User guide

LBA-USB User Guide Document No: 11294 Page 105
3. Line Aperture – Use the Top Hat Line Aperture to analyze the data lying
along the axes of a Drawn Aperture. As in Area Aperture above this should
be limited to regions lying on the upper surface of the Top Hat beam. Except for
Top Hat Factor, a separate result will be computed for each axis of the
aperture. Use a Drawn Aperture to isolate regions of the beam and compute
results over only the orthogonal axes of the aperture, (the clip level is ignored
for all but the Effective Area and Diameter results). This method is intended for
use with a Drawn Aperture. However, it will work without an aperture and with
an Auto aperture. See the note below.
Note: In general it is not advisable to use the Auto Aperture feature when making Top
Hat measurements.
4.18.1 Top Hat Mean and Standard Deviation
The computation of the Mean and Standard Deviation are described in the
equations below.
For the Mean:
Z
Z
n
=
∑
Where:
Z
= Mean intensity
n = Number of summed pixels
Σ Z = Sum of the pixel intensities above the clip level,
or in the area, or on the line being evaluated.
For the Deviation:
(
)
2
1−
−
=
∑
n
ZZ
σ
Where: σ = Standard deviation
N = Number of pixels summed
Σ (Z -
Z
)² = Sum of the square of the differences between the
mean intensity and the pixel intensity values
above the clip level, or in the area, or on the
line being evaluated.
4.18.2 Top Hat Minimum and Maximum Intensities
The values appearing here represent the highest and lowest energy intensities
that are found within the Top Hat area, as defined by the Data, Area or Line
Aperture selection.
Note: In Data mode, the Minimum will often be the clip level value determined by the
Beam Width Method.