Upper limit of sharpness score

Question

I am calculating sharpness scores of images (from same fov) using this algorithm:

def calc_sharpness_score(image): 
image=gaussian_filter(image,sigma=3)
kernel_size = 7 
scale = 1 
delta = 0 
lplImage = cv2.Laplacian(image, cv2.CV_64F, ksize=kernel_size, scale=scale, delta=delta) 
(mean, std) = cv2.meanStdDev(lplImage) 
return std[0][0]**2 

So, how can I calculate the upper limit of this score? Or is there any upper limit for it?