User guide

LBA-USB User Guide Document No: 11294 Page 97
 In the case of Percent of Peak, the included pixels are those that are greater than the
Clip% level.
 With Percent of Energy, the included pixels are those that are greater than or equal to
the Clip% level.
If the 4 Sigma or one of the Knife Edge methods are chosen and the Top Hat
calculations are not checked, the clip level is set to 86.5% of energy. If the Top Hat
calculations are checked, then an 80% of Peak clip level is set.
The following equations describe the X and Y centroid locations from the collection of
data points that satisfy the above energy clip level criteria:
(
)
∑
∑
×
=
z
zX
centroidx
(
)
∑
∑
×
=
z
zY
centroidy
Where: X = x locations of selected pixels
Y = y locations of selected pixels
z = value of selected pixels
4.11 Beam Widths and Diameters
To some extent, beam width is a term that describes how you have decided to measure
the size of your laser beam. LBA-USB is designed to provide a set of measurement tools
that will allow you to make this measurement as you see fit. During the past few years
there has been some movement toward a consensus regarding a standard definition of
beam width. This definition has grown out of laser beam propagation theory and is
called the Second Moment, or D4-Sigma beam width. (The D erroneously stands for
Diameter.) Sigma refers to the common notation for standard deviation. Thus an X-axis
beam width is defined as 4 times the standard deviation of the spatial distribution of the
beam’s intensity profile evaluated in the X transverse direction. A measurement taken in
the Y transverse direction will yield the Y-axis beam width.
Note: For a TEM00 (Gaussian) beam, 2-Sigma is the 1/e² radius about the centroid.
The term Diameter implies that the beam is radially symmetric or circular in shape. The
term Width implies that the beam is non-radially symmetric, but is however axially
symmetric and characterized by two principal axes orthogonal to each other. Beams that
are asymmetric, distorted, or irregularly shaped will fail to provide significantly
meaningful or repeatable beam width results using any of the standard methods.