An abstract model for DIP came across my mind. It contains four sections viz; Acquire, Transfer, Display and Interpret. This post is belongs to Display section of abstract model. Refer earlier posts for Acquire and Transfer sections. This article deals with the rise of display devices and social factors that helped for the phenomenal rise. This post does not elaborate the principles of display technologies.
Display systems are one of the wonders of the 21st century. Origins of this wonder are blood soaked. Brutal World War II killed and maimed millions of people and destroyed many cities. Scientists of fighting nations, worked day and night to improvise their countries' war machine with new inventions. Two such inventions namely, computing machines and radar display units helped display systems to blossom into a multi-billion dollar industry. After the war, radar display units were modified to show pictures. This resulted in the birth of Television. After the war bulky computers paved the way to personal computers that possess Video Display Unit (VDU) as output section.
In earlier days Cathode Ray Tube (CRT) was “the display technology.” The shadow-mask colour CRT came to the market way back in 1950. Yet most information displays and imaging applications used monochromatic displays until 1970. Microprocessors in arrived in the market mid 1970s and facilitated the introduction of CRT colour displays for computers. The processing power of microprocessors enabled to encode, manipulate colour in these devices. In the next twenty five years, exponential growth in terms of technology and application of colour occurred [1].
An ideal display system is expected to possess the following features, namely, high contrast, maximum brightness, high resolution and lower cost. Moreover, consumer expects larger display, larger colour gamut and better colour saturation [1]. Customer expectations for color display have risen in rapid pace, driving the development of display technologies and allied colour control and image processing algorithms [1]. Till date no display technology possesses such venerated features. Thus diverse colour display technologies have evolved to support wide variety of applications. It is imperative to build taxonomy (i.e. classification) of display systems and technologies to comprehend them.
VDU classification
Classification of Visual display units can be based on duration, number of viewers, technology incorporated and targeted applications. Movie screens are viewed for minimum 90 minutes and in another extreme digital signage are viewed for a maximum of 90 seconds. VDU can be broadly divided into public-viewer, multi-viewer and mono-viewer displays based on number of simultaneous viewers. Display systems can be segmented into the following categories based on underlying technologies used. They are CRT, Digital Light Processing (DLP), Plasma Display Panel (PDP), Liquid Crystal Display (LCD), Light Emitting Diode (LED), e-ink (electronic ink). Targeted applications like entertainment, consumer, automotive, informative etc can be the basis of classification. I strongly feel classification like this is very fuzzy.
As categorization based on number of viewers is not standardized, let us define in the following way. Public-viewer displays are capable of exhibiting the content to more than 100 people at a time. The movie theatre projection systems perfectly fall into this category. Public address systems are used to reach large gathering of audience via loud speaker. Likewise public-viewer display reaches large viewer via projection systems. Multi-viewer display is suited for few people at a time. Television is the best example. As the name suggests mono-viewer displays are suited for single viewers. Instrument panels, automotive panels, Personal Digital Assistant (PDA), e-readers, tablets, mp3 player, cellphone and digital camera display falls into this category.
Display Characteristics
Comparison between display technologies can be performed based on the following parameters; brightness, contrast, field of view, colour gamut, resolution, physical dimension and cost [2]. Brightness is light perceived by the human eye and luminance refers to the amount of light emitted by a source say VDU. Luminance is measured in Candela per meter squared (cd/m2). Dynamic range refers to the luminance difference between white and black pixels [2]. Contrast also refers to the difference in range but with respect to images. But Contrast and dynamic range is used interchangeablely in every day use. I believe contrast is connected to perception of human eye and dynamic connected to luminance (i.e. measurable quantity). Field of view measured in degrees, determines the number of people can view the display device simultaneously. Colour gamut is an enclosed triangle in chromacity diagram. Area of triangle represents the VDU unit's ability to display range of colours. For details refer to the earlier post of this blog [3]. Resolution refers to the number of pixels and provides rough estimates of the height and width of image.
Rapid proliferation of digital technology has affected conventional projection of movies through films. Today digital projection systems are widely used. It is a combination of projector and cinema screen which is made up of silver halide. Digital projectors can be broadly classified into three categories based on technology; DLP, Liquid Crystal on Silicon (LCoS) and Grating Light Valve (GLV). DLP is made up of tiny mirrors which can move -10 to 10 degrees [4]. Small rotation of mirror causes grey scale values. LCoS is similar to LCD and acts as a 'Window Blinds.' Thus passage of light is controlled by LCoS. Size of silver screen will be in the range of 14 meters to 20 meters (diagonal). The lamps used to display pictures on the screens consume around 1kW of power. The resolution of ranges starts from 2K, 4K and reaches 8K. The 2K resolution is equivalent to High Definition Television (HDTV) resolution. The luminance and dynamic range is very much higher than Television systems. Movies are viewed in dark environment. These reasons make 2K resolution is sufficient for the average movie patron. If a patron sit in the front rows and watch movies then he or she can spot the difference between 2K and 4K systems. Patron sitting in last row cannot distinguish the difference unless he or she is a digital cinema expert [citation required].
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| Figure 1. Display device classification. (a) Public-viewer display - Movie screen (b) Multi-viewer display (c) Mono-viewer display - smart phone Image Courtesy: Wikipedia |
Television (TV) is the apt example for multi-viewer display. At present, the preferred technology for production of TV set is CRT. But within a decade, other technologies like LCD or PDP may take over.
