The earliest measuring instruments used to evaluate wet and dry coatings were the human eye and hand. Today's instruments do more, do it faster and do it better, but terms such as thumb tack, touch dry and rubout serve to remind us of the old approach. Today we take it for granted that a thickness measuring instrument will store hundreds (or thousands) of data points readily, thus saving us valuable time which can be used later in trying to make sense of statistical analyses of the data. That's no longer news - so what is? Company names are provided in this review to assist readers in identifying whether a particular technology may be of interest. This does not imply (unless it is specifically indicated) that the named companies are the only providers of any of the techniques or instruments described.
High resolution ultrasonic thickness gauges have been described as a form of 'acoustic microscope'. The technology uses a drop of liquid to provide acoustic coupling between the measuring head and the coating surface.