def nrmse(ref_im, test_ims):
    """
    Euclidean normalised mean square error.
    
    Parameters
    ----------
    ref_im : 2-D array
        Reference image.
    test_ims : ndarray
        Images to compare.
    
    """
    im1 = ref_im.astype(float)
    im2 = test_ims.astype(float)
    n = (((im1 - im2) ** 2).mean((-2, -1)) / ((im1 ** 2 + im2 ** 2).mean((-
        2, -1)) / 2)) ** 0.5
    return n
