User guide

LBA-USB User Guide Document No: 11294 Page 111
4.24 Frame Averaging
The signal to noise ratio of the digitized data can be improved by using Frame
Averaging. The amount of the improvement is roughly the square root of the number
of frames being averaged. LBA-USB can average a maximum of 256 frames for at best a
16 times improvement of the signal to noise ratio. The amount of improvement is also
limited by the noise content of your camera. In general you will begin to receive
diminishing returns when you average more frames than the square of the number of
noise counts. For CCD cameras the noise is about 4-5 counts, so averaging more than
16 to 25 frames will be of little benefit.
Pulse to pulse variations for a pulsed laser will be reduced by a like amount.
When Frame Averaging is enabled, the display will update with the averaged results only
after all frames have been received. Any calculations will similarly be performed only
after all frames have been received.
Note: When Frame Averaging is enabled and Stop! is clicked, LBA-USB will
immediately abort the collection of frames for averaging and will display the last
completed set of averaged data. Any frames that were in the process of being
averaged are discarded, thus when you again click Start! a totally new averaging
process is begun.
Important: Do not use Frame Averaging if your beam suffers from poor pointing
stability, and you want to make accurate beam width measurements. Instead,
enable statistics and find the mean beam width by using results averaging.
This is independent of centroid position.
4.25 Frame Summing
Use Frame Summing to observe the cumulative effect of a pulsed laser. LBA-USB can
sum a maximum of 256 frames. Make sure that the pulse rate does not exceed the
capabilities of LBA-USB and the camera system, and that the total energy does not
exceed the available dynamic range.
Note: It may be necessary to use Block Mode to ensure that no pulses are missed.
Frame Summing will cause fixed pattern noise to increase in proportion to the number of
frames summed, while temporal noise will increase only as the square root of the
number of frames being summed. Thus some improvement in the signal to noise ratio
will be realized. To further improve the signal to noise ratio, try using Frame Summing
together with Frame Averaging.
When Frame Summing is enabled, the display will update with the summed results only
after all frames have been received. Any calculations will similarly be performed only
after all frames have been received.
Note: When Frame Summing is enabled and Stop! is clicked, LBA-USB will
immediately abort the collection of frames for summing and will display the last
completed set of summed data. Any frames that were in the process of being
summed are discarded, thus when you again click Start! a totally new summing
process begins.