Calculation parameters

There are several settings which control and effect the velocity correlation measurements. These are found in the Settings menu whilst the Linescan window is active.

Calculate cbv

This menu item needs to be checked to enable cbv calculation in the linescan window. If it off, then only the grey level pattern along the line is recorded.

Subtract scans

This is required to be on for most measurements except very slow flows. When checked each grey level profile is subtracted from the grey level profile of the previous field. This removes any static pattern which might give an overwhelming zero velocity correlation.

Set sample rate

The grey level profile along the line is, by default, sampled on every field of the video sequence. This setting alows a lower sampling rate for lower velocity measurements. It is recommended to leave this at the default value of 1 unless you are measuring for a long period (eg directly from the "live video" window with truely live video or from video tape) and so need to reduce the memory requirements of the linescan window.

CBV calculation parameters

This dialog box enables fine tuning of the velocity measurement.

cbv method from linescan

This contains some experimental options. Please leave at the default "Spatial Correlation" setting

Number of correlations to smooth

This provides a smothing function to the correlations. Without smoothing, the cbv trace would have many intermittent short spikes. Increasing the number of corrleations smoothed, reduces the peak correlation coeficient. Therfore the correlation coeficient limit will need to be adjusted to a lower value.

Linescan sample periods between correlations

Increase this value for lower velocities, to improve resolution.

Correlation coeficient lower acceptable limit

Use this parameter to remove noise from low correlations. Values below this limit will give a zero cbv value. Note that zero values are not included in the average calculations (but this can be changed see artifact filter).