according to a [de:Wikipedia][1] [user:Rotkaeppchen68][2] it is according to an old german physics book like this:

there is a relationship between resistance R and illumination level E like this: $$R \sim E^{\frac{\log R_1 - \log R_2}{\log E_1 - \log E_2}}$$


  [1]: https://de.wikipedia.org/w/index.php?title=Wikipedia:Auskunft/Archiv/2015/Woche_12&oldid=216917511#Finstere_Gedanken
  [2]: https://de.wikipedia.org/wiki/Benutzer:Rotkaeppchen68