Dear Listers,
to clarify the HV vs. Gain on FluoView FV1000 topic and to follow Stanislav 
Vithas request for someone from Olympus to answer:

1. Adjust HV of the PMT. This regulates how many counts are generated by a 
photon hitting the PMT. When you have a weak sample with a good S/N you 
can raise the HV to a value where the brightest Point on your image generates 
4095 counts, therefore using the complete dynamic range. (Of course also 
with a strong sample it is desirable to use the complete dynamic range for your 
sample, it simple generates a better data-quality).
But be aware: If you raise the HV too much you reach the non-linear range of 
the PMT, leading to an unequal amplification of areas of different brightness. 

2. The Gain is an alternative way to facilitate the full dynamic range of the AD-
Converter. To give an example: By using only the HV your sample has intensity 
values from 0-1000 counts, rising the HV to top end would take you into the 
nonlinear range of the PMT. If this signal is converted by the ADC, you do not 
use the whole dynamic range of 4096. If you add a Gain of e.g. 4, you get 
everything multiplied by 4 – the PMT noise as well as the actual analog signal. 
The PMT noise will be higher, but now you can use a much larger dynamic 
range – up to 4095 without sacrificing the linearity of the image. Another 
advantage is that the quantization noise will be smaller in comparison to the 
signal. 

Plainly, if you have a weak sample that can not be imaged with the full 
dynamic range in the  linear range using the HV, you can use Gain to amplify 
the image and to keep the HV in the linear range. If you don´t care about 
using the full dynamic range, there is no need to use the Gain.

Best regards
Florian Eich
Olympus Life Science Europa GmbH