Question
What is the difference between Absolute Symmetry and Directional Symmetry algorithms in myQA Daily? [Software version 1.3 and newer]
Answer
Summary
The advantage of the Directional Symmetry over the Absolute Symmetry algorithm is, that it is not only showing absolute deviations in symmetry but also is giving information about the direction of the tilts of the profiles.
This is why the Directional Symmetry algorithm is our recommended way to analyze the symmetry of beam profiles.
Prerequisites
In the General Settings of myQA Daily users with Administrator role can choose the desired algorithm for the calculation of the symmetry of profiles:
If the symmetry algorithm has been changed, the reference measurement may no longer be valid and is it recommended to re-measure the baseline.
General Hardware Layout
There are 25 and 57 chambers for the symmetry and flatness measurement for field size, 10 × 10 cm² and 20 × 20 cm², respectively:
Illustration of the chambers used for symmetry and flatness tests (highlighted with a purple box), left: for 10 cm × 10 cm field (25 chambers); right: for 20 cm × 20 cm (57 chambers).
Absolute Symmetry Algorithm (according to IEC 60976:2007)
The minimal possible value is 100% and every deviation from an ideally symmetric beam leads to an increase of this number.
Parameter | Symbol | Formula |
Measurement signal* | M_{i,j} | |
Ratio for crossline symmetry | R_{x,i,j} | = M_{i,j} / M_{-i,j} |
Ratio for inline symmetry | R_{y,i,j} | = M_{i,j} / M_{i,-j} |
Beam crossline absolute symmetry | S_{x} | = 100% × max{R_{x,i,j}} |
Beam inline absolute symmetry | S_{y} | = 100% × max{R_{y,i,j}} |
* Uniformity and kTp corrected measurement signals of the chambers. i and j denote the chamber position in x and y direction, with i = j = 0 at the center position.
Directional Symmetry Algorithm
The result of this algorithm also gives information about tilts of the profiles. A tilted beam corresponds to values above or below 100%, depending on the direction of the tilt.
Parameter | Symbol | Formula |
Measurement signal* | M_{i,j} | |
Ratio for crossline symmetry | R_{x,i,j} | = M_{i,j} / M_{-i,j} |
Ratio for inline symmetry | R_{y,i,j} | = M_{i,j} / M_{i,-j} |
Ratio for crossline symmetry with largest diff. to one | R_{x,max} | = max{|1-R_{x,i,j}|} |
Ratio for inline symmetry with largest diff. to one | R_{y,max} | = max{|1-R_{y,i,j}|} |
Beam crossline directional symmetry | DS_{x} | = 100% × R_{x,max} |
Beam inline directional symmetry | DS_{y} | = 100% × R_{y,max} |
* Uniformity and kTp corrected measurement signals of the chambers. i and j denote the chamber position in x and y direction, with i = j = 0 at the center position.