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

Algorithm

The algorithm work 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 instruments that use or detect it. Optics usually describes the behaviour of visible, ultra ...

with very little depth of field. This can be realized if a

microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot 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 mi ...

like optics and a microscope objective is used. These objectives have a high numerical aperture 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=Burmudez , 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 * can deliver color information * can measure on rough surfaces Disadvantages: * 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

Standardisation

ISO committee is working on a new series of ISO standards that is called the

ISO 25178 ISO 25178: Geometrical Product Specifications (GPS) – Surface texture: areal is an International Organization for Standardization collection of international standards relating to the analysis of 3D areal surface texture. Structure of the stan ...

series. The part 6 document describes the available methods for roughness measurement. Focus variation is one of the described methods.

References