Clearer specification required. Plastic films have IR absorbance, e.g. at 3.4 um, may be acceptable though. Thin IR glass or fused silicia should work. What's the application?
Sounds like you didn't yet fix an exact specification, e.g. of required transmittance. Respectively don't expect clear answers.
For optical sensors, we often choose window materials with sufficient rather than perfect transmittance if they offer better mechanical properties. But your application may require a flat transmission spectrum for some reason.
I am placing this Transparent Film on the sensor for EMI Shielding. My sensor wavelength is 400nm to 4000nm. My required transmittance is 400nm to 4000nm.