
DInput beam diameter
M²Beam quality factor
Overview
The spot size of a focused laser beam is determined by diffraction. Even with perfect lenses, a beam cannot be focused to a infinitely small point, but rather a diffraction-limited spot known as an Airy disk or Gaussian waist.
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Pro Tips
Smaller focal lengths (faster lenses) produce smaller spots.
Larger input beam diameters produce smaller spots (due to lower f/#).
Short wavelengths (UV/Violet) focus to smaller spots than long wavelengths (IR).
The theoretical limit is d = (4 * λ * f * M²) / (π * D).
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Fun Facts
"Wait, did you know that the focus isn't a point, but a cylinder-like 'waist'? This region of high intensity is called the Rayleigh range."
"Blu-ray players use blue-violet lasers (405nm) specifically because the shorter wavelength allows for a smaller focused spot, enabling more data to be packed onto the disc compared to DVDs."
"In laser surgery, the spot size determines the precision and the 'fluence' (energy per unit area) applied to the tissue."
