A computer monitor is an electronic device that outputs information in pictorial form. It acts as an interface, connecting the viewer with the computer.
Formerly known as visual display units, computer monitors were used for only data processing. Advances in electronics in the late 1970s enabled monitors to support both data processing and entertainment tasks.
In the mid of the 20th century, electronic computers had light bulb panels to indicate the on/off state of a specific register. They were used to monitor the machine’s internal state. As technology developed, scientists came up with new types of monitors that could show more information.
At presents, monitors come in various designs with tons of smart features. However, based on the core technology, they can be categorized into three types.
1. Cathode-Ray Tube (CRT)
The first commercially made electronic televisions with cathode-ray tubes were manufactured in Germany in 1934. These displays became more popular in the mid 20th century.
The same display technology was then used in personal computers in the late 1970s. The CRT was integrated with keyboard and other computer components in a single large chassis.
Oscilloscope CRT | Credit: Soviet Digital Electronics Museum
A CRT is basically a vacuum tube comprised of an electron gun at one end and a fluorescent screen at another end. The electron gun emits a strong beam of electrons which travel through the tube (made of electromagnetic devices) and eventually strikes the fluorescent screen, forming an image.
By the end of the 1990s, CRT displays were capable of producing thousands of different colors and a resolution of up to 2048 x 1536 pixels. However, they had low refresh rates (approximated 60 Hz) so flickering was quite noticeable, especially when viewed with peripheral vision.
Other than this, CRT displays have numerous shortcomings –
- They emit a very small amount of X-ray radiation.
- Their illumination levels can strain the eyes and reduce visual acuity.
- Prolonged used can cause headache, nausea, and confusion.
- Users can feel extremely tired due to electromagnetic radiation and can even hear ringing in the ears and suffer from memory loss.
In the late 2000s, Flat panel displays started replacing CRTs due to their better picture quality, less bulkiness, additional features, and reducing costs. In 2008, Flat panel displays exceeded CRTs, and by the end of 2015, not a single manufacturer was producing or recycling CRTs.
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2. Liquid Crystal Display (LCD)
LCD flat panel
LCDs have been in used in several devices since its inception. In the 1990s, for instance, they were used in laptops where smaller size, lighter weight, and lower power consumption of LCDs justified their higher price versus liquid crystal displays.
The picture quality of LCD surpassed the picture quality of CRTs in 2007. As a result, the worldwide sales of LCD TVs surpassed CRT TVs in the last quarter of 2007. Today, LCDs have replaced CRTs in nearly all applications.
LCD uses the light-modulating characteristics of liquid crystals. They cannot produce light on their own: liquid crystals use a reflector or backlight to generate images in monochrome or color.
A liquid crystal display is comprised of a layer of monochrome or color pixels aligned schematically between two transparent electrodes and a couple of polarizing filters (perpendicular and parallel). To achieve the optical effect, the system polarizes the light in varying amounts and passed through the liquid crystal layer.
There are two types of liquid crystal display available in the market right now: passive matrix and active matrix.
Passive matrix LCDs were mostly used in laptops and Nintendo Game Boy in the early 1990s. They are still used in handheld devices where less data needs to be displayed, such as inexpensive calculators.
Active matrix structure (or TFT), on the other hand, generates better image quality. They are used in high-resolution color displays, including modern televisions and computer monitors.
Advantages and Disadvantages of LCDs
Compared to CRTs, LCDs have low power consumption, higher refresh rate and resolution. They are not affected by magnetic fields and do not emit any harmful electromagnetic radiation. However, they do have the following shortcomings:
- Limited viewing angle.
- Features poor black on dark imagery, and motion blur is common.
- Inconsistent backlighting, which causes distortion in brightness, especially towards the corners.
- Low response time and loss of contrast in low and high-temperature environments, respectively.
This type of display is currently used in various applications, ranging from computer monitors and televisions to instrument panels and airplane cockpit displays. They are also used in small electronic devices such as smartphones, smartwatches, calculators and digital cameras.
Now, liquid crystal displays are gradually being replaced by an advanced technology — OLED — that has wider color gamut, greater viewing angle, and faster response time.
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3. Light Emitting Diode (LED)
Samsung 146-inch modular TV built using extremely tiny LEDs
They are the latest type of monitors in the market today. While LCD monitor uses Cold Cathode Fluorescent for backlighting, LED display uses light-emitting diodes for backlighting.
A light-emitting diode is capable of producing light from electricity. Unlike an incandescent lightbulb, it lasts longer and produces several different colors.
Most modern digital displays in devices such as smartphones, computer monitors and television screens, use organic light-emitting diode (OLED). An OLED consists of an organic compound film that emits light in response to an electric current.
Since OLED emits visible light, it works without a backlight. The color of the device can be infrared, visible, or near-ultraviolet: it depends on the chemical composition of the semiconducting material used.
This type of display can be driven with either an active matrix (AMOLED) or passive matrix (PMOLED) control scheme. The AMOLED scheme uses TFT backplane to directly access and switch individual pixels on or off, whereas in the PMOLED, each row in the display is controlled sequentially.
Active Matrix control scheme used in the Samsung Galaxy smartphones
AMOLEDs have faster refresh rates than PMOLED, consume less power, and are suitable for larger displays with higher resolution. They are, however, more complicated and expensive to fabricate.
Advantages and Disadvantages of OLEDs
OLEDs have a greater contrast ratio, wider viewing angle, and deeper black levels compared to LCDs. They also have a much faster response time than other types of displays.
And since OLEDs can be fabricated on flexible plastic substrates, it is possible to build foldable and roll-up displays embedded in fabrics or clothing.
Despite decades of research and testings, OLED technology still has several disadvantages. For example,
- It consumes about 60% more power than LCDs to display a normal image.
- Displaying images with a white background, such as a website or document, can take more than 3 times as much power.
- Manufacturing OLEDs require an extra ‘sealing process’ since organic materials can be instantly damaged by water.
- Their performance depends on the ambient temperature of the operating environment.
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Several private companies and institutes are searching for efficient OLED materials that make displays more efficient and less expensive. A lot of work remains to be done in the realms of color shift, thermal management, light distribution, and system reliability.
