To extend the UV sensitivity of commercial image sensors, the front cover glass can sometimes be removed. 

A company even offers this as a service: https://www.eureca.de/files/glasscoverremoval_2019-09-24.pdf

The cover glass is most likely N-BK7 with an AR coating for the visible range. Here's a microscope slide demonstrating that it is highly transmissive even at 337.1 nm light provided by a N2 laser: