Advantages of Electronic Paper Display (EPD)

Electronic Paper Display (EPD) is a screen display which uses electronic ink molecules to form images on a plastic panel. Its mechanism is a little bit similar to a printer which gradually sprays ink molecules line by line onto a paper , but a much faster one. It uses electric field to control the position of ink molecules so images can form almost immediately. Therefore, EPD is more like a reusable printed matter rather than a screen display. Liquid Crystal Display (LCD) is the kind of screen which we encounter every day. It's widely used for television, computer and mobile device displays. It uses light of primary colors to form images and project them directly into our eyes.

EPD has several advatanges when compared to LCD:

• Protecting our vision.

There are three reasons why EPD can protect our vision:

• EPD images are formed by reflected light, not by direct light sources. Human eyes should not recieve light with great intensity. Thus if we stare at the sun or light bulbs for just a short period of time our vision will be impaired; much like our hearing will be impaired if we stay in a loud environment for a long time. And the intensity of reflected light is always less than direct incident light if we keep the light source at the same traveling distance. (With the exception of a polished metal surface, which the intensity is almost the same.) Among visible light, blue light has higher energy per photon and therefore we can ruduce the amount of blue light if light is reflected before entering our eyes.
• EPD avoids using light intensity greater than reflected ambient light. Although both the screen surfaces of EPD and LCD can reflect ambient light, the surface of LCD will reflect images of the incident light while that of EPD does not. Because LCD uses a polished glass panel to protect its contents while allowing its backlight to be transmitted through, the surface reflection of LCD is mostly mirror-like reflection (specular reflection). EPD does not need a transparent covering to protect its contents and it ususally uses plastic panel instead. Plastic panel of EPD has a paper-like surface characteristics and surface reflection of EPD is mainly diffuse reflection. Due to the above-mentioned difference, LCD needs a backlight intensity stronger than the reflected ambient light to prevent interference by the reflected images, otherwise we will see nothing about the content but the reflected images.

Video demo: The stronger the ambient light, the higher the intensity of backlight used by LCD

• No flicker. Flicker is the phenomenon that light intensity changes regularly and periodically. Medical studies have shown several adverse effects such as headache, dizziness, nausea, blurred vision and eye fatigue when human eyes are exposed to low-frequncy flicker. It can also induce a seizure to people with epilepsy. Although currently medical studies can not prove that flicker can cause any permanent health issues, it is known that the retinal cells of human eyes can detect flicker frequency up to 200 Hz. Most recent studies have shown that it is safer to stay in a flickering environment with frequency above 800 Hz if long time exposure is necessary. IEEE (Institute of Electrical and Electronic Engineers) has made a recommendation of flicker frequency above 1000 Hz for safety. LCD products do not always have flicker issues because most of the time the backlight intensity is modulated by the liquid crystal alignment. But some brands use pulse width modulation (PWM) to modulate light intensity when the intensity is lower than 10 to 20 percent of the maximum; flicker issues thus arise. Newer generation high-end mobile devices such as iPhone 12 use OLED screen. OLED with no exception uses PWM to modulate light intensity. Although the flicker frequency of OLED screen is 240 Hz, it's still far below the recommended value, let alone future screens which utilize Mini LED or Micro LED to achieve a better image quality. We can't make sure if LCD and LED manufacturers will strictly follow the recommendation. In contrast to LCD or LED screens, EPD products by no means have flicker issues because light intensity of EPD is controlled by the ambient light and color brightness is adjusted using different concentrations of ink molecules. Although the color saturation of EPD products can't be too high, images produced in this way have a unique style, much more like a piece of newspaper or a painting.  Despite the fact that there's no definitive conclusion about the long term effects of flicker on our health, it's safer to use a flickerless screen. I think no one wants to be a guinea pig under the circumstances that the incidence of eye diseases is increasing every year. Humans use ink to write and read for more than a thousand years; this method seems more reliable than using a light-emitting screen.***

Video demo: iPhone 12 screen filmed by slow motion shooting, showing the phenomenon of screen flicker.

Warning: this video may cause discomforts due to low-frequency flicker!

• Energy conservation 

• EPD only consumes electricity when there's a need to change the position of the ink molecules. On the contrary, LCD or other light-emitting screens keep consuming energy to produce light. Because of this difference, it's quite inappropriate and energy wasting  to use LCD for some applications:
• Digital note-taking, writing and word processing.
• Reading. (Either using a web browser or a reading software)
• Electronic labels.
• Posters, advertisements, billboards, bulletin boards and public transport information boards and timetables.
• Digital photo frames.
• Digital blackboards and whiteboards.

