Focus variation
Encyclopedia
Focus variation is a method to calculate a sharp image and to measure the depth with an optics with limited depth of field
.
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.
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.
Disadvantages:
series. The part 6 document describes the available methods for roughness measurement. Focus variation is one of the described methods.
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
.
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 opticsOptics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
with very little depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
. This can be realized if a microscopy
Microscopy
Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...
like optics and a microscope objective is used. These objectives have a high numerical aperture
Numerical aperture
In optics, the numerical aperture 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, NA has the property that it is constant for a beam as it goes from one...
which gives a small depth of field.
Usage
The use of this method is for optical surface metrologySurface metrology
Surface metrology is the measurement of small-scale features on surfaces, and is a branch of metrology. Surface primary form, surface waviness and surface roughness are the parameters most commonly associated with the field...
and coordinate-measuring machine
Coordinate-measuring machine
A coordinate measuring machine is a device for measuring the physical geometrical characteristics of an object. This machine may be manually controlled by an operator or it may be computer controlled. Measurements are defined by a probe attached to the third moving axis of this machine...
. This means measuring form, waviness
Waviness
Waviness is the measure of the more widely spaced component of surface texture. It is a broader view of roughness because it is more strictly defined as "the irregularities whose spacing is greater than the roughness sampling length"...
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 and Disadvantages
Advantages:- can be used on samples with steep flankes. 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 25178ISO 25178
ISO 25178: Geometric Product Specifications – Surface texture: areal is an International Organisation for Standardisation collection of international standards relating to the analysis of 3D areal surface texture.-New features:...
series. The part 6 document describes the available methods for roughness measurement. Focus variation is one of the described methods.
External links
- Roughness
- Surface metrologySurface metrologySurface metrology is the measurement of small-scale features on surfaces, and is a branch of metrology. Surface primary form, surface waviness and surface roughness are the parameters most commonly associated with the field...
- Alicona http://www.alicona.com . Manufacturer implementing focus variation into a product.
- http://www.alicona.com/cms/upload/pdf/magazine/PaperAlicona_NonDestructiveTestingLaibach.pdf FOCUS VARIATION – A NEW TECHNOLOGY FOR HIGH RESOLUTION OPTICAL 3D SURFACE METROLOGY