As the war progressed, growing Allied air superiority
made daytime travel nearly impossible for armored units. Furthermore,
daytime fighting had become an increasingly dangerous proposition.
As early as 1942 an infra-red gunsight had been created for the
7.5cm Pak40 self-propelled gun (which used an IR headlamp in addition
to a viewer). The reports from troop testing were favorable,
therefore it was decided that sighting devices should be developed
for tanks as well. The effect of massed armored assault under
the cover of night (as the Allies had no night-fighting capability)
could have been potentially devastating.
The drawing below shows a typical IR emitter. Series production reached roughly 100 pieces per month during the last few months of the war. At the end of the war, when 1000 IR units were available, there were too few tanks and too little fuel to carry out the great night operations envisioned by Guderian.
Drawing from Panther and Its Variants, translation by Michael McSwiney