Added intensity correction due to field view angle.
This requires field of view angles (fovx, fovy) calculated from
  fovx, fovy, focalLength, principalPoint, aspectRatio = \
    cv2.calibrationMatrixValues(np.asarray(self.cameraMatrix),
                                tuple(self.imageSize),
                                self.sensorSize[0],
                                self.sensorSize[1])
  where sensor size of Canon D700 is 22.3mm x 14.9mm