def luminance_level_adaptation_factor(L_A):
    """
    Returns the *luminance* level adaptation factor :math:`F_L`.

    Parameters
    ----------
    L_A : numeric
        Adapting field *luminance* :math:`L_A` in :math:`cd/m^2`.

    Returns
    -------
    numeric
        *Luminance* level adaptation factor :math:`F_L`

    Examples
    --------
    >>> luminance_level_adaptation_factor(318.31)  # doctest: +ELLIPSIS
    1.1675444...
    """

    k = 1 / (5 * L_A + 1)
    k4 = k ** 4
    F_L = (0.2
           * k4 * (5 * L_A) + 0.1 * (1 - k4) ** 2 * (5 * L_A) ** (1 / 3))
    return F_L