Thermal focus change

I've been schooled by my mentor, R. Juergens, for using the term "Abbe number" for an IR doublet. The term "Abbe" denotes a visible solution. Therefore when discussing dispersion in a non visible material one should use the term "V number"

Invert the therm optic coefficient, β, and you get the thermal V number or VT. Predicting thermal defocus based on the thermal V number is analogous to calculating axial color defocus.

For a derivation of this equation and the therm optic coefficient see here.

Here is an editorial view: V numbers are an “inverse” type parameter:
  • Higher the number the lower the defocus
  • Lower the number the more defocus
  • True for color correction and thermal effects
This fact may contribute to the therm optic coefficient’s popularity and wider use.  
Lens designers are calibrated to Abbe numbers from centuries of experience with color correction.
I contend that lens designers should not use the therm optic coefficient, β
Rather they should use the thermal V number, VT,  described here.
Scott Sparrold,
Mar 12, 2012, 11:50 AM