These applications listed above should be displayed by EPD either due to eye protection or energy conservation. 

• It's quite normal for the battery of an EPD device to last for days and sometimes even a week if you don't turn on WiFi and Bluetooth communications and turn off background app runtime. Since the global trend is reducig carbon dioxide and lower energy consumption, EPD is definitely a green technology for sustainable development and a display technology which humans should rely upon in the future.

iPhone 7 screen under microscope (100x), the sceen is made of light-emitting units.

Printed matters under microscope (100x), the ink molecules were sprayed onto the paper surface.

Electronic Paper Display under microscope (100x). Though the ink molecules are covered by a layer of plastic film, this picture is similar to that of printed matters.

• Content of EPD is still visible under strong ambient light. 

• As mentioned above, LCD needs a backlight intensity stronger than the reflected ambient light to prevent interference by the reflected images. If the backlight can't be stronger, then what you can see is the reflected images and the content displayed will be totally invisible. Therefore, you can't read the content of LCD under the sun at noon (and you shouldn't). But you can still read the content of EPD and the content is even brighter because it uses reflected light. (It's still recommended not to read under sunlight.) Here I want to share a personal experience. I visited Boston Longwood area in 2018. At that time some of the bus stop information boards and timetables had been changed to EPD and some of them were still LCDs. I came to one of the bus station in the afternoon when it's facing the sun. I was quite impressed that information displayed on those LCD monitors couldn't be seen while that on EPD monitors was clearly visible. 

Under strong light source, the content of LCD is not visible over the area of strong reflection (glare caused by mirror-like reflection).

Under strong light source, the content of EPD is still visible over the area of strong reflection (glare caused by diffuse reflection)

Test done under different incident light angle.

Test done under different incident light angle.

• EPD is durable. 

• Because EPD doesn’t need a glass panel as a covering but a plastic panel, it has more pleasurable material properties. Plastic panel is not brittle so it won’t break if you drop it on the ground accidentally. Moreover, it can endure pressure and does not break if pierced by a sharp pen.

• EPD is thin and lightweight and if properly designed can be bent.

• EPD module is as thin as a pile of 7 pieces of papers and its weight is equivalent to 15 pieces of A4 papers. On the other hand, merely the glass covering of LCD module is thicker and heavier than the whole module of EPD.

Conclusion

Electronic Paper Display has unique features with obvious advantages over liquid crystal display. Both type of screen have distinct characteristics and when choosing the most suitable screen display, these differences should be put into consideration according to the applications. Recently most operating systems have issued working environments with black background. The core idea behind this change is to help lengthening the battery life of LCD devices and protecting our eyes. However, if the working mechanism of LCD does not change, improvements to these aspects are quite limited. Although EPD is often regarded as a monitor, it's more accurately classified as paper. In 105 A.D. Cai Lun, an court official of the Eastern Han Dynasty, made great improvements to the making and composition of paper. Since then, the modern form of paper has been established. The development of EPD technology will revolutionise the modern form of paper again and it's most likely to be adopted as the next-generation paper for human societies (the documentations of which mainly go digital in recent decades). Only when more people start to care about the development of EPD and start to support EPD will the improvement of this technology be accelerated. In this way, we can lower the speed of deforestation and protect our children's vision.

*** If the ambient light source is fluorescent light or LED, flicker issue may also appear. Because modern electricity system use alternating current, if the bulb suppliers do not use AC-DC converter or design a current stablizer in the circuit, the alternating current will cause flicker. Someone may argue that since the ambient light may flick, why bother to worry about the flickering light of LCD? There may be a reason: one is direct incident light and the other is reflected light. Because our visual system also responds to the amplitude of the light signals, small fluctuation may not make the system to work hard to adjust than big fluctuation. Or there's a possibility that both flickering reflected light and flickering incident light might cause a problem and we just can't find a proper method to prove this yet.

Further Reading:

1. Why Pulse-Width Modulation (PWM) is such a headache
2. Flicker Beyond Perception Limits - A Summary of Lesser Known Research Results
3. New e-paper arrival boards for bus stops
4. Tiny Screens Can Cause Big Vision Problems
5. Computer vision syndrome