Table II. Optical Characteristics of transmissive optical materials
 Material  Symbol
Refractive Index
Long wavelength 
Limit, µm
  Comments
    n5µm T<6mm T50mm  
Sodium chloride NaCl 1.52 18 15  
Potassium chloride KCl 1.47 23 19  
Potassium bromide KBr 1.54 30 25  
Cesium bromide CsBr 1.66 42 38  
Cesium iodide CsI 1.74 60 48 easy to scratch
           
Lithium fluoride LiF 1.33 7 6  
Magnesium fluoride MgF2 1.34 8 6.5 birefringent
Calcium fluoride CaF2 1.40 10 8.5  
Strontium fluoride SrF2 1.41 11.5 10  
Barium fluoride BaF2 1.45 12 11  
           
Silver chloride AgCl 2.00 27 21 darkened by UV
Silver bromide AgBr ~2.15 34 30 darkened by UV
Thallium bromochloride KRS-6 2.19 25 21  
Thallium bromoiodide KRS-5 2.38 45 40 orange-red
           
Magnesium oxide MgO 1.64 8 6 birefringent
Sapphire aAl2O3 1.62 6 4 birefringent
Crystal quartz aSiO2 1.47@4µ 4 2.6 birefringent
Zirconia ZrO2 2.13@½µ 5.5 5  
           
Zinc sulfide ZnS 2.25 14 10.5  
Zinc selenide ZnSe 2.43 20 15 yellow
Cadmium sulfide CdS 2.24 15 12 birefringent
Cadmium selenide CdSe 2.45 20 15 visually opaque, birefringent
Cadmium telluride CdTe 2.70 28 ~20 visually opaque
           
Diamond C 2.39 *80 N/A  
Silicon Si 3.42 *50 8.5 visually opaque
Germanium Ge 4.02 23 14 visually opaque
Gallium arsenide GaAs 3.82 17 12 visually opaque
           
Silica glasses vSiO2 1.40@4µ 4 2.6 OH absorption varies
Borosilicate glasses BSCG 1.50@2µ 2.5 2 strong OH absorption
Fluorozirconate glass HMFG 1.50@½µ 8 6.5 little OH absorption
Arsenic trisulfide glass As2S3 2.41 13 ~8  
AMTIR-1 glass AMTIR 2.51 16 11.5 visually opaque
GASIR1 glass GASIR1 2.51 14 12 visually opaque
GASIR2 glass GASIR2 2.60 14 12 visually opaque
           
Poly-ethylene HDPE 1.54 d - visually translucent
Poly-methylpentene TPX 1.43 d - visually transparent
Polyester PET ~1.54@½µ d - visually transparent
Poly-tetrafluoroethylene PTFE 1.35@½µ d - visually translucent/opaque
Fluorinated EPC FEPC 1.34@½µ d - visuallytransparent/translucent
Refractive index values are given for room temperature and 5 µm wavelength, except as noted.Approximate wavelength limits are given for two material thicknesses, <6mm and ~50mm
Wavelength limits designated "d" (discontinuous) indicate interrupted regions of transparency.
Diamond has a narrow absorption band at 5µm.
Silicon has a narrow 2-phonon absorption band at 16.4µm.
Czochralski-grown silicon has an additional absorption band due to oxygen at 9µm.
Semiconductors (e.g., Si, Ge, CdTe, GaAs) display free-carrier absorption at elevated temperatures, limiting their use at high temp.
These data were collected from a variety of sources, so this table probably contains errors. Use cautiously, and empirically verify critical values.
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