[hornychz]

My formula seems to produce over estimated results, especially at the lower part of the scale, but this compensates the losses due to reflexion at high incidence angles (sunset and sunrise).

One more time I say, this is a result of a practical observation, done to obtain readings comparable to the readings from other nearby weather stations.

Just another thought: Using your formula increases memory usage in more than 40 bytes. As much as possible, avoid complex floating point calculations in Arduino it's a good rule. If you gonna use it, I'm sure it can be simplified.

Hi, Werk_AG,
and thank you for your reply. I'm saying again - you are right, definitely ...
But, there are min. three resons to use a new formula:
1) it is a bit more refined and more versatile
2) respects the fact that the curve passes through zero
3) but mainly: it enables a very simple amateur calibration
Formula could be a little simplified:

IndiceUV = (( ADC1 * 1.074 / Transmission - 135 ) * 0,011

but it doesn't mean, in my opinion, nothing for decrease memory use ...
Here is my results for today:

and this for today from CHMU:

Now I'm using only CD cover and my TX case is only down in the garden, not yet on the roof (I'm still waiting for some components).
Have a nice summer ...