def datt1(b4, b5, b8):
    """
    Vegetation Index proposed by Datt 1 (Datt, 1999a).

    .. math:: DATT1 = (b8 - b5) / (b8 - b4)

    :param b4: Red.
    :type b4: numpy.ndarray or float
    :param b5: Red-edge 1.
    :type b5: numpy.ndarray or float
    :param b8: NIR.
    :type b8: numpy.ndarray or float

    :returns DATT1: Index value

    .. Tip::
        Datt, B. 1999a. Remote sensing of water content in Eucalyptus leaves. \
        Australian Journal of Botany 47, 909-923. doi:10.1071/BT98042.
    """

    DATT1 = (b8 - b5) / (b8 - b4)
    return DATT1