© 2011 Thorlabs
108 Thorlabs Beam 4.0
Beam Width
The calculation of the 4 Diameter follows the ISO 11146-1:2005 standards.
According to ISO11146-1, if the ellipticity is larger than 0.87, the beam profile
may be considered to be of circular symmetry at the measuring location - in this
case a 4 simplified calculation can apply.
It is strongly advisable to select the 4 option to ensure that the M² measurement is
in conformity with ISO11146-1.
Note
There are two different types of 4 diameters considered:
When measuring the Beam Profile, in the Calculation Results window the 4
diameters are related to X and Y direction (lab system - see Coordinate systems)
When measuring the M² beam Quality, 4 diameters are calculated for X' and Y' of
the transformed coordinate system, which have in general an non-vanishing angle to
the lab system.For more information see the ISO11146-1:2005 standard.
The Approximated Ellipse beam width is based on the 13.5% clip level. At the
beginning of a measurement a reference angle is determined by averaging over 10
frames. This angle is then used to evaluate all following frames and ellipses.
Since the clip level width suffers from more noise and unsteadiness than the 4
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