BNC 1420 - Video Microscope - Manual (Page 37)

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d. If the probe is located in a region of fast flow, the probe
size must be large enough that the correlation does not fall
beyond the window. The Cross Correlation field (see
below) can be an aid in setting the size.
NOTE: If you cannot achieve sufficient signal-to-noise ratio with a
required probe size you may need to adjust the illumination,
particle feed, etc. If the probe is in an area of steady flow then
increasing time averaging may also help.
3. The Averaging percent per frame values help separate the useful
flow information from the background data by weighing the
previous frames of data versus the current frame. The default
values of 95% are acceptable for most flows. Some guidelines for
setting the percentages are:
a. The Background (mean) percentage determines how much
of the image field is considered background based on its
steady presence over multiple frames. Increasing this value
increases the amount of information that is ignored, such as
stuck particles. Check the Subtract Mean box (see below)
to apply this calculation and remove the data.
b. Decreasing the Correlation value improves time
resolution; increasing the value improves noise control.
4. The Calculations Options control how the flow parameters are
calculated:
a. Enter 1 in the Frames Skipped box to calculate cross
correlation for every sequential pair of frames. Enter 2 to
use every other frame, 3 for every third frame, etc. This
option is useful for examining very slow flows.
b. Subtract Mean subtracts the background (non moving)
data from the flow calculations. The amount of data that
will be subtracted is based on the Background % value
(see above).