import numpy as np
import matplotlib.pyplot as plt

fname = 'dH2si.png'
img = plt.imread(fname)[220:290, 590:660, :3].sum(axis=2)
img = img/img.max()
a, b = np.histogram(img.flatten(), bins=255)
plt.figure()
plt.subplot(1, 2, 1)
plt.imshow(img, cmap='gray', vmin=0, vmax=1)
plt.subplot(1, 2, 2)
plt.plot(b[:-1], a)
plt.ylim(0, 40)
plt.show()