def XYZ_to_uv76(X, Y, Z):
    """ convert XYZ to CIE1976 u'v' coordinates
    :param X: X value
    :param Y: Y value (luminance)
    :param Z: Z value
    :return: u', v' in CIE1976 """
    denominator = (X + (15 * Y) + (3 * Z))
    
    if denominator == 0.0:
        u76, v76 = 0.0, 0.0
    else:
        u76 = (4 * X) / denominator
        v76 = (9 * Y) / denominator
    
    return u76, v76     # u', v' in CIE1976