OPTICAL TERMINOLOGY

MATERIAL TRANSMISSION VS TRANSMITTED WAVEFRONT
The portion of the light that meets a glass substrate that is not reflected or absorbed is transmitted (passes through) the glass. Transmission refers to the percentage of light at a given wavelength and angle of incidence that is transmitted through a glass substrate. Transmission curves for typical optical window substrates are available at https://www.advancedoptics.com/optical-materials.html to help with the selection of a suitable substrate.

TRANSMITTED WAVEFRONT
Is expressed in waves just as surface flatness. However, one must be careful to not confuse transmitted wavefront and surface flatness. Transmitted wavefront measures how much the light path is distorted as it passes through an optical window and is a function of surface flatness of both sides of the optic, the purity and homogeneity of the material as well as the parallelism. Transmission measures how much light is passing through a substrate at a given wavelength.

REFRACTIVE INDEX
In optics, this is the number indicating the ratio between the speed of light in a vacuum to the speed of light in an optical medium such as an optical glass substrate. It is a measurement used to show the extent to which an optical substrate bends and therefore slows down light waves passing through an optical substrate. It is expressed as a number using Snell’s Law of refraction. The higher the index of refraction of an optical glass, the more the light will bend at a specific angle where the light comes in contact with the optical surface.

ABBE NUMBER
The Abbe number, also known as the V-number, is a measure of a materials light dispersion (dispersion is the scattering of light at different angles into its component colors) in relation to the refractive index with wavelength. Abbe numbers are used to classify optical materials. A high Abbe number indicates the material will disperse less light.

COEFFICIENT OF THERMAL EXPANSION
The coefficient of thermal expansion is a number that describes how the size of an object changes with temperature. Softer materials exhibit a higher CTE whereas harder materials exhibit a low CTE. This is important when choosing a substrate that will be exposed to extreme temperatures and/or fast temperature fluctuation.

KNOOP HARDNESS
Knoop hardness is defined as the ability of a substrate to resists indentation. A Knoop hardness number (HK) is based upon the force divided by the projected area of the indentation. Test loads are typically in grams-force (gf) and indentation diagnosis are in micrometers (µm). A rhombic-based pyramidal diamond is pressed into a substrate with a fixed force. The resulting depth of the indentation is measured to provide an HK number. Higher HK numbers indicate a harder material that has left a smaller indentation. Likewise, brittle materials will have a smaller HK number.

DENSITY VS OPTICAL DENSITY
The physical density of a glass substrate determines how much the optical component weighs which may be a consideration for an overall optical assembly. The optical density of the medium refers to the sluggish tendency of the atoms of a material to retain the energy absorbed from the electromagnetic wave in the form of vibrating electrons before being reemitted as an electromagnetic disturbance. The speed of electromagnetic waves in a material with a higher optical density will be slow causing the material to absorb more light. The higher the optical density of a material, the more it will absorb light.