T265 IRCUT filter
An ir-cut filter can block out light frequencies that are in the invisible range. With T265 specifically, a stated intention for providing an ir-cut filter was to block out projected patterns from the 400 Series depth cameras and so allow D4xx and T265 cameras to be used together.
According to the Chief Technical Officer of the RealSense Group at Intel, the D415 and D435 depth camera models can see the visible, 950 and 940 nm IR ranges because those models do not have an IR filter and so are "sensitive to ALL wavelengths in the visible to near IR, following the sensitivity curve of the CMOS sensors". Please see the link below for the full discussion that this quotation was sourced from.
The ir-cut filter on the T265 fisheye sensors could complicate things though if it can block out the higher light frequencies like those from your external IRLED. If you have not already bought a T265 and wish to do tracking, an alternative solution may be to get a D435i instead, as it has the same IMU tracking component as the T265 but its IR imager (ordinary IR, not fisheye) does not have an ir-cut filter.
The T265 data sheet document does not have specifications for the ir-cut filter. Its cover materials section on page 23-24 specifies though that the transparent cover material in front of the imager is designed to admit light in the 400 to 900 NM range.
Intel supports using your own cover material to change the light properties entering the camera to your requirements. I wonder if putting a filter film over the front of the T265 might lower the IRLED light's wavelength to one visible to the IR imager.
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