Material properties of Optical window

Surface quality:

The surface quality of an optical window can be defined as an evaluation of the imperfection present on the surface that may be caused during handling or manufacturing. These defects typically result in a small decrease in throughput and a small increase in scattered light, which has little or no adverse effect on overall system performance in most imaging or light-gathering applications.

Although there are some types of surfaces that are more sensitive to these types of defects which includes surfaces at image planes because surface defects are in focus.

Windows that have high strength levels are also vulnerable in nature to surface defects as they can increase the absorption of energy and damage the window. The precision optical manufacturing companies available in the market uses optical lensing mirrors and laser optics windows for the production of mirrors.

The scratch-dig specification present in the U.S. Standard MIL-PRF-13830B is used to describe the surface quality. We can determine the scratch designation by comparing the scratches present on a surface to a set of standard scratches under controlled lighting conditions.

It is not a direct measure of the dimensions of the scratch. On the other hand, the designation of the excavator is directly related to the size of the excavator. We can easily calculate the dig designation simply by taking the diameter of the dig in micron-scale and then dividing it by 10.

Surface Flatness:

Surface fatness is simply the measure of the deviation of the window from a perfectly flat surface. The surface flatness of a test piece can be measured using an optical flat, which is a highly accurate flat reference surface.

Once the surface of the test is placed against the optical flat, then fringes start appearing whose shapes dictate the surface flatness of the window under the inspection.

The surface of the window should be as flat as the reference flat if the fringes are equidistant apart, straight, and parallel. If the fringes are curved, the flatness error is indicated by the number of fringes between two imaginary lines: a tangent to the center of a fringe and a tangent through the ends of the same fringe.

Usually, the value of waves is used to measure the deviations in flatness or multiples of the testing light source’s wavelength. Each fringe corresponds to a half-wave. 1λ flatness can be used for specific applications, but high precision applications such as high-power laser systems require flatness values up to divided by 20.