MOOR INSTRUMENTS moorLDI2 RESEARCH USER MANUAL
114
19.3 THE DISTANCE FACTOR
A full detailed description of the distance factor procedure performed during manufacture and
checked at installation is in
Appendix 1
.
To obtain laser Doppler blood flow values that are independent of skin-to-scan head distance a
correction table is used to scale raw values. This table is defined by the distance factor.
The specified range of measurement distances for flux measurements is 30cm to 100cm; however
measurements can be made from 30cm to 20cm and from 100cm to 120cm.
For the distances above 100cm,where the detected light will be of relatively low intensity, it is
important to take measurements in low ambient lighting conditions to avoid the possibility of signal
contamination from background lighting.
The moorLDI is calibrated with the motility standard (see the Calibration section of this user manual)
at 30cm from the 'laser aperture'. This is from the front panel of MK1 imagers and is measured from
the vertical raised mark on the side of the scanner case for the MK2 imager.
The distance factor scales all values to those obtained at 30cm. At distances other than 30cms the
flux value is automatically multiplied by a distance factor. Note that this automatic multiplication
can only occur if the distance between skin and aperture is accurately measured (to +/- 2cm).
The
distance factors
are stored as tables in software for each instrument and for each of the three
normalisation conditions, i.e.
unnormalised
dc normalised
and dc squared normalised.
dc normalised
is the
default normalisation
for the moorLDI. This is the division of the flux
signal by a quantity proportional to the dc signal (the measure of the detected laser light intensity). It
has been shown to be a more accurate way of measuring flux differences, over a curved area of skin,
than using dc squared normalisation (as used in optic fibre systems).
Using dc normalisation the typical difference between flux measured at 30cm and that at 100cm is
approximately 20%.
The default distance factor (the multiplier) at 100cm is x 1.2 and the distance factors for distances
between 30cm and 100cm can be determined to a good approximation (typically better than +/- 5%)
assuming a linear increase in multiplier from 30cm to 100cm. So at 65cm (halfway between 30 and
100cm) the multiplier is x 1.1.
To set the distance factor for a moorLDI, it is necessary to run the mldise29u service software or
above (available from Moor or your local distributor). Close all other moorLDI measurement
software before use.
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