In this equation Φ Φ is the Full Width at Half Maximum of the beam, δ δ is the geometrical projection of the beam and σ σ A is the Airy diffraction term. θ θ is the Heaviside step function used here to indicate that the presence of the diffraction term depends on the value of the Fresnel number. Note that there are variations of this equation depending on what is defined as the "beam width" (for details compare and ). Due to the small wavelength of the helium beam, the Fraunhofer diffraction term can usually be omitted.