All Endurance sensors are fitted with a Schott BK-7 front window (field replaceable). When specifying any additional sight glass window it is a good idea to use the same material.
If it is necessary to choose a material with reduced expansion properties take care to avoid Pyrex glass.
The IR transmission is not consistent over the spectral range of the E1R and this will result in a false reading due to an "E-Slope" error (different integrated transmission values for wide and narrow bands).
You may see the reading increase with the glass in between the sensor and the object.
An easy check for any proposed window material for use with a ratio sensor is to place the material in between the sensor and a black body and ensure that the reading does not change.
Here is an example of Pyrex vs Schott SF8 and SF10:
Other suitable window materials include Quartz and Sapphire, even Calcium Fluoride is suitable as the transmission is equal at the two wavelengths See Article