Behaviour of e-paper with temperature

As part of our contract work and internal research and development, we have made several detailed measurements of the temperature-dependent behaviour of low-power LCD - so-called e-paper. For products of this type of LCD, a significant technological shift can be seen over time, especially towards its applicability towards negative temperatures. In the two attached videos you can compare for yourself.

Today's status - usable down to -15 C

When we started experimenting with this imaging medium in 2017, we were very disappointed with it (see the section "2017 status″ and the attached video below). It turned out to be almost unusable in negative temperatures, or it must "melt" inside when redrawn in winter. At that point, its use as a low-power medium loses its meaning.Subsequently, when we implemented a project of 25 e-paper stop signs in 2020, it was already clear that new e-paper production technologies have advanced (improved) the behaviour of e-paper in negative temperatures by as much as -5 to -10 °C. i.e. the data from the video below is valid for -5 °C up to at least -10 °C.

The latest technology from late 2021 already demonstrates that the temperature limits of working with e-paper displays have shifted considerably and as shown in the opening image, even at -15 C the display will redraw to a "readable state" - see video of testing the new e-paper markers - see below. It is clear from the video that although the time and therefore the energy required to redraw the display is relatively longer at negative temperatures, this type of media becomes useful for passenger information.

Example of display redrawing at -15°C from 2022:

2017 status

At the time, the prevailing opinion was that e-paper as a low-power medium was capable of fully replacing real paper. These views were valid for temperatures above -5 °C. As part of the ITIS II research task, we have carried out a verification of these properties together with the Brno University of Technology - Institute of Telecommunications , which is presented below. At that time, these results indeed showed the difficulty of use.

The measurements were carried out in a temperature chamber at various temperatures from -15 °C to + 80 °C and this was recorded on video. The aim of the test was to play back the plain text displayed on the screen and change this to images converted to 16 degrees greyscale (beginning of the video). For the measurements we used an e-paper display - type ES133UT2 with the following result:

• -15 °C - The display is completely "frozen". Attempting to change the image on the LCD is not possible even after many attempts of rewriting - in this case the e-paper becomes UNUSABLE !!! The changed image is only displayed in the background with a certain probability and transparency.
• -10 °C - The display is backed up by the overwritten image and this repeated overwriting helps. It is necessary to do many rewrites in succession to get the image to a state where the newly rewritten image is more legible than the original one (in this case the text). Again, it can be stated that the e-paper becomes UNUSABLE for this temperature.
• -5 °C - The display is redrawn with the newly transcribed image. After several redraws, the new image becomes readable. The LCD is no longer usable for this temperature, however, a longer than normal redrawing time should be assumed.
• 0 °C to +60 °C - The display is already working normally
• +70 °C - The e-paper display is already starting to error - white areas do not return to bright white and the edges of the letters are blurred. A slight "ghost" of the original image also appears on the LCD (transcription not completed), but refreshing the image removes it (re-transcription). Therefore, if the e-paper is placed in a summer sunset and contains multiple dark spots, the display may overheat and begin to exhibit this error.
• +80°C - The heat inside the chamber has melted the plastic cover and the display has slipped down. Most of the display area turned gray, new image rendering did not help. Most of the display is difficult to read.

A sample of the above is shown in the following video:

From the above, the e-paper of the time was not really suitable in the freezing winter months for regular transcription without heating the LCD to around 0°C and for panels that will receive direct side sunlight in the summer months around 5pm where there will be a lot of black on the LCD. However, if we need to update the e-paper display regularly in freezing weather (on-line operation), we will need to heat it and at that point we need to have power to heat it - hence the display is no longer "low power"at this time. If we update it once in a "while", e.g. when changing schedules, then it can be heated for a short time , the display overwritten and then the heating activity stopped. For this solution, e-paper was suitable in our climatic conditions even then.