Basics of video and various types of display technologies
By Abrie du Plooy
The earliest colour CRT (Cathode Ray Tubes) televisions used technology where primary-coloured electrons were fired at a phosphor coated surface. The charged electrons would energize the phosphor which then emitted the desired colour.
These days, with digital signals and much more signal information, pixels are created by using various different technologies in order to display an image.
Each pixel needs to be displayed in a specific colour to form part of the overall image or frame. It is very interesting to understand how images and videos are created and displayed. In a nutshell, a single image is made up of a certain amount of adjacent pixels, each displaying a dedicated colour and in concert, creating an image. Pixels are configured in rows and columns to create the shape of the image. Each image needs to be a standard shape. The shape is defined in a width to height ratio, known as the Aspect Ratio. There are a few standard aspect ratios used in various video applications, the most common is a wide screen, 16:9 aspect ratio which is the shape of a standard consumer flat screen television. A slightly wider screens in cinemas uses 1:1.85 aspect ratios (19:10.02) and 21:9 ratios.
Social media platforms seem to focus more and more on 1:1 square images because it flawlessly adjust when you rotate your device from portrait to landscape mode. The number of pixels defines the resolution. The more pixels in a single frame create the ability to display finer details of an image. The higher the resolution, the better the quality or definition of an image. The most common resolution used these days is UHD (Ultra High Definition) with a resolution of 3840 x 2160 at a 16:9 aspect ratio. It is recognized as 4K even though true 4K has a resolution of 4096 x 2160 in a 1:1.85 aspect ratio.
In Video technology, continuous static images are flashed consecutively to create a ‘moving picture’ effect. Conventional video uses a frame rate of 25 frames per second (fps) whilst more modern content can go higher to 30fps or 60fps.
All video signals consist of only the 3 primary colours, red, green and blue (RGB). Various screen types use different display technologies to manipulate these 3 colours into a full colour image. The 3 colours combined at 100% produce white, or clear transparent light, as we know it. If white light gets diffracted through a medium, a rainbow of colours become visible. In nature, light rays diffract and filters through moisture or particles in the sky. A beautiful example would be sunrise and sunsets or well, rainbows. These 3 colours, at various intensities will mix and create millions of other colours.
How images are displayed. Not all display technologies create images the same way. Some create the individual primary colours while others use a single white light source and create the primary colours through diffraction filtering. The final process is creating the 3 primary colours in each pixel with their various intensities in order to mix them into the desired pixel colour and create the image. The various display technologies are broken up into 3 categories based on the method of light utilization in order to create the image to the viewer. These categories are:
1. Emissive Display – The pixels inside the screen panel create the light and primary colours in the overall image. Each pixel would create the red, green and blue colours and these are then energized at various brightness levels to mix into the desired colour for each pixel. Examples of emissive displays are LED screens used for large displays in retail, sport stadiums and outdoor advertising/billboards. Each pixel has 3x individual LEDs either mounted separately or grouped into a single dot. Organic LED TVs are also emissive as well as conventional plasma screens.
2. Reflective Display – The light is directed from a single light source, which filters into individual red, green and blue before being directed through an optical lens system onto a passive screen surface at a distance away. The reflected light from the screen surface creates the image to the viewer. Any picture created by front projection displays are classed as reflective, irrelevant if the pixel creation is DLP or LCD. The type of light source, UHP Lamp, Laser or LED is also irrelevant. This could be corporate, cinema or residential application using projectors and projection screens.
3. Transmissive Display - The light source is positioned right behind the display panel which is then transmitted through a Liquid Crystal Display screen. The individual pixels manipulate the back light into the 3 primary colours. This technology is used in all LCD devices, conventional and current LCD display screens as well as laptops, PCs and mobile devices.