Tracked particle trajectories superimposed on an image of the experiment. White lines indicate particles in or just above the focal plane, which appear brighter in the image compared to the background. Dark lines indicate particles below the focal plane, which appear darker. The tracks appear and disappear as particles go in and out of focus. On the left is an optical trap surrounded by a optical shield, on the right is an unshielded trap. The tracks of the freely diffusing particles can be seen to enter the right hand, unshielded trap, whereas the optical shield around the left hand trap prevents such an event. (b) and (c) are images of the sample at the time of the measurements with and without a shield respectively.