SUNAs ordered with the 5mm path length coupler as a factory option will perform much better in low light transmission waters due to the shorter length the light needs to travel leading to less absorption. Equipping your SUNA with the factory bio-wiper option will also perform better and be less susceptible biofouling or buildup of other material that can reduce light transmission.
There are also some maintenance practices and device settings that can give SUNA a better probability of being able to capture enough light for a sample. Enable adaptive integration will trigger the SUNA to increase the lamp on time when light received by the spectrometer is low. It is also important to clean the windows as frequently as possible and monitor lens for scratches. Finally, you want your maximum light spectral counts at the peak wavelength (around 240nm) to be between 45,000 and 55,000 counts in pure or deionized water. This can be viewed in the "Spectra" tab in UCI when sampling or replaying data. If your peak spectrometer output is below 45,000 counts after cleaning the window, you may increase the integration period by 25 to 50 ms if needed (but not more; further changes require a factory recalibration). After adjusting the integration period, always perform a reference spectrum update per the instructions in the SUNA manual.