Calcium Fluoride (CaF2)
Bi-Concave Lenses

Bi-concave ko Double-concave (DCV) ruwan tabarau mara kyau ne waɗanda suka fi kauri a gefen fiye da na tsakiya, lokacin da haske ya ratsa su, yana rarrabuwa kuma wurin mai da hankali yana kama-da-wane. Bi-Concave ruwan tabarau suna da daidaitaccen radius na curvature a ɓangarorin biyu na tsarin gani, tsayin dakawar su mara kyau ne, da kuma radius na lanƙwasa na saman masu lanƙwasa. Tsawon tsayi mara kyau yana haifar da haɗuwa da hasken da ya faru ya bambanta, galibi ana amfani da su don karkatar da katako mai jujjuyawa. Saboda fasalullukansu, ana amfani da ruwan tabarau bi-concave gabaɗaya don faɗaɗa haske a cikin masu faɗaɗa haske irin na Galilean ko ƙara ingantaccen tsayin daka mai haɗawa da ruwan tabarau ta amfani da nau'i-nau'i a cikin tsarin da ake dasu kamar tsarin hasashen haske. Hakanan suna da amfani idan ana batun rage hoto. A cikin tsarin gani, ya zama ruwan dare a zaɓin na'urorinsu a hankali domin ɓarnar da aka gabatar da ruwan tabarau masu kyau- da mara kyau-tsawon-tsawon ido kamar soke. Ana amfani da waɗannan ruwan tabarau mara kyau a cikin na'urorin hangen nesa, kyamarori, lasers ko gilashin don taimakawa tsarin haɓakawa ya zama ɗan ƙaramin ƙarfi.