'How to calculate the area of a shape in an image with Python OpenCV?

I would like to calculate the area of a shape in an image produced from infra red cameras.

I have a large set of matrices which i produce from an infra- red camera. In each matrix/image i have mostly background with an image of a spot, which is a source of infra red radiation. The way i proceeded with it is to use Python OpenCV in order to isolate the image of the source by nullifying the background and calculating the number of pixels in the shape. The problem is that in each image, parts of the image is also turned into background and so i can't get the full image, as i would like.

import cv2
import numpy as np
from matplotlib import pyplot as plt

PPmm = 81/55 #Pixel per mm


img = np.genfromtxt('Image 5 Z_plane = 141.0_contour_plot.csv', delimiter= ',')

img_cv = cv2.resize(img,(81,81))
np.savetxt('testing.csv', img_cv, delimiter= ',')

img = (img_cv*255).astype(np.uint8)









edges = cv2.Canny(img,150,250)

se = np.ones((7,7), dtype='uint8')





# Perform morphology

image_close = cv2.morphologyEx(edges, cv2.MORPH_CLOSE, se)

# Your code now applied to the closed image
cnt = cv2.findContours(image_close, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)[0]
mask = np.zeros(img.shape[:2], np.uint8)
cv2.drawContours(mask, cnt, -1, 255, -1)





non_zero_pixel_count = (np.count_nonzero(mask))

Area_in_mm = non_zero_pixel_count*(1/PPmm)**2
print("Area of shape = {0:1f}mm^2".format(Area_in_mm))






plt.subplot(121)
plt.imshow(img,cmap = 'gray')
plt.title('Original Image')
plt.xticks([])
plt.yticks([])
plt.subplot(122)
plt.imshow(mask,cmap = 'gray')
plt.title('Edge Image'), plt.xticks([]), plt.yticks([])
[enter image description here][1]
plt.show()

the shape area is: 58.093278mm^2. If i calculate manually, approximatly i will get 68mm^2. In circular images this is even worse and I get an area twice as small

Circular image