ZnSe vs Germanium - which one makes a more 'scratch resistant' lens?

[Liam G]

Would also be interested to know how ZnSe, Germanium etc. compare to normal glass in the 'scratch resistance' department.

[Fraser]

Germanium is a hard material and when covered in a “diamond like” AR coating, it is very resistant to scratches. Such lenses are used in fire fighting thermal cameras.

ZnSe is a relatively soft material that is easily scratched, even when AR coated.

The hardness figures for Germanium and ZnSe is available via a Google Search

We then have Chalcogenide IR Glass which is common in thermal cameras these days. I do not know how hard these lenses are but Google should help find specifications for such material.

Do remember that AR coatings are normally used on thermal imaging lenses and these may be damaged by poor handling and cleaning practices. The “Diamond like” AR coating used on high end thermal camera optics is very resistant to damage but all AR coatings are vulnerable to damage if abused or exposed to chemicals/water for long periods of time. AR coating delaminating can result from inappropriate handling and long term storage.

Fraser

[Fraser]

BK7 Crown Glass Knoop HK kg/mm2: 610
Germanium Knoop HK kg/mm2 : 800
ZnSe Knoop HK kg/mm2 : 105
GASIR Chalcogenide IR glass Knoop HK kg/mm2 : 155

Take a look at page 5 here…..

https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/10/Optomechanical-Properties-ofInfrared-Materials-desroches.pdf

Fraser

[Liam G]

Much thanks