def convert_diameter_sigma_to_fwhm(diameter):
    """
    Converts a beam diameter expressed as the full with at half maximum (FWHM)
    to a beam diameter expressed as 2-sigma of a Gaussian distribution
    (radius = sigma).

    :arg diameter: 2-sigma diameter diameter.
    """
    # d_{FWHM} = 1.177411 (2\sigma)
    return diameter * 1.177411