- Merits of CRT technology are as follows. (i) It is a century old technology. So, it is very matured. (ii) Display is bright. So, it is not very much affected by stray external lights. (iii) Long life and have high reliability. (iv) Inexpensive. Cost per pixel is lowest among the display technologies. (v) In 1980s, 640 x 480 was high resolution. Today HDTV (1920x1080?) resolution CRTs are available. (vi) Viewing angle is high. This enables multiple people to view the TV comfortably.
- The demerits of CRT technology are as follows. (i) It is very bulky i.e. voluminous. Weight is also high compared to other display technologies. (ii) Size of the screen is limited. Seeing 36” and above is rarity or even a technical wonder. (iii) Consumes high power. Minimum power consumption is 100W per hour. (iv) Emits electromagnetic radiation.
Introduction of HDTV standards (in USA, Japan) created a need for larger display. Next proliferation of digital satellite broadcasting helped to get crisp video signals. Satellite TV channels earmarked for sports and movies created a need for large screen. Conventional large CRT screens spoilt the aesthetics of the room in which it was housed. Slim form factor of new technologies resembled wall hanging paintings. Thus Techno-savvy people preferred to shift their loyalty to LCD and PDP technologies. They were ready to shell out extra money for aesthetics.
The other merits of new technologies are as follows, (i) Rich colour (ii) Consumes less power (particularly LCD) (iii) Suited for high resolution (iv) Viewing angle is high for PDP and relative low for LCD (v) No electromagnetic radiation (vi) Suited for mobile environment (like in car, van, lorry i.e. truck)
Mono-viewer Display
In earlier days, computer screens were the dominant mono-viewer display. Automotive and industrial instruments used mechanical dials only. Rapid automation and digitization of the industry paved way for electronic displays. Rapid increase in oil (i.e. gasoline) price made automobile manufacturers to opt for fuel efficiency. All mechanical controls were replaced with electronic controls and electronic displays were introduced to assist the drivers with timely information (for example GPS enabled car).
Prices of computers were falling and after the advent of Web, non-programmers started using the computers extensively. A huge pool of volunteers put up required content for the Web. The content is in digital form. Internet connects geographically separated computers and enables seamless flow of digital data between them. Thus digital delivery favoured display of information, rather than print form of information [5].
Parallelly, cell phones which were built to transfer speech were improvised to transfer digital data and act as low-end digital cameras. In earlier days, most of the mobile displays were expected to display alphanumeric and limited graphical icons. This was satisfied with monochromatic screens. The requirement for colour screens emerged because of the need of view finder in digital camera and on-board monitors to display captured pictures. The requirement was emboldened by picture phones and embedded digital camera in cell phone [1]. As the mobile phone user base far exceeds the Personal computer user base, large amount of small-screen display are manufactured. Internet enabled smart phones were introduced and size of the screen increased to 5” (pocket size of an adult). These screens have very good resolution to facilitate reading.
Seven inch electronic readers help to house electronic books. E-readers can hold as much as e-books as memory can hold. Font size of text can be customized. Search facility is possible [6]. It is a very handy method for globe trotters to take their favourite book shelf. Thus E-readers made a niche market for itself. The E-readers extensively rely on e-ink technology. It is a bistable technology. So power is not required to display content. Power is used only when a page is turned (i.e. previous or next). It consumes very low power. Present day battery technologies augment e-reader dominance. Full colour e-ink is emerging. Present day e-readers are Sony reader, Amazon Kindle and Barnes & Noble Reader.
Mobile devices need display units that are capable of visibility under diverse illumination environments, small form factor, less power consumption and longer battery life. Under this conditions LCD is the dominant technology. Organic Light Emitting Diode (OLED) is a promising technology [1].
Display of high quality image is a great engineering challenge. The colour gamut of mobile colour displays is compromised compared to TV and computer screens. This is due to the limitations of mobile computing power. Mobile devices have limited processing resources. They are expected to provide ability to handle out-of-gamut colours, contrast stretching and saturation enhancement. This has to be carried out by image processing algorithms [1].
- [1] Louis D. Silverstein, “Color Display Technology: From Pixels to Perception,” The Reporter, vol. 21, no. 1, pp. 1–12, Feb. 2006.
- [2] Paul Anderson, “Advanced Display Technologies,” JISC Technology & Standards Watch.
- [3] A to Z of Digital Image Processing: Abstract DIP Model - Part III (Science of colour) [Online] http://diwakar-marur.blogspot.in/2014/01/abstract-dip-model-part-iii-science-of.html
- [4] A to Z of Digital Image Processing: Digital Cinema Projection Technologies [Online] http://diwakar-marur.blogspot.in/2013/05/digital-cinema-projection-technologies.html
- [5] ADT Michael Kleper, Advanced Display Technologies, A Research Monograph of the Printing Industry Center at RIT, Rochester, New York, USA, October 2003.
- [6] Eva Siegenthaler, Laura Schmid, Michael Wyss and Pascal Wurtz, “LCD vs. E-ink: An Analysis of the Reading Behavior,” Journal of Eye Movement Research, 5(3):5, pp. 1–7, 2012.
Note
All articles related to science and technology glamourize the technological feats. This 2000 worded article is written in a technology deemphasised and human centered approach. In this approach, technology is seen as a tool to achieve human goals and technology per se is not human goal. I want to acknowledge Mr. Varun Vinod for doing proof correction for this article.
