Can a filter extend a FLIR One thermal camera beyond its 100°C maximum range?

Not in the simple “subtract 200°C” sense. Filters don’t shift temperature readings by a fixed amount; they only reduce or block some of the infrared radiation reaching the sensor.

In principle, an infrared neutral-density filter can attenuate the incoming IR energy, and FLIR has suggested this approach for some applications. But that does not guarantee your FLIR One will accurately measure a new higher temperature range unless the camera and its software are designed or calibrated for that setup.

A standard filter also won’t recover wavelengths the camera doesn’t detect, and a notch filter only blocks selected wavelength bands rather than creating a simple temperature offset.

So the practical answer is: you may be able to reduce the IR signal with an IR ND filter, but you cannot reliably turn a 0–100°C camera into a 200–300°C camera just by adding a filter. For accurate high-temperature measurement, you generally need a thermal camera model rated and calibrated for that range.