Last to come off of the display is the edge-lit LED backlight—a common type of backlight found in smartphones.
An edge-lit backlight, like the name suggests, only has LEDs on one edge of the backlight. Light from these diodes is spread evenly over the screen by the diffuser—a multi-layer, semi-transparent plastic sheeting.
As mentioned earlier, the reflective mylar layer was previously one of the layers of the backlight.
In this iteration of the iPhone the mylar layer now serves two purposes; the new use, to obscure the 3D Touch sensors from view, and the traditional use, to reflect diffused light outward instead of wasting light on the internals of the iPhone.
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