Materials/Glasses and optics/Collections

Optical crystals

optical-crystals

9 materials across 1 types

About optical crystals

Optical crystals (sapphire, ruby, calcium fluoride, lithium fluoride, zinc selenide, zinc sulphide, caesium iodide, cadmium selenide): transmission range, hardness and thermal data with sources.

Optical crystals are used as windows, lenses and detectors from the ultraviolet to the far infrared. The range of wavelengths they pass, their hardness and how they cope with heat decide where each one is used.

At a glance

Highest density: Cadmium selenide (CdSe) reaches 5.81 g/cm³. Lowest: Technical gemstone minerals starts at 1.9 g/cm³. (9 of 9 grades have a sourced value.)
Highest melting point: Sapphire (Al2O3) reaches 2,054 °C. Lowest: Caesium iodide (CsI) starts at 621 °C. (8 of 9 grades have a sourced value.)
Highest thermal conductivity: Sapphire (Al2O3) reaches 42 W/(m·K). Lowest: Caesium iodide (CsI) starts at 1.1 W/(m·K). (8 of 9 grades have a sourced value.)

Ranges run across the conditions each source reports (temper, heat treatment, form), measured at room temperature where the source says so.

How they compare

The same measurements for every material in the collection. A range means the grade has values under several conditions; "at least" means the source gives a specified minimum. "no data" means no sourced value yet. For ranked tables, open a benchmark.

Optical crystals: how the grades compare
Material	Density	Melting point	Thermal conductivity	Young's modulus	Flexural strength	Thermal expansion
Sapphire (Al2O3)optical window, substrate and insulator crystal	3.97 to 3.98 g/cm³	2,040 to 2,054 °C	23 to 42 W/(m·K)	335 to 470 GPa	480 to 895 MPa	5.6 ppm/K
Ruby (Cr:Al2O3)solid-state laser crystal, optical window and jewel bearing material	3.98 g/cm³	2,040 °C	33 to 35 W/(m·K)	no data	no data	4.78 to 5.31 ppm/K
Calcium fluoride (CaF2)UV and IR optical crystal for windows, lenses and prisms	3.18 g/cm³	1,360 °C	9.71 W/(m·K)	75.8 GPa	no data	18.9 ppm/K
Lithium fluoride (LiF)VUV and X-ray optical crystal, with dosimetry and battery-interphase uses	2.64 g/cm³	848 °C	11.3 W/(m·K)	65 GPa	no data	37 ppm/K
Zinc selenide (ZnSe)infrared optical window and lens for CO2 lasers at 10.6 µm	5.27 to 5.42 g/cm³	1,525 °C	18 W/(m·K)	67.2 GPa	55.1 MPa	7.1 x10-6/K
Zinc sulphide (ZnS)infrared window and dome material, and electroluminescent phosphor base	4.08 to 4.09 g/cm³	1,827 to 1,830 °C	27.2 W/(m·K)	74.5 GPa	68.9 to 103 MPa	6.5 ppm/K
Caesium iodide (CsI)IR optical crystal, and X-ray and gamma scintillator (CsI(Tl) and CsI(Na))	4.51 g/cm³	621 °C	1.1 W/(m·K)	5.3 GPa	no data	48.3 to 51 ppm/K
Cadmium selenide (CdSe)birefringent infrared crystal for waveplates and long-wave optics, also used in quantum dots and photoconductors	5.81 g/cm³	1,240 to 1,537 °C	6.2 to 6.9 W/(m·K)	no data	no data	2.45 to 4.4 x10-6/K
Technical gemstone mineralsgem-quality minerals used as hard, transparent or decorative crystal material; beryl is also a beryllium feedstock	1.9 to 3.27 g/cm³	no data	no data	no data	no data	no data

