def xy_to_uv76(x, y):           # CIE1931 to CIE1976
    """ convert CIE1931 xy to CIE1976 u'v' coordinates
    :param x: x value (CIE1931)
    :param y: y value (CIE1931)
    :return: CIE1976 u', v' """
    denominator = ((-2 * x) + (12 * y) + 3)

    if denominator == 0.0:
        u76, v76 = 0.0, 0.0
    else:
        u76 = (4 * x) / denominator
        v76 = (9 * y) / denominator

    return u76, v76     # CIE1976 u', v'