Display Tech(s)

2019-07-11

This article will discuss about the various display technologies used in both analog & digital displays (TV, Monitor, Mobile & Laptop displays) in recent to previous years.
Most common displays includes

  • CRT (Cathode Ray Tube)
  • Plasma
  • LCD / LED
  • QLED
  • OLED
CRT (cathode ray tube) Link to heading

NTSC(National Television System Committee) was the broadcast TV standard for North America, Canada and a handful of other countries, set back in the day before LCD screens, when CRT’s ruled. NTSC was 525 lines (the vertical resolution - think the ‘Y’ resolution in pixels of a modern LCD) and 30 frames per seconds redraw (refresh).

PAL(Phase Alternating Line) was the European broadcast standard - 625 lines at 25 frames per second.

Simplified, a TV picture was drawn by bending a beam of light to scan from (as you looked at the screen) the top left corner to the top right. The beam was then cut off, and it would move down a line to draw again from left to right. For NTSC this was all 525 lines 30 times a second.

Though NTSC was an analog techinque for CRT displays but it is still being used.

Plasma Link to heading

Plasma screens work by exciting tiny pockets of gas (Xenon and Neon), changing them to a plasma state. In that state, the electrons of that gas emit ultraviolet light, which is not visible to the human eye. The ultraviolet light is then absorbed and re-emitted into the visible spectrum of light by the phosphor inside each cell. Each pixel consists of three sub-pixels: one red, one blue, and one green. The more excited the gas, the brighter the color produced.

Because each pixel emits its own light, the blacks are really deep. When the television wants to display black, it simply emits no light at all for the selected pixels. Also, there is less of the motion blur sometimes seen on an LCD screen, because the gas stops emitting light as soon as it is not excited.

  • Less motion blur
  • Image retention problem
LCD (Liquid Crystal Display) Link to heading

An LCD screen is composed of two parts: the actual liquid crystal display and a light source at the back of the screen (called backlight).
Based on the backlight LCD displays are of two types

  • CCFL
  • LED

Regular LCD = LCD + CCFL (cold-cathode fluorescent lamp) backlit
LED = LCD + LED Backlit (full array, edge-lit, and direct lit)

CCFL Link to heading

When someone refers to an LCD TV, they usually mean a CCFL (cold-cathode fluorescent lamp) backlit LCD screen. This is how a normal LCD screen works. The backlight is a series of light tubes placed behind the screen. These tubes are very similar to the fluorescent lamps used almost everywhere, but smaller.

LED (Light Emitting Diode) Link to heading

An LED screen is actually an LCD screen, but instead of having a normal CCFL backlight, it uses light-emitting diodes (LEDs) as a source of light behind the screen. Marketing made a lot of fuss about LED TVs, but it is only the backlight that changed, so there is actually no picture quality improvement over a normal LCD screen.

There are three main configurations of LED as backlights for television screens:

  • full array
  • edge-lit
  • direct lit.

Full Array
In a full array LED screen, the LEDs are distributed evenly behind the entire screen. This produces a more uniform backlight and provides a more effective use of local dimming, where it can change the luminosity of only a specific part of the screen. This is mostly expensive displays.

Direct Lit
This is how the lower-end LED TVs are constructed. Similarly to the full array method, the LEDs are directly behind the screen. However, there are very few of them and they cannot be controlled separately to match the luminosity of the picture. These TVs are not very thin because of the space required behind the screen to add the LEDs and to diffuse the light over a big area.

Edge Lit
This is the most common method for LED screens. With an edge lit LED screen, the LEDs are placed at the peripheral of