Confocal profilometry is a point scanning optical technique used to image the sample surface. It works by using a small aperture, known as a confocal aperture. Highly focused light is projected through the aperture and only surfaces within the focal plane will provide a useful signal. The optics are scanned up and down above the surface until a bright spot is observed. Once a bright spot is observed the instrument calculates the distance to that point on the surface, giving it a point in Z. The optics then scan along the surface laterally while maintaining a bright spot to reconstruct the surface.
Pattern Projection with Focus Detection
Typically, software used on optical instruments can have difficulties determining what is in focus and what is out of focus. The accuracy of the optics may limit how effectively the focal plane can be resolved.
By using advanced optics to project a sharp pattern that will only be in focus at a precise distance, the software and optics are able to determine what is in the focal plane more easily and with greater accuracy. This also allows for more flexibility in measuring transparent samples as the projection will only be in focus on a real surface.
This combination technique is slightly more complicated than focus detection but it provides more accurate measurements and simpler software detection.
Pattern projection with focus detection is generally faster than the other scanning techniques because only Z motion is required. This method also improves height resolution to be comparable or better than scanning techniques. Pattern projection with focus detection is the essence of the ZETA 3D optical profiler.
Pattern projection profilometry works by projecting a known pattern onto the sample and comparing what is projected to what is reflected by the sample. By comparing these two the surface of the sample can be reconstructed by software.
Focus detection works much like how we see the world. The instrument determines what is in focus and what is out of focus by looking for sharp contrasts. Areas with sharp contrast are deemed in focus, and by knowing this focal length of the optics you can determine the distance. This generally works by scanning the optics in the Z direction and using the camera in order to create a three-dimensional volume of data. After the data is acquired, the parts that are out of focus are cut out and you are left with a representation of the surface of the sample.
This technique can also be performed by using a zoom microscope with a variable focus to create the volume. This technique is generally simpler with less resolution at a lower cost. By using regular microscope objectives and precision Z movements, the X, Y and Z resolution can be greatly enhanced; the resolution is still lower than confocal or interferometry instruments.