Starting with firmware version 1.4 in May, 2017, the LISST-200X
automatically measures and subtracts light that is not produced by
scattering from its laser. This Ambient Light Rejection (ALR) prevents
sunlight or other bright sources from contaminating the scattering
measurements. Without ALR, as in earlier LISST instruments, high
ambient light levels, if not physically blocked from the LISST’s optical
aperture, could falsely elevate the apparent concentration of small
particles.
You can verify that ALR is active by connecting to the instrument with
LISST-SOP200X, then opening the terminal window. Type DS and
press enter, and look for “Ambient light subtraction: on” in the output. If
it is not shown, you should upgrade the firmware. Contact Sequoia
Scientific for assistance.
ALR works by measuring the signal on the LISST-200X’s 36 detectors
first with the laser on, then with the laser off. This on-off process is
repeated 3 times per second. Because of the time required for the laser
and electronics to change between the on and off states, rapidly
changing ambient light may not be completely rejected. Therefore, if
working in bright sunlight, near the water surface, and in conditions
where the sunlight is rapidly modulated by waves, ambient light could
still have an effect. ALR will reduce the average effect, but the
changing ambient conditions could increase sample-to-sample noise. In
those worst-case conditions it is still advisable to shade the LISST-
200X optics from ambient light. The same is true if operating under
intense artificial light modulated by AC power.
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