def nbr(b8, b12):
    """
    Normalized Burn Ratio \
    (García and Caselles, 1991. Named by Key and Benson, 1999).

    .. math:: NBR = (b8 - b12) / (b8 + b12)

    :param b8: NIR.
    :type b8: numpy.ndarray or float
    :param b12: SWIR 2.
    :type b12: numpy.ndarray or float

    :returns NBR: Index value

    .. Tip::

        García, M. J. L., Caselles, V. 1991. Mapping burns and natural \
        reforestation using thematic Mapper data. Geocarto International \
        6(1), 31-37. doi:10.1080/10106049109354290.

        Key, C.H., Benson, N., 1999. The Normalized Burn Ratio (NBR): A \
        Landsat TM Radiometric Measure of Burn Severity. United States \
        Geological Survey, Northern Rocky Mountain Science Center.
    """

    NBR = (b8 - b12) / (b8 + b12)
    return NBR