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Displays have proliferated globally. We are increasingly surrounded by tablets, computers, e-readers, smart phones, interactive kiosks, digital signage, touch screen devices, smart watches, televisions, etc.

This display landscape is composed of countless materials, technologies, and complex engineering, from the organic layers of OLED displays to the liquid crystals in LCD screens to nano-scale quantum dots. This article will outline the classification of monitors, how each type of monitor works, and their differences.

Lighting is common to all electronic displays, because there must be some light source to create the digital image that the user sees on the device screen.

A key difference in display types is the difference between display types based on non-emissive technologies (so they rely on separate light sources) and display types based on emissive technologies that generate their own light.

Each pixel in the display is an emitter of a light-emitting display, which is an element that outputs light when a current is applied. For example, OLED displays consist of millions of tiny diodes that produce red, green, blue, or white light, which combine to create an image on the screen.

Light-emitting displays, where each LED (light-emitting diode) is a pixel, includes many types:

LEDs and their smaller counterparts, miniLEDs, are also used in non-luminous displays, for example, LCDs are used as backlights to illuminate display pixels from behind.

The structure of a typical non-luminous LCD (top), where LEDs are used for the backlighting of subsequent layers, and a light-emitting OLED (bottom), where the "light-emitting layer" of the organic LED produces light on the screen for the high-resolution image being viewed . Image source: Above: FlatpanelsHD, below: Android Authority.

The popularity of light-emitting displays like OLEDs is due to the quality of the visual experience they provide. Most light-emitting displays are Lambertian emitters, which means that the brightness at different viewing angles is the same, resulting in wide viewing angle performance. 

Due to the self-luminous characteristics, these displays can be used under low ambient light conditions. Since it is completely dark when turned off, light-emitting displays usually have extremely high contrast. 1 

Another type of light-emitting display that uses tiny colored fluorescent pixels (red, green, and blue) to form an illuminated image on the screen is a plasma display panel (PDP). Fluorescence comes from plasma and is produced by gas excited by electric charges. 

Vacuum fluorescent displays (VFD, now obsolete) and field emission displays (FED) are also part of the emissive display category. Quantum dot (QD) displays are called light emission. 

The blue (or ultraviolet) backlight illuminates the QD semiconductor nanocrystal layer and makes the dots emit a pure primary color. Electrical emission (or electroluminescence) QD displays using QD light-emitting diodes (QDLED or QD-LED) are still in the experimental stage. 

Most non-luminous displays are liquid crystal displays (LCD). A layer of liquid crystal molecules is sandwiched between two thin layers of polarized glass with light sources, such as reflectors or backlights that illuminate pixels. 

The LCD display can be operated with three different lighting configurations, making it suitable for a variety of ambient light conditions, as described below. 

Ambient light provides illumination. Usually, the mirror is located behind the liquid crystal layer, it receives the light, and then reflects the light back to the LCD. The advantage of these displays is that they are highly readable in outdoor environments, even in bright sunlight, and their energy consumption is very low. 

LCD displays can be reflective, such as liquid crystal on silicon (LCoS) panels, but electronic paper (e-paper) displays are the most common application of reflective lighting today. 

The illumination source reflects ambient light, but the image on the electronic paper display is produced by manipulating charged black and white particles (or the nearest colored particles). 

The light from the backlight passes through the LCD, and the LCD panel or glass acts as a "light switch", where the light from the backlight passes through the LCD unit according to the direction of the liquid crystal molecules. The electric field can be used in the "open" or "close" direction. 2 

The backlight brightens the displayed image by generating a lot of light. However, since they are always "on" even if the image content is not displayed (for example, the TV is turned on but displays a black screen), the traditional backlight also consumes a lot of energy. 

This utilizes a combination of transmissive and reflective light sources. For example, LCDs have a reflective layer, each pixel has a hole to reflect ambient light when needed, and a semi-transparent reflective layer that allows the backlight to pass through when needed. 

This allows the display to switch from transmissive mode to reflective mode to optimize image visibility based on ambient light conditions (for example, from night to day).

Compare the light source mechanisms of reflective, transflective, and transmissive displays. Image source: New visual display.

Other non-luminous displays work by using various lighting mechanisms, including: 

A light scattering display from Lux Labs, which uses a window to display a series of images. Video source: Lux Labs

The visual classification of the display; the different lighting mechanisms are orange. The most common LCDs are transmissive, reflective or transflective. E-paper displays also use reflected lighting. Transparent displays can use absorption technology, scattering technology using LCD panels with a mechanism for absorbing light, or they can use emissive display elements. Holographic displays usually rely on light diffraction to create 3D images. Image source: Radiant Vision Systems

For many applications, display technologies like OLED are popular, in part because of their contrast and brightness (visibility under a range of ambient light conditions) and vivid color rendering. 

However, some medical experts worry about the impact of strong light on human health, especially blue light. Problems such as interrupted circadian sleep patterns and eye fatigue are partly attributed to the display. 

Reflective technology is not so tiring for our eyes, which is one of the reasons why e-paper displays are becoming more and more popular in the e-reader market (such as Kindle and Nooks). 3 Human vision has evolved to be able to perceive reflected light (such as light from the sun) leaving the surface as an object. 

For example, a red apple looks red to humans because only the wavelengths of the red spectrum will be reflected from the apple to our eyes, while all other wavelengths of light (yellow, green, indigo, etc.) are absorbed. Just like the printed paper of a book, electronic paper displays reflect light to our eyes, so our eyes can perceive it more naturally.

The new Onyx BOOX Note Air e-reader device has an e-paper display and touch screen note-taking functions. Image source: © Onyx.

There is also a new reflective LCD (RLCD) display under development. They benefit from the cost-effective and versatile LCD technology and its sophisticated manufacturing supply chain, but use reflection (for example, from front lighting technology) to reduce the amount of glare directly hitting our eyes. 

Human ingenuity has brought us a variety of lighting methods and display technologies. Each has its advantages and disadvantages, and most of them 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 the audience in the clearest and clearest form, while balancing considerations such as manufacturing costs and energy efficiency. 

During the development and production process, display manufacturers rely on visual inspection to ensure that display performance and quality meet customer expectations and brand standards. 

An automatic visual inspection system using photometric imaging can quickly provide calibration brightness, chromaticity, and defect data for any type of display to achieve the highest level of accuracy and efficiency. 

No matter what technology the display device uses, Radiant has the expertise and solutions to measure it. For thirty years, they have been providing industry-leading software and hardware solutions for automatic visual inspection of displays. These include:

Radiant's multifunctional ProMetric® imaging photometer and colorimeter offer a variety of high-resolution sensor options to optimize the speed and accuracy of automatic display inspections in production lines and laboratories. 

When combined with TrueTest™ software, their ProMetric solution allows display manufacturers to quickly and easily measure and correct uniformity, identify defects, and evaluate multiple visual performance parameters to ensure that the final product provides the desired user experience.

Radiant's display inspection solutions include (from 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 viewing angle performance. Image source: Radiant Vision Systems  

Made of materials originally created by Anne Corning of Radiant Vision Systems.

This information is derived from materials provided by Radiant Vision Systems and has been reviewed and adapted.

For more information on this source, please visit Radiant Vision Systems.

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