Thickness of a plastic sheet / glass plate

a) To find the thickness of a plastic sheet/glass plate, a simple approach is to use the triangulation technique. For this, you need to have a light source and a detector positioned like as shown in the figure.

b) In the previous approach (a), you always need to focus light on a particular point of the plastic sheet with the reference point of the surface to get the correct thickness. Here in this approach, you can focus light anywhere on the plastic sheet without a reference point.