104
Revision 00 EN - 01/2021
HP-MSM-HB
35,0(6
23.3
Measurement errors
Regardless of the measuring principle, there are many sources of errors in determining beam radius.
• Determination of the zero level
• Finite size of the measurement window
• Resolution in x and y directions
• Intensity resolution
23.3.1
Error in determining zero level
Calculation of the beam waist radius is very strongly dependent on changes in the zero level. It doesn’t mat-
ter if the 86% or 2. moment method is used.
V
1
= 0.86 (V
1
+ V
2
)
V
1
V
1
V
2
V
2
Positive offset
b
∆r+
∆r-
r
o
Negative
offset
= Fill factor
r
b
Fig. 23.6:
Gaussian Intensity Distribution, zero level lowered (left) and raised (right)
Fig. 23.6 on page 104 illustrates this. When the zero level is lowered (left side), the total volume between
the measurement values and the zero level increases. Because of this increase, a larger beam radius is cal-
culated using the curve equation. Conversely, if the zero level is raised (right side), the volume decreases and
the computed beam radius will be too small.
23.3.2
Saturating the signal
High signal amplitudes are clipped by the limited dynamics of the system. If the high power densities are
missing from the calculation of beam geometry, the algorithm always computes a beam that is too large. This
can be compensated for by increasing attenuation.
