How should the display window shielding be designed for HMI (Human Machine Interface) systems?

The design of HMI display window shielding needs to balance light transmittance and electromagnetic shielding effectiveness. A common approach is to fabricate a transparent conductive layer on glass or transparent plastic, with indium tin oxide (ITO) film being the most prevalent. ITO film offers high light transmittance and surface resistivity down to tens of ohms per square meter, effectively attenuating high-frequency electromagnetic waves. For ITO glass, the conductive frame around its perimeter needs to be tightly connected to the metal frame of the device's front housing using conductive tape, conductive foam, or metal springs to form a continuous shield. Another approach is to use a metal mesh to shield the window. This involves weaving extremely fine metal wires into a mesh and sandwiching it between two layers of glass or plastic. This offers even higher shielding effectiveness, but slightly lower light transmittance and clarity.

For non-touch, purely display windows, a transparent shielding film can also be applied to the display surface. During the design process, it's crucial to ensure low overlap impedance between the shielding layer and the housing, and to maximize the overlap perimeter to reduce overlap inductance. If the window size is large, it may be necessary to consider adding overlap points in the middle. Furthermore, the display driver cable enters the device from the window area, which also requires special handling. Typically, the cable needs to be shielded, and the shielding layer must be securely connected to the window's shielding layer and the device housing. By selecting an appropriate type of shielded window and ensuring its reliable grounding, the impact of this largest opening (the display window) on the overall shielding effectiveness of the HMI can be minimized.