First let's talk about Borosilicate glass: it is a good high strength glass, it is tough, has good thermal shock capabilities, and is relatively inexpensive. Borosilicate glass is a mixture of ~ 13% boric oxide (B2O3) and ~80%silica (SiO2) (the balance is a mixture of metal oxides). Glasses are noncrystalline, amorphous materials, distinctly different from solid state crystals. Typically glass measuring cups, pots, and pans are made out of borosilicate because of its thermal shock resistance (Pyrex is a trade name for borosilicate glass). It is not particularly clear and tends to be less homogenous and exhibits striae and bubbles more than optical glasses. A 15mm thick borosilicate glass window absorbs about 20% of the visible light traveling through it.

Sapphire (synthetic is our case) is a remarkable material composed of crystallized Al203 , it is not a glass. Natural sapphire and rubies are essentially the same thing (corundum) except that trapped impurities give them coloring. Synthetic sapphire does not have the impurities which add color but weaken the natural material. Sapphire's incredible hardness is the reason it is used for bearings in watches and high precision instrumentation. Sapphire is about 4 times as transparent to visible light as borosilicate glass, but because it is significantly stronger a thinner window can be used. Our 3.3 mm window absorbs about 1/6 of the light that a competitors 15mm borosilicate glass window would and still provides comparable strength. In addition sapphire has about 4 times the working temperature (up to 1900C). Sapphire is significantly harder than glass, it exhibits a Knoop hardness of 2000 compared to borosilicate glass's Knoop hardness of 418. On the Mohs scale, borosilicate glass is about 5 (appetite), where sapphire measures 9 (about the same as silicon carbide just under diamond). One of the most significant properties of sapphire in this application is its coefficient of thermal expansion. Sapphire expands only about 1/6th as much as borosilicate glass does per degree when it is heated. This is the principle cause of thermal stress cracking in glass. A thick piece of glass, which is hot on one side (lamp side) and cold on the other (water side), is subject to very high mechanical stress because of the difference in thermal expansion. The thicker a window is, the higher the stress. Additionally, the thermal conductivity of sapphire is about 20 times higher than borosilicate glass, it is roughly the same as the stainless steel and titanium housings. Because of this higher thermal conductivity and lower absorption of light, the sapphire window efficiently moves heat from inside the housing out into the water.

Among the many qualification tests we put our design through was a wave slap test determined to measure the strength of the window in its seat in the face of repeated impacts. We hammer the window with a 500psi water pulse that is designed to generate and propagate cracks in the window if there are any design problems. Our sapphire window has been tested over 250,000 cycles when mounted in a THSL without damage. Each pulse was held for about 1 second and then released for about 1 second exerting a 1500 pound force on the window every cycle.