Here is a picture of the coherence length holograms for a 5 and 20 mW He-Ne's. Check out the link above to read the background on how they were made.
- From the link earlier in the thread.
The coherence of the 5 mW seems to come and go, with definite nodes placed 70 cm apart. The coherence of the 20 mW sort of comes back but nowhere near as bright or even usable like the 5 mW, with the nodes 52 cm apart. Pretty close for practical purposes.
The zero path length distance is slightly to the left of the line in the center of the bar. The Gaussian distribution of the light is also evident. There is approximately 20 cm on either side of the zero path that is usable for making holograms with the 5 mW, plus/minus about 17 with the 20 mW. I would guess the 40 mW you have for sale would be similar to the 20 mW if the tube lengths are about the same.
During the run of the experiments, 3 or 4 holograms were shot at different exposure times, in the 10's of seconds for both of these lasers. They all looked pretty much the same.
It is great to hear that Laser Sam is also looking at this phenomenon. And sometimes someone has to actually look at what happens in the lab!
See the for sale page if you are interested in buying one from 1 mW to over 40 mW. Long live Helium Neon Lasers! Phil and Sam
but the thoery is basically correct. 
