Because the spectral response of the thermopile detectors is significantly flatter and broader than the spectral transmission windows of the protective covers used on the sensors, the spectral responses of thermopile sensors is essentially that of the protective windows.
Below are the protective windows for various thermopile sensors:
Thallium Bromoiodide (KRS-5) IR
Barium Fluoride (BaF2) Visible-Near IR
The KRS-5 window response is nearly flat from 600 to 30000 nm; whereas, the BaF2 window response is nearly flat from 300 nm to 10000 nm. The BaF2 is preferred for measuring both the visible and infrared; whereas, the KRS-5 will only measure into the yellow part of the visible, but will measure further into the infrared.
The time constant of all of these PASCO thermopile sensors is ~85 ms.