Reducing the distance z further causes the region of the glass bottle getting inside the two ROIs on the two sides of the assigned blue rectangle. As a result, these two ROIs collect unwanted image data, leading to the incorrectly calculated CR and its corresponding chlorine concentration. On the other hand, moving the smart mobile phone further away reduces the size of the region of the glass bottle and some small parts of the reference scene can get into the region of the central ROI. In this case, the image data associated with the water and the unwanted image data from the reference scene are mixed together. Again, this result leads to the incorrectly calculated CR and its corresponding chlorine concentration.