Baader IR pass filter 1.25″ – for infrared photography (685 nm).
The IR pass filter (substrate thickness 2mm) blocks almost all of it
visible light off.
At 685 nanometers (beyond the red light) it has 50% transmission
and the transmission curve rises steeply. The passage starts very
steep at 685 nanometers, so you can just about focus and
sees a deep red image. The human eye is in this range
hardly sensitive anymore, so this filter is purely for
photographic applications.