Light splitting plastic film in phone display screen

Question

I was recently taking apart a button-press phone (like this one) and right under the glass protective screen was a thin plastic film.

This is what happens when I place it over a single dot:

As far as I can tell:

• The light that passes through the film is separated into two beams, inclined at the same magnitude but in opposite directions to the normal.
• The intensity of each beam is halved
• The film is striated in a particular direction, and the light splits into two directions, perpendicular to the direction of striation.
• At first I thought that this might be caused due to birefringence, however, I have no idea why a phone display should contain a birefringent layer. Edit: If the two beams that split were of orthogonal polarizations (as birefringence would imply), then passing one of the split beams through another similar film, we shouldn’t see any splitting again. However, it does split again when I tried to do so. I think birefringence can be ruled out.
• After thinking about the striations, I think that this behaviour could also be because of wavy curvature on the plastic surface; half of the light refracts in one direction and the other half in another.
• Such phones generally have TFT-LCD or IPS-LCD screens.

Can anyone think of a better explanation for why this might be happening? Also how such a layer could be useful for a phone display?

EDIT:

After going through the remains of the phone display again, I found another plastic film which shows the same behaviour.

Torch light split by single film:

After both films were placed (one upon another, as placed originally on the phone display):

Answer 1

These are prism sheets that commonly found in LCD displays to direct light perpendicular to the screen towards the viewer. (As noted in this video on "The Crazy Physics of LED Displays", such sheets are also found in LED displays.) A prism sheet is like a lenticular lens (shown below), but with prismatic instead lens lines. These are the striations you noticed.

Since a single prism sheet only works along one axis, there are often two sheets (as you found) with the prism lines at right angles to each other to direct light from both horizontal and vertical screen axes towards the viewer. According to a Society for Information Display article on "Getting the Light Through: TFT-LCD Optical Films", a single prism sheet can increase brightness by 60%, and two sheets by 120%.

A point light source (or dark spot) will be split into two when viewed through a prism sheet. For example, here are some light rays traced through a single 45° tip angle prism sheet using the https://phydemo.app/ray-optics simulator. (The actual simulation is here if you want to modify it yourself.)

If your eye is at point D$1$, you will see images of light source at points H$1$ and L$1$ on the sheet. If you move your eye farther away, such as at point D$4$, the images would move closer together to be seen at points H$4$ and L$4$. For this sheet, the minimum separation between the images is roughly the distance between the light source and the sheet (d), so that will be the separation if your eye is even farther away. If you move your eye closer to sheet (e.g. D$0$ in the open area), the light source should disappear since the light rays are scattered off to the side. This is similar to how lenticular sheets are used to make the invisibility shields that appear in many YouTube videos.