LBP2 User Guide
Document No: 50306-001
Rev G 3/12/2020
Page 80
adjusted by the orientation angle. The default clip level is 13.5% which will yield
an accurate second moment beam width for a TEM
00
beam.
ISO 11146-3 section 10.4.3 describes this method. It is assumed that M²=1 and
thus the summed data is from a perfect TEM
00
Gaussian distribution. As a result,
the moving slit method will return an accurate second moment beam width for a
TEM
00
beam. For higher mode mixes, the accuracy is, at best, a second moment
approximation. In many cases, this is not very accurate.
5.9.6
Encircled Power Methods
Depss(X/M) 95.4, Depss(Y/m) 95.4
The epss method employs a symmetrically adjustable slit centered at the beam
centroid, and sized in both the X/Y or M/m axial directions. The widths are found
that contains 95.4% of the beam power/energy. The separation distances
between the slit edges are the reported beam widths.
The above fixed percentages will return an accurate second moment beam width
result for a TEM
00
beam. For higher mode mixes, the accuracy is, at best, a
second moment approximation. In many cases, this is not very accurate.
5.10
Rotated Beams
Orientation, Ellipticity, Eccentricity
LBP2 can compute and display the Orientation, Ellipticity and Eccentricity of beams
rotated with respect to the normal X and Y axis. This includes elliptical or rectangular
shaped beams. When any one of the above results items is activated, the cursors will
change to a rotated mode of operation. In this mode, the cursors will align to the
orientation of the input beam.
If the beam is more circular than elliptical, the beam axes will gyrate uncontrollably
indicating that the elliptical results items should probably be turned off.
The Orientation is defined as the angle formed between the Major axis and the
horizontal, pointing to the right. If the Major axis points above the horizontal, the
angle is positive (+); below, the horizontal is negative (-). The Major and Minor axes
are perpendicular to each other. The method for computing the orientation is
effected by the chosen beam width basis. To achieve an ISO orientation result, one
of the ISO indicated beam widths must be chosen as the beam width basis. Choosing
a non-ISO (legacy) beam width as a basis will apply various clip level criteria for
analysis of the orientation.
Beam Width Basis
Orientation
Algorithm
D4Sigma
Azimuthal (ISO)
Smallest Slit
Azimuthal (ISO)
Knife Edge 16/84
Azimuthal (ISO)
