Chris, sorry this is a long read but here is some info on 1990s imagers and a few webpages regarding the development of night vision systems (including thermal imagers). Your player should probably read those webpages too. The following is info from various books and websites but also my own personal experience with thermal imagers in the early 1990s with the Australian Army.
Thermal imagers of the 1980s and 1990s were big bulky things commonly found only mounted in vehicles or on tripods. They required coolant gases to increase their heat sensitivity which meant they required several seconds of "cool down" time before they could be used. Thermal Weapons Sights for small arms were in the early stages of development and production and were scarce in military service (they were incredibly expensive, somewhat fragile and there just weren't that many made at the time to allow widespread distribution)
It was not until the mid-late 1990s that the FPA (Focal Plane Array) allowed thermal imagers to get small enough for widespread use and the technology was actually pushed forward by commercial applications (police, fire, rescue and security) rather than military. For the Twilight War, those commercial demands are highly unlikely to exist so the research that lead to the FPA would be in its infancy at best.
Someone mentioned the AN/PAS-13, it's a hell of lot handier than earlier generations of thermal imager but unfortunately it didn't come into service until the late 1990s (I think it was 1997 or 1998).
My ruling would be that if your player has a thermal imager for a weapon sight and you have the Twilight War starting before the late-1990s, then it's probably a Hughes Thermal Weapon Sight (TWS) or a Magnavox Short-Range Thermal Sight (SRTS) - either way, it's big, it's bulky, it makes a high pitched whining noise like a mosquito when it's operating, it's only able to display in green, white and black, it uses unique batteries and it's heavy, like several kilograms heavy.
And the player should consider it to be rarer than rare and should feel blessed that they have such a high-tech piece of gear!
http://www.nvl.army.mil/history.html This is a US government run site and they have a strict "if you use this site expect us to check you" kind of warning at the start.
http://www.strategypage.com/dls/arti...-3-18-2011.asp
http://www.thermal-eye.com/about_us/index_history.htm This page has a short history of thermal imaging development. Note that L-3 Infrared Products was formerly Raytheon Commercial Infrared so they're a defence industry company.
http://www.globalsecurity.org/milita...-22-9/apph.htm This page is from one of the US Army manuals and it deals with night fighting - long but worth a read for the different aspects of aided and unaided night viewing.
Some basic specs for the two thermal sights I mentioned:
Hughes TWS - Light Weapon Thermal Sight (for rifles & carbines)
Weight: 1.7kg
Effective Range; 550m
Battery Life: approx 10 hours*
Hughes TWS - Medium Weapon Thermal Sight (for GPMGs)
Weight: 1.9kg
Effective Range; 1100m
Battery Life: approx 8-10 hours*
Hughes TWS - Heavy Weapon Thermal Sight (for HMGs, Mk19 AGL, sniper rifles)
Weight: 2.1kg
Effective Range; 2200m
Battery Life: approx 8 hours*
Magnavox SRTS
Weight: 1.8kg
Effective Range; 550-600m*
Battery Life: 10 hours
* = estimated (I haven't got any solid data, just some estimated figures from a 1993 source)