Fraunhofer diffraction as an aperture Fourier transform
When the input-aperture quadratic phase in the Fresnel integral is negligible, the far-field amplitude is proportional to the two-dimensional Fourier transform of the aperture field evaluated at spatial frequencies x/(λz), y/(λz).
Key knowledge
When the input-aperture quadratic phase in the Fresnel integral is negligible, the far-field amplitude is proportional to the two-dimensional Fourier transform of the aperture field evaluated at spatial frequencies x/(λz), y/(λz). A suitable thin lens produces this Fourier pattern at its focal plane without a physically distant screen.
Values and equations
E_far(x,y)∝∫∫E₀(ξ,η)e^[−i2π(xξ+yη)/(λz)]dξdη
- E_far,E₀
- Output/input field amplitudes; proportionality includes geometric normalization
- x,y,ξ,η,λ,z,a
- Coordinates, wavelength, distance and aperture scale m
- i
- Imaginary unit
Conditions and scope
- Scalar paraxial coherent illumination, aperture extent a satisfying a²/(λz)≪1 for free-space far field, or a correctly arranged Fourier lens system.
- Retain the output phase and prefactors when reconstructing complex amplitudes; intensity patterns alone discard phase. The aperture field includes both illumination and transmission.
Sources · 1
References
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Evidence and review
Optics 2.71: Diffraction lecture 14 (2014): Pages 1–2: quadratic propagation phase and Fraunhofer limit; pages 4–6: lens Fourier plane and normal-incidence grating equation. Scalar source normalization is supplied by the separate recitation kernel.
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Entry details
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