Pete, There is a thermal sensitive paint used for indicating over temperature conditions of electronic power components, perhaps this will help you focus the search. And speaking of focussing, I think a lens would be needed to implement such an indicator.
Luke Pete Swanstrom wrote: > A new topic! > > Is anyone aware of a durable (and inexpensive?) thermal-indicating film, > similar to that on side of Duracell batteries, which would be sensitive > enough to infrared to be used on a sundial face to indicate the gnomon's > shadow even on cloudy days? > > Pete.