A weight determining mechanism and a method for determining the weight of a piece of loaded luggage, particularly a backpack. The light guide plate has a lower surface, a light output surface opposite to the lower surface, and at least one light input side surface connected between the lower surface and the light output surface. The piston assembly is in contact with the piston tube. Since the linear motors drive the blades in the direction corresponding to the synchronous movement of the reticle and the wafer, the linear motors do not cause vibration when the blades move at a constant speed. The cover is made from a material which restricts ultraviolet light from contacting the fingernail, thus preventing yellowing when the user is tanning.