def srnirnarrowre1(b5, b8a):
    """
    Simple NIR and Red-edge 1 Ratio (Datt, 1999b).

    .. math:: SRNIRnarrowRE1 = b8a/b5

    :param b5: Red-edge 1.
    :type b5: numpy.ndarray or float
    :param b8a: NIR narrow.
    :type b8a: numpy.ndarray or float

    :returns SRNIRnarrowRE1: Index value

    .. Tip::

       Datt, B. 1999b. Visible/near infrared reflectance and chlorophyll \
       content in Eucalyptus leaves. International Journal of Remote Sensing \
       20, 2741-2759. doi:10.1080/014311699211778.
    """

    SRNIRnarrowRE1 = b8a/b5
    return SRNIRnarrowRE1