Reiji Murai, Reuters:
Two supply chain sources said display panel production suffered a setback after the backlight that helps illuminate the screen had to be revised, putting screen assembly on hold for part of June and July. One said Apple, aiming for the thinnest phone possible, initially wanted to cut back to a single layer of backlight film, instead of the standard two layers, for the 4.7-inch screen, which went into mass production ahead of the 5.5-inch version.
But the new configuration was not bright enough and the backlight was sent back to the drawing board to fit in the extra layer, costing precious time and temporarily idling some screen assembly operations, the source said.
According to Murai who quotes “the sources” the display manufacturers supplying the screens for the iPhone 6 are Japan Display Inc. (JDI), LG Display (LGD), and Sharp.
There are many types of backlight films but those that specifically enhance brightness are BEFs or Brightness Enhancement Films.