What is knoop hardness for lens?

Knoop hardness is a measure of a material's hardness, specifically used for brittle materials like ceramics and glass, including lenses. It is determined by pressing a diamond indenter with an elongated pyramidal shape into the material's surface under a specific load, creating an indentation. The hardness value is calculated based on the length of the diagonal of the indentation, allowing for a precise evaluation of the material's resistance to deformation. This property is important for assessing the durability and scratch resistance of optical lenses.