Multivariate Regression of Curve fitting approach?

I was wondering what may be the best approach to model the following situation.

I have two set of sensors readings, both sets are measuring the same amount of light but they are done with a different kind of sensor. I would like to calibrate one of the set of sensors based on the readings of the other ones. Thissituation leads to having the following graph

So I would like to fit data from sensors S1 and S2 to the called Ref variable, but I am not sure whether the best model is a multivariate regression or just a curve fitting approach, or maybe a different one.

Could you give me some idea of what kind of model fits better in this situation?

Thanks in advance,

regards!

[John D'Errico]

I will claim only that your problem is not one of curve fitting at all.