def fraunhofer_distance(d=1.0, wavelen=550e-6):
    """computes the Fraunhofer distance from a diffracting aperture. It is the
    limit between the near and far field
    
    Parameters
    ----------
    d : float
        largest dimension of the aperture. i.e. aperture width/diameter (in mm)
    wl : float
        wavelength of light (in mm)
    
    Returns
    -------
    fdist : float
        the Fraunhofer distance in mm
        
    Notes
    -----
    This is the more stringent condition for the Fraunhofer Approximation. 
    Therefore the far-field is the region where ``z > fraunhofer_distance``
    """
    return 2.0*d**2 / wavelen