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Diffraction envelope, or diffraction envelope pattern, is the smooth intensity distribution that bounds a diffraction pattern produced by a finite aperture. In many experiments, especially those involving an array of slits or a grating, the observed intensity pattern is the product of a sharp interference pattern from multiple coherent elements and a broad, smooth envelope arising from the finite width of each element. The envelope is determined by the aperture function; in the Fraunhofer (far-field) regime it is proportional to the squared modulus of the Fourier transform of the aperture.

For a single rectangular slit of width a, the envelope is I(θ) ∝ (sin β / β)^2 with β = π a sin θ / λ. Zeros occur when a sin θ = m λ (m ≠ 0). For N identical slits of width a and spacing d, the intensity is I(θ) ∝ (sin β / β)^2 [sin(N δ / 2) / sin(δ / 2)]^2 with δ = 2π d sin θ / λ. The factor (sin β / β)^2 is the diffraction envelope; the second factor yields the sharp interference maxima.

The concept extends to optical systems and Fourier optics: the envelope reflects the aperture's spatial content and sets the angular extent of detectable energy, thereby influencing resolution and the visibility of higher-order fringes. Understanding the diffraction envelope helps analyze laboratory diffraction patterns, gratings, and telescope optics.