Q: Is a Pyrex window suitable for Infrared energy transmission?
A: The Pyrex window is just boron-doped high temp glass, it has no IR transmission above 3um and also is bad with ratio sensors, but certainly a no-go for LT (8-14 um) sensors. It is also not-suited to ratio or two-color sensors due to a "kink" in the transmission curves at the E1R or EF1R wavelenths.
Additional info:
BK7 is a form of borosilicate crown-glass as is Pyrex (more doping with Boron to achieve high temperature thermal shock resistance).
MT sensors choose Sapphire or Calcium Fluoride -sapphire is more rugged, Calcium Fluoride has a lower cost.
Data from Crystran in the UK for typical IR window materials:
G5, MT, P7. 3M, 2M, 1M, HT
1R, 2R, 1M, 2M, 3M, MT, CO2, CO, NOx, HT, P3
1R, 2R, 1M, 2M, 3M, HT
1R, 2R, 1M, 2M, 3M, HT
All sensors
1R, 2R, 1M, 2M, 3M, HT, P3
All sensors
P7 LT (with care)