You also need to account for the difference in resolution along both axis. For your y-axis you have 640 data points for 6 micrometers and for the x-axis 1024 data points for 35 micrometers. Matplotlib assumes both to be equal.
data = np.random.rand(640,1024)
fig, axs = plt.subplots(2,1, figsize=(10, 4))
aspect = 6 / 35
axs[0].set_title('aspect: %1.2f' % aspect)
axs[0].imshow(data, aspect=aspect, interpolation='none')
aspect = (6/35.) * (1024 / 640)
axs[1].set_title('aspect: %1.2f' % aspect)
axs[1].imshow(data, aspect=aspect, interpolation='none')