Use of a transparent lens would unavoidably create rear surface reflection of the incoming radiation, which would mix with {HEAD ~ the sample} reflected radiation traveling toward {TAIL ~ the detector} 
Use of a transparent lens would unavoidably create rear surface reflection of the incoming radiation , which would mix with { HEAD ~ the sample } reflected radiation traveling toward { TAIL ~ the detector }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL