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Originally Posted by mobilewilier
i recall that amazon said that it perforates the bottom side of the light sheet to make the light diffract downwards. If there are inaccuracies in execution this might lead to a non-uniform diffraction/distribution of light and also coloration.
Didn't the Sony PRS-505 have a more rudimentary lighting sheet which just has light shine through the sheet? Maybe Kobo glo and Nook Glowlight use a more basic approach, too? Just speculating.
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The approach does seem to be "simpler" in the Kobo Glo and the Nook Glowlight. The main visible difference is that the PW looks like the eInk page "glows" while the others look like they are being "lit" by a layer above the eInk. The effect of a glowing page with a front light is tricky. It's almost like looking at a backlit display with the paperwhite.
You still need diffraction grating or something similar to get it to work in both cases. But the imprinted design you choose is fairly important to get the effect you want. Different designs can look fairly different in practice. But I wouldn't be surprised at this point if some designs introduce chromatic aberration. It's not that uncommon to deal with it in optical design.
Inaccuracies may not even factor in, or be due to tolerances that are so tight that they are having problems scaling up manufacturing while still meeting those tolerances. The reality is in optics, you can't simply make everything perfect. Light will do what light will do, and as an optical engineer, you have to make the right set of trade-offs. For example, it's pretty much impossible to create a 1 or 2 lens refractor telescope that is "free" of chromatic aberration unless the focal length is very long.