The instruments used in dual beam microscopy are primarily used to measure surface roughness. It can also be used to measure the thickness of transparent and translucent overlays, especially anodized films on aluminum.
The instrument works by illuminating a beam at a 45° angle of incidence onto the surface of the overlay, and part of the beam is reflected back from the surface of the overlay. Another part penetrates the cover and reflects back from the cover-substrate interface. Two separate images can be seen from the microscope eyepiece, the distance being proportional to the thickness of the overlay, and the distance can be measured by adjusting the scale control knob.
This method can only be used when enough light is reflected back at the coating-substrate interface to obtain a clear image in the microscope.
For transparent or translucent overlays, such as anodized films, the method is nondestructive. In order to measure the thickness of the opaque cover layer, a small piece of the cover layer must be removed, so that a step that can refract the light beam is formed between the surface of the cover layer and the substrate, so the absolute value of the thickness of the cover layer can be measured. In this case, the method is a destructive testing method.
The measurement error of two-beam microscopy is usually less than 10%.
