def cigreen(b3, b8):
    """
    Chlorophyll Index Green (Gitelson et al., 2003a).

    .. math:: CIGREEN = (b8/b3) - 1

    :param b3: Green.
    :type b3: numpy.ndarray or float
    :param b8: NIR.
    :type b8: numpy.ndarray or float

    :returns CIGREEN: Index value

    .. Tip::

        Gitelson, A. A., Gritz, Y., Merzlyak, M. N. 2003a. Relationships \
        between leaf chlorophyll content and spectral reflectance and \
        algorithms for non-destructive chlorophyll assessment in higher plant \
        leaves. Journal of Plant Physiology 160(3), 271-282. \
        doi:10.1078/0176-1617-00887.
    """

    CIGREEN = (b8/b3) - 1
    return CIGREEN