Where they are used

Uses
Use	Grades
Lunar Orbiter Laser Altimeter (LOLA) on NASA's Lunar Reconnaissance Orbiter, sapphire optic at 1064 nm	Sapphire (Al2O3)
Front elements of spacecraft optical systems	Sapphire (Al2O3)
LED substrate for blue, green, ultraviolet and white LEDs	Sapphire (Al2O3)
Cr:Al2O3 laser rods for solid-state lasers	Ruby (Cr:Al2O3)
Pulsed ruby laser ranging to the Apollo 11 retroreflector (Lick Observatory, 1969)	Ruby (Cr:Al2O3)
Pulsed ruby laser lunar-ranging station at McDonald Observatory (2.7 m telescope)	Ruby (Cr:Al2O3)
Infrared spectroscopic windows, prisms and lenses	Calcium fluoride (CaF2)
Excimer laser windows and UV optics	Calcium fluoride (CaF2)
Lithography optics at 193 nm and 157 nm	Calcium fluoride (CaF2)
Special UV optics and vacuum-ultraviolet windows	Lithium fluoride (LiF)
X-ray monochromator plates	Lithium fluoride (LiF)
Thermoluminescent dosimeters (LiF:Mg,Ti)	Lithium fluoride (LiF)
CO2 laser lenses and output windows at 10.6 µm	Zinc selenide (ZnSe)
Infrared optical coating for low Earth orbit (ground test only)	Zinc selenide (ZnSe)
X-ray and gamma-ray detectors (tellurium-activated ZnSe scintillator)	Zinc selenide (ZnSe)
Exterior infrared windows in the 8 to 12 µm thermal band	Zinc sulphide (ZnS)

When each grade fits

Best use and weak spots of each grade
Material	Best for	Weakest at
Sapphire (Al2O3)	Hard, radiation-tolerant optical windows and substrates that must pass light from the ultraviolet to the near infrared (NASA 1981 ranked sapphire first for radiation resistance)	Brittle fracture and edge chipping: flexural strength is 480 to 895 MPa in one supplier's sheet and 690 MPa in another, and Crystran's safety sheet warns about chipping and sharp edges
Ruby (Cr:Al2O3)	Pulsed and tunable solid-state laser rods that need a hard, well-characterised crystal with a known pump band	Little space-qualification data: the sources read cover ground lasers, lunar ranging from Earth and one datasheet, with no ruby-specific radiation, outgassing or thermal-vacuum result
Calcium fluoride (CaF2)	Windows, lenses and prisms that must pass light from the vacuum ultraviolet to the infrared, including excimer-laser optics and Raman spectroscopy (Crystran)	Brittleness: the crystal cleaves on the (111) plane, and it reacts with strong mineral acids to give hydrogen fluoride gas (Crystran)
Lithium fluoride (LiF)	Optics that must transmit into the vacuum ultraviolet, down to 121 nm (Crystran)	Brittleness: the crystal cleaves easily on (100), and it reacts with strong mineral acids (Crystran; SDS)
Zinc selenide (ZnSe)	Lenses and windows that must pass 10.6 µm CO2 laser light with very low absorption, and infrared windows that need transmission out to about 21 µm	Softness and toxicity: Knoop hardness of 120 and a rupture modulus of 55.1 MPa (Crystran and Coherent), and a supplier safety sheet that lists it as toxic if swallowed or inhaled (Angstrom Sciences)
Zinc sulphide (ZnS)	IR windows and lenses in the 8 to 12 µm thermal band that must also pass visible light (water-clear multispectral grade transmits 0.4 to 12 µm)	Softness and a 250 °C use limit in air: Crystran says windows should not be used above 250 °C in normal atmosphere, and the material oxidises at 300 °C
Caesium iodide (CsI)	IR windows that must reach far into the infrared, and X-ray and gamma scintillators read by silicon photodiodes or SiPMs (CsI(Tl))	Softness and moisture: the crystal is extremely soft and hard to polish, and it is slightly hygroscopic and soluble in water
Cadmium selenide (CdSe)	Polished, birefringent infrared parts and waveplates from 0.75 µm to about 25 µm, where a crystal is needed rather than a film	Toxicity and softness: the supplier safety sheet and Wikipedia classify it as toxic if swallowed or inhaled with a repeated-exposure hazard, and Mohs hardness is 3.25 (Tydex)
Technical gemstone minerals	Hard, transparent mineral material where hardness and density matter (beryl and forsterite are the hardest, at Mohs 7 to 8)	Brittleness and cleavage: the Handbook lists beryl, orthoclase and forsterite as brittle, and orthoclase cleaves perfectly on two planes. Opal and turquoise are softer, at Mohs 5 to 6