The background measurement is the best source of information on the
current health of the LISST-200X. It provides information on the
current functionality of all of the major systems including laser, ring
detector, data collection electronics, and optical alignment. The LISST-
200X is a sensitive optical device. The laser must be aligned such that
the focused spot is centered on the hole in the center of the ring
detector. If this alignment in not correct the instrument will not function
correctly. The background measurement can easily provide the user
with information about the status of the alignment.
The first indicator of an alignment problem is a severe drop in the
currently transmitted laser power when compared to the factory laser
power. The laser power and laser reference values will change over
time but they should track together. If the value of the laser
power:laser reference ratio for your current background measurement
is less than 85% of the laser power:laser reference ratio from the
factory background file, misalignment may be occurring.
The second indictor of misalignment is high value on the inner rings.
The inner rings are shown on the left side of the background display.
The most important rings for misalignment indication are rings 1-4 in
the background scatter file. As the focused laser starts to move away
from the center of the ring detector it will scatter more light onto the
inner rings. This will cause the inner ring values to be much higher
than the factory values, and is also the reason for the decrease in
transmitted laser power.
It must be noted that low laser power or high inner rings may not
always indicate misalignment. Low laser power can also occur
because the windows or water is dirty or if there are bubbles on the
window. Large particles or bubbles can cause the inner rings to be
higher than factory values. All of the other possibilities must be
eliminated before it can be concluded that the instrument is misaligned.
The transmit optics on the LISST-200X do allow for the possibility of the
user adjusting the alignment. A written procedure can be provided for
this procedure. However, it is highly recommended that the need for
re-alignment be discussed with a trained Sequoia Scientific technical
service representative before attempting this procedure.
As was discussed in earlier sections, the LISST-200X uses a custom
detector to measure light scattered at small angles from particles in
water. In order to measure only the scattered light contributed by the
particles, a measurement of the background scattering must be
obtained. This background scattering can come from a number of
areas. Scratches on the windows, imperfections on the optics, and
other sources all contribute to the scattered light. By subtracting this
background scattering from the measured data, a true measurement of
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