Displays are proliferating around the globe. We are increasingly surrounded by tablets, computers, e-readers,smartphones, interactive kiosks, digital signage, touchscreen appliances, smart watches, televisions and more.
This display landscape is made up of myriad materials, technologies and complex engineering, from the organic layer of OLED displays to the liquid crystals in LCD screens to nanoscale quantum dots. This article will outline the taxonomy of displays, how each display type works and their differences.
Emissive Vs. Non-emissive
Illumination is the common thread among all electronic displays as there has to be some light source to create the digital images which a user sees on the device screen.
A key distinction in display types is between those that are based on technologies which are non-emissive, so they rely on a separate light source, and on emissive technologies which produce their own light.
Each pixel in the display screen is an emitter for emissive displays, an element which outputs light when electric current is applied. For instance, an OLED display is made up of millions of tiny diodes which generate red, green, blue or white light, which combine to create the images on the screen.
Emissive displays, where each LED (light emitting diode) is a pixel, include multiple types:
- MicroLED displays are emerging into the market, currently found mainly in ultra-high-resolution, high-end televisions but are growing in popularity because of their visual performance which exceeds OLED
- LED displays like large-format video screens and outdoor signage
- OLED displays (organic LED, including AMOLED, active matrix and passive matrix, PMOLED) have become a common technology for computer and smartphone display screens of today because of their vivid colors, brightness and energy efficiency
LEDs, and their smaller cousins miniLEDs, are also utilized in non-emissive displays, for example, LCDs as the backlights which illuminate the display pixels from behind.
LCD Display Layers
OLED Display Layers
The structure of a typical non-emissive LCD (top) where LEDs are used for backlight illumination of subsequent layers, and an emissive OLED (bottom) where the “emitting layer” of organic LEDs creates light for the high-resolution images that are viewed on-screen. Image Credit: Top image: FlatpanelsHD, bottom image: Android Authority.
The popularity of emissive displays like OLED is because of the quality of the visual experience they supply. The majority of emissive displays are Lambertian emitters, meaning luminances from different viewing angles are the same, leading to a wide viewing angle performance.
These displays can be employed in low ambient light conditions due to self-emissive characteristics. As they are completely dark when turned off, emissive displays usually possess extremely high contrast ratios.1
Another emissive display type which utilizes tiny colored fluorescent pixels (red, green and blue) to form the illuminated images on screen is Plasma Display Panels (PDP). The fluorescent light comes from plasma and is generated by a gas that is excited by electric charge.
Vacuum Fluorescence Displays (VFD, now obsolete) and Field Emissive Displays (FED) are also part of the emissive display category. Quantum Dot (QD) displays are known as photo-emissive.
A blue (or UV) backlight irradiates a QD semiconductor nanocrystal layer and causes the dots to emit pure basic colors. Electro-emissive (or electroluminescent) QD displays which utilize QD light emitting diodes (QDLED or QD-LED) are still in the experimental phase.
LCDs and Non-Emissive Displays
Most non-emissive displays are liquid crystal displays (LCD). A layer of liquid crystal molecules is sandwiched between two thin layers of polarized glass with a light source, for example, a reflector or backlight panel which illuminates the pixels.
LCD displays are able to operate via three different illumination configurations, making them suitable for a large variety of ambient light conditions which are outlined below.
Ambient light supplies illumination. Usually, a mirror is located behind the liquid crystal layer which receives, and then reflects, light back through the LCD. These displays have the benefit of being highly readable in outdoor settings, even in bright sunlight and they have low energy use.
LCD displays can be reflective, for instance, liquid crystal on silicon (LCoS) panels, but electronic paper (e-paper) displays are the most common application of reflective lighting today.
The illumination source is reflected ambient light, but the images on an e-paper display are produced by manipulating electrically charged black and white particles (or more recently, colored particles).
Light from a backlight source passes through the LCD and the LCD panel or glass acts as an ‘optical switch’ where, depending on the orientation of liquid crystal molecules, light from the backlight passes through the LCD cell. An electrical field can be used to ‘switch’ the orientation on or off.2
Backlights make the display image bright by generating a lot of light. Yet, as they are always “on” even if there is no image content being displayed (for instance, a television that is turned on but displaying a black screen), traditional backlights also utilize a huge amount of energy.
This utilizes a combination of both transmissive and reflective light sources. For instance, an LCD with a reflective layer with a hole for each pixel to reflect ambient light when needed, and a semi-transparent reflective layer which enables the backlight to shine through when required.
This allows the display to switch from transmissive mode to reflective to optimize image visibility depending on ambient light conditions, for instance, from nighttime to daytime.
Comparing the light source mechanism of reflective, transflective, and transmissive displays. Image Credit: New Vision Display.
Other non-emissive displays work by utilizing a variety of lighting mechanisms, including:
- Diffraction – Diffractive displays do not need a surface or panel to show image content, unlike other displays. Holographic images can be projected (for example, by lasers) into the air, exciting the molecules so that light is diffracted into 3D shapes which can be viewed from any angle.
- Absorption – Some transparent displays utilize an absorptive approach. Liquid crystals are transparent, so light in varying intensities can be projected on a clear LCD panel to generate images. Some transparent head-up display (HUD) and augmented reality (AR) screens work in this way.
- Scattering – Some transparent displays are created with nanoparticles which scatter light selectively to produce images. For this type of display, applications can include HUDs and retail windows. For instance, the application of special light-scattering clear film can turn any glass surface into a display.
Lynx Fitness Club
A light-scattering display from Lux Labs that uses a window to show a series of images. Video Credit: Lux Labs
A visual taxonomy of display screens; the different lighting mechanisms are in orange. LCDs are most often transmissive, reflective, or transflective. E-paper displays also use reflective illumination. Transparent displays can employ absorptive techniques, scattering techniques using LCD panels with an absorptive light mechanism, or they can use emissive display elements. Holographic displays typically rely on light diffraction to create 3D images. Image Credit: Radiant Vision Systems
Reflective Display Technology for Vision Health
For a number of applications, display technologies like OLED are popular in part because of their contrast and brightness (for visibility in a range of ambient light conditions) and their vivid color rendering.
Yet, some medical experts are concerned about the influence of bright light on human health, particularly blue light. Issues like interruption of circadian sleep patterns and eye strain have been attributed in part to display screens.
Reflective technology being less tiring for our eyes is one reason that e-paper displays have become more popular in the electronic reader market, for example, in Kindles and Nooks.3 Human vision has evolved to perceive the light (e.g. from the sun) reflected off surfaces as objects.
For instance, a red apple looks red to humans because only the red spectrum of wavelengths are reflected off the apple into our eyes and all other wavelengths of light (yellow, green, indigo, etc.) are absorbed. Just like the printed paper of a book, E-paper displays reflect light to our eyes and so, are a lot more natural for our eyes to perceive.
The new Onyx BOOX Note Air e-reader device with e-paper display and touchscreen note-taking capabilities. Image Credit: © Onyx.
There are also new reflective LCD (RLCD) displays in development. They benefit from cost-effective and versatile LCD technology with its well-established fabrication supply chain but utilize reflection (for instance, from front-lighting technology) to mitigate the amount of bright light shining directly into our eyes.
Ensuring Display Brightness, Color, and Visual Performance
Human ingenuity has brought us a wide variety of illumination methods and display technologies. Each has its strengths and weaknesses and most are suitable for a range of display applications.
A common factor for all electronic displays is a prerequisite for visual performance: all displays must present digital content to a viewer in the most legible and clear form, while balancing considerations like fabrication costs and energy efficiency.
During development and production, display manufacturers rely on visual inspection to make sure that display performance and quality adhere to customer expectations and brand standards.
Automated visual inspection systems which use photometric imaging supply calibrated luminance, chromaticity and defect data for any type of display at speed, for the highest level of precision and efficiency.
Whatever technology a display device uses, Radiant has the expertise and a solution to measure it. They have supplied industry-leading software and hardware solutions for automated visual inspection of displays for thirty years. These include:
- LCD televisions
- Flat panel monitors
- Transparent HUDs
- Curved automotive displays
- Virtual reality headsets
- Emissive displays down to the pixel and subpixel level
The versatile ProMetric® Imaging Photometers and Colorimeters from Radiant are available in a variety of high-resolution sensor choices which optimize speed and accuracy for automated display inspection on production lines and in labs.
When combined with their TrueTest™ Software, their ProMetric solution permits display makers to measure and correct uniformity, identify defects and assess multiple visual performance parameters quickly and easily to ensure the end product supplied the user experience desired.
Radiant’s display inspection solutions include (left to right) ProMetric® I Imaging Colorimeter, ProMetric Y Imaging Photometer, TrueTest Software (with multiple modules for special unique applications such as AR/VR devices and automotive displays), and ProMetric I with our FPD Conoscope Lens to evaluate display view angle performance. Image Credit: Radiant Vision Systems
- Roehrig, H., “Displays Chapter 2: Physical Characterization & Image Quality.” Displays for Medical Imaging, published by the Society for Imaging Informatics in Medicine, Krupinsky, E. Editor. (Retrieved July 15, 2021)
- “What LCD Modes Mean: Reflective, Transmisssive, Transflective.” New Vision Display, March 14, 2017.
- Casper, M., “Developments in New IT Products Using Reflective LCDs.” Presented at Society for Information Display (SID) Display Week 2020 Business Conference.
Produced from materials originally authored by Anne Corning from Radiant Vision Systems.
This information has been sourced, reviewed and adapted from materials provided by Radiant Vision Systems.
For more information on this source, please visit Radiant Vision Systems.