Focus variation is a method used to sharpen images and to measure surface irregularities by means of optics with limited

depth of field The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus (optics), focus in an image captured with a camera. See also the closely related depth of focus. Factors affecting depth ...

Algorithm

The algorithm works as follows: # At first images with difference focus are captured. This is done by moving the sample or the optics in relation to each other. # Then for each position the focus over each plane is calculated # The plane with the best focus is used to get a sharp image. the corresponding depth gives the depth at this position-

Optics

Focus variation requires an

optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

with very little

. This can be realized if a

microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical mic ...

like optics and a microscope objective is used. These objectives have a high

numerical aperture In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, has the property ...

which gives a small depth of field.

Usage

The use of this method is for optical surface metrology and coordinate-measuring machine. This means measuring form, waviness and roughness on samples.{{cite web , last1=Bermudez , first1=Carlos , title=Active illumination focus variation , url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11056/2525981/Active-illumination-focus-variation/10.1117/12.2525981.short?SSO=1 , accessdate=22 October 2020 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 and disadvantages

Advantages: * Can be used on samples with steep flanks. This is because a ring light can be used to extend the illumination aperture * Very vibration insensitive * Can deliver color information * Can measure on rough and smooth surfaces Disadvantages: * Normal focus variation can not be used if the surface of the sample does not give structure in the image. This means it can not be used for wafers and glass. Focus variation according to ISO 25178-606 can also measure smooth surfaces.

Standardisation

The

ISO The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. Me ...

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

References

Optical metrology Metrology