def afri_16(b8a, b11):
    """
    Aerosol Free Vegetation Index 1.6 \
    (Karnieli, Kaufman, Remer, and Wald, 2001).

    .. math:: AFRI_16 = b8a - 0.66 * (b11 / b8a) + 0.66 * b11

    :param b8a: NIR narrow.
    :type b8a: numpy.ndarray or float
    :param b11: SWIR 1.
    :type b11: numpy.ndarray or float

    :returns AFRI_16: Index value

    .. Tip::

        Karnieli, A., Kaufman, Y. J., Remer, L., Wald, A. 2001. \
        AFRI - aerosol free vegetation index. Remote Sensing of Environment \
        77,10-21. doi:10.1016/S0034-4257(01)00190-0.
    """

    AFRI_16 = b8a - 0.66 * (b11 / b8a) + 0.66 * b11
    return AFRI_16