“With its irradiance responsivity factor and conversion time, the AS7331 supports an overall huge dynamic range up to 3.43E+10 (resolution multiplied by gain range). It achieves an accuracy of up to 24-bit signal resolution (internal via I²C and shifter 16-bit), with an irradiance responsivity per count down to 2.38 nW/cm² at 64 ms integration time. Via an integrated divider, the 16-bit I²C output can be adjusted to the significant bits of interest.”
i use the sparkfun library 
But if i change in SfeAS7331.cpp , the code at end of the file (with : _sensorGain = GAIN_1; for example) the gain is low (with gain = 2048 too) , i test different value but the more efficient is with gain=1 .
I test with uv diode with minimum power/visibilty , my eye see the uv light, but the as7331 no (test in one shoot mode ) , (6 cm between the UV diode and the sensor )
I search the nanowatt range , but what modification to make
Edit OK , i change de delay in the ino code to 50 ,the serial monitor is more efficient , 
, i see the change ,with 1048 and 2048, i see a little UVA:0.01 from time to time
for the idea i see,with difficulty, but i see, the 395nm uv diode at 3 meters