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