Newport LBP2 Series, Руководство пользователя

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LBP2 User Guide Document No: 50306-001 Rev G 3/12/2020 Page 75
5.8 Centroid Location
Centroid X, Centroid Y
The Centroid location is found by calculating the first moment (center of mass) of all
the pixels that are selected to be analyzed. The selection process is controlled by the
aperture settings. When no apertures are enabled, the centroid is computed over the
entire area of the imager. When a manual aperture is present, the centroid
calculation only involves the data contained within the manual aperture. When Auto
Aperture is enabled, then it defines the region of the centroid calculation.
An Auto Aperture takes precedence over a Manual Aperture.
The following equations describe the X and Y centroid locations from the collection of
data points that satisfy the above criteria:
 




z
z X
centroid x
 




z
z Y
centroid y
Where:
X
= x
locations of selected pixels
Y
= y
locations of selected pixels
z
=
value of selected pixels
5.9 Beam Widths and Diameters
To some extent, beam width is a term that describes how the user has decided to
measure the size of a laser beam. LBP2 is designed to provide a set of measurement
tools that will allow the user to make this measurement as they see fit. The ISO
standards have created 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” stands for Diameter but is also
used to denote Beam Widths.) 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 TEM
00
(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 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.