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.

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.