A laser confocal microscope has the ability to isolate a single plane of focus from thick samples or overlapping objects. Therefore it is a powerful tool to image the surface or bulk (in transparent media) features of a specimen.
To study the surface features, e.g. roughness is quite straightforward, although resolution selection, e.g. suitable N.A., is the key to obtain meaningful results. This post is about the measurement of sub-surface features in a transparent specimen.
The above figure shows a schematic view of the light path. The important part is the confocal pinhole which allows in-focus light pass through to the detector while blocking majority of out-of-focus light, as shown in the following figure. Therefore it has high z direction resolution and obtain a sharp 3-D image.
Both Keyance VK-X series and Carl Zeiss LSM 700 can be used to measure the sub-surface features (the following picture is from Keyence company).
The VK-X measures thickness by analyzing the light reflected from two locations of a transparent object—the top surface and the bottom surface. The topmost and bottommost layers of the film are converted to a 3D image, and the thickness is measured from the image’s cross-sectional shape.
The LSM 700 from Carl Zeiss can generate information about sub-surface structures in multilayer systems and semi-transparent materials. Inhomogenities, cavities, inclusions and interfaces can all be analyzed. If the refractive index of the medium is known, layer thickness and optical path lengths can be measured (reference).