User guide

LBA-USB User Guide Document No: 11294 Page 57
 Far-Field – This method requires two measurements of the beam’s widths at
two known locations in the beam’s far-field. The difference in size is used to
compute the rate of beam divergence in mili-radians.
First, collect reference data at the smaller beam width location. Once identified,
click Stop! and note the location of the camera in the beam path.
Open the Computations dialog box from the Options menu.
Check the Divergence box and select Far Field in the drop down box. Click the
Set button to transfer the current Reference beam widths into the provided
edit control boxes.
Note: Enter these values from a previous measurement if desired. The Set
button is provided to facilitate making this entry.
Finally, move the camera to the larger beam width sample location, noting the
distance traveled by the camera. The distance traveled is the separation distance
between the two sample locations. Enter this distance in the Separation field, in
meters. Click OK and then Start! The divergence results will be computed for
both the X and Y-axes based on how the current beam size has increased from
that of the Reference sample.
Note: It is not recommended that Elliptical calculations be enabled when
making this measurement. Rather, rotate the laser/camera to be X and Y-
axis aligned. Under certain conditions the rate of divergence could cause
the major and minor axes to swap in the far field region, yielding false
results.