It's built in temperature sensitive vision. Activating it is as simple as blinking for the drow, it lets them see warm things as reds, and cooler things as greys or blues. 

[img]http://i288.photobucket.com/albums/ll173/tylerawhirtley/1243977423_7f4bcc3cd3_m_zpsmvydkhqc.jpg[/img]