ePaper Displays

Because a shelf label changes a few times a day and then has to sit there. ePaper draws power only during an update, so a label can run for years on a coin cell, where an LCD would need its backlight or at least its drive electronics powered continuously. ePaper is also readable from any angle under store lighting and looks like a printed tag.

The trade-offs that matter in that application are update speed, a limited colour palette, and behaviour at low temperature, which is why chilled and frozen aisles need parts rated for it.

Start with how often the screen changes.

ePaper holds an image with no power at all and only draws current when it updates. If the display shows a reading that changes every few seconds or less often, ePaper beats everything else on battery life by a wide margin, and it is the most readable of the three in sunlight. It cannot do video, updates take a noticeable fraction of a second, and color options are limited and slower still.

OLED / PMOLED is self-emissive, so its power depends on how many pixels are lit and how brightly. For sparse content on a black background it is very efficient. For a mostly-white screen it is not. It gives excellent contrast and a wide viewing angle and needs no backlight; its weaknesses are limited lifetime at high brightness and image retention with static content.

TFT LCD is the answer when you need full color, a real interface or moving content. Power is dominated by the backlight, which is on whenever the display is on regardless of what is shown. A transflective TFT recovers much of the outdoor readability and some of the power.

A useful test: if the device spends most of its time showing the same thing, use ePaper. If it shows a little bright information on a dark screen, use OLED. If it shows a colour interface, use a TFT.