def sri(b4, b8):
    """
    Simple Ratio Index (Jordan, 1969).

    .. math:: SRI = b8/b4

    :param b4: Red.
    :type b4: numpy.ndarray or float
    :param b8: NIR.
    :type b8: numpy.ndarray or float

    :returns SRI: Index value

    .. Tip::

       Jordan, C.F. 1969. Derivation of leaf-area index from quality \
       of light on the forest floor. Ecology 50, 663-666. \
       doi:10.2307/1936256.
    """

    SRI = b8/b4
    return SRI