Focus variation

Focus variation is a method to calculate a sharp image and to measure the depth with an optics with limited depth of field.

The algorithm work as follows:

Focus variation requires an optics with very little depth of field. This can be realized if a microscopy like optics and a microscope objective is used. These objectives have a high numerical aperture which gives a small depth of field.

The use of this method is for optical surface metrology and coordinate-measuring machine. This means measuring form, waviness and roughness on samples. With optimized hardware and software components a lateral resolution of 500 nm (limitation of wavelength of light) and a vertical resolution of several nm can be reached.

Advantages:

Disadvantages:

ISO committee is working on a new series of ISO standards that is called the ISO 25178 series. The part 6 document describes the available methods for roughness measurement. Focus variation is one of the described methods.

...
Wikipedia