Well - none of us (except may IR-V who hasn't posted here in years - and
@Max_R ) are thermal engineers, but what do we "know" from our experience ??
1 - lower Net-D definitely seems to convert the 2D images I saw in the old FLIR tau2 apollos into 3D images. The contrast in those olde FLIR cores was "stupendous" ... I would let neighbors (inexperienced thermal users) look thru my Armasights and Pulsars side by side ... inevitably they would say that they could detect smaller critters faster with the Armasights, but could see the terrain better with the Pulsars.
How much of that is lower mK ... IDK.
The problem is there are a lot of "layers" in the electro-optical stack from front to rear of a thermal.
The front lens - which has its own "quality" variable and this includes focusing where it is present
The sensor - technically where the net-d lives - more variables here
The core/image processing - where a lot of variables live !
The display - more variables here
The rear len(s) - more variables here
So with all those variables - if one thermal "looks better" for a single given scene, why is that ?? Not super easy to tell. I've heard from former FLIR employee that FLIR purposely optimized "hot spot pop" - with high contrast.
Also, back when we were comparing Armasights to early pulsars, the pulsars were auto nucing and the armasights weren't.
So just turning them on and holding them up, the nucing and the contrast by themselves are going to show a significant difference. By adjusting both thermals and working to make them "look the same" for a given scene - it was possible to get them much closer.
But the 'texturing" we see in thermals like the BAE/OASYS cores or higher end chinese thermals is not present in most of the older armasights, pure FLIR units, and lower end chinese units. So, I do attribute the bulk of that "texturing" to lower mK, but probably some of it is "image processing" also.
2. Bad thermal conditions
Well, these include
Rain, Snow, Ice, wind, mist, frozen mist, fog, etc. Most of these boil down to various flavors of "water in the air". And "water in the air" can be a show stopper.
I've been out in "Noah's Rains" a number of times with a number of thermals. These are heavy heavy rain. And while I can see hot things, like critters or my 4-wheeler which I had just be riding ... out to distances of like 100yds plus or minus - even in Noah's rains ... the only terrain I can see is the "tops of the treelines" ... just barely ... against the sky. And with NV, not much different, the tops of the tree lines only, though with more magnification, I can see the 4-wheeler or buggy farther away.
Same in "mist" or "fog" ... apparently "fog" officially is "heavy mist". In "heavy mist" the critters are "floating in a sea of grey" .. because while we can detect the critters out for some distance (100-ish yards - like for a field full of turkiyes) but can't see the terrain ... even cranking up the UTC gain 3 clicks ... couldn't see fence posts at 100yds).
So, "bad thermal conditions" can be bad enough to effectively shut down all the electronic night viewers. We can actually see about the same with our mk1eb, or maybe even better. And conditions can be bad enough that ALL electronic night viewers get shut down, whether they are UTCs or TPL peep sights.
There must be a "curve" charting thermal usability - say detection distance for a primate - as conditions deteriorate - but measuring "deterioration of conditions" might be tough. X amount of water in the air per cubic meter ? Maybe somebody can measure that - but I can't.
But in theory there is such a curve. And at some point all the thermals are converging to "zero". But it definitely could be that some converge to zero faster than others. And it could be that lower mK in general approach zero more slowly than higher mK units. But can we usually tell the difference ? And how wide is this window where they are different ?
My gut is the window isn't very wide and we can't usually tell much difference. That's based on 8 years of going out avg 10 hours per week, with dozens of different thermals in wide range of conditions - at least wide for my terrain - in "East/Central" KS. Though I've now moved to SW MO.
Summary, I think all thermals "suck" in bad thermal conditions
all the ones I've had for sure. TPL peep sights up to UTC or Mk3 60mm.
If I ever get my hands on a 25 mK unit - I'll have to think about taking it out in the rain. If the chinese say it is Noah's rain proof (IP67 ish) then I'd try it. And I'll post about it. In the mean time, maybe
@Max_R can summarize what he knows about this !!??