def ngrdi(b3, b5):
    """
    Normalized Green Red Difference Index (Zarco-Tejada et al., 2001).

    .. math:: NGRDI = (b3 - b5)/(b3 + b5)

    :param b3: Green.
    :type b3: numpy.ndarray or float
    :param b5: Red-edge 1.
    :type b5: numpy.ndarray or float

    :returns NGRDI: Index value

    .. Tip::

       Zarco-Tejada, P. J., Miller, J. R., Noland, T. L., Mohammed, \
       G. H., Sampson, P. H. 2001. Scaling-up and model inversion methods \
       with narrowband optical indices for chlorophyll content estimation in \
       closed forest canopies with hyperspectral data. IEEE Transactions on \
       Geoscience and Remote Sensing 39, 1491-1507. doi:10.1109/36.934080.
    """

    NGRDI = (b3 - b5)/(b3 + b5)
    return NGRDI