Newport LBP2 Series, User Manual

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LBP2 User Guide Document No: 50306-001 Rev G 3/12/2020 Page 73
prerequisites. Such a prerequisite might be the need to utilize as an input the second
moment beam width. If the user were to choose a non-ISO beam width basis, such
as a 50% of Peak, then it is likely that another result will not yield an ISO compliant
answer.
Another area where one may fail to achieve a good ISO result could be in the
application of proper baseline correction. While the Ultracal technique is an ISO
compliant method, it could be improperly applied or not monitored for thermal drift or
ambient light effects. All of which could lead to an incorrect result, even though the
computations themselves are performed according to ISO equations.
See Appendix A for a table of ISO computational definitions with a reference to the
applicable ISO standards.
5.4 Clip Level
The clip level is a processed power/energy pixel value. Various input dialog box
entries apply to these results items. Only those pixel values that exceed the clip level
are used in computing the following results:
 Certain Beam Width methods that have programmable clip levels such as
those shown below:
 Elliptical beam Orientation
Depending on the Beam Width Method selected, the clip level value is determined as
follows:
 With the Percent of Power/Energy method, LBP2 totals the pixel
energy values in descending order until it finds the pixel that causes the
sum to exceed the set Clip% of the total energy value. The energy value
of this pixel becomes the clip level.
 With the Percent of Peak method, LBP2 sets the clip level to the value
that is equal to the set Clip% of the current peak energy value.
The number of pixels with values above the clip level establishes the Effective Area of
the beam. In some cases, the selected Beam Width Basis determines which Clip Level
is used in the calculation of the Centroid and Orientation of elliptical beams. These are
shown in the table below.
Beam Width Basis
Clip Level used for
Orientation and Centroid
D4Sigma None
Smallest Slit None
Knife Edge 16/84 None