Measuring light intensity over a percentage of the image

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Does opencv2 have a way of measuring light intensity, of a binary image, over a percentage of the image?

I have a movement detector that is working well until the movement crosses a light source; when it wrongly picks up on the adjustments the auto brightness, within the camera, makes.

I thought i could somehow ignore quick and big light intensity changes that occur over say 50% of the image

I'm looking into cv2.calcHist() and np.histogram() but need some expert guidance on a possible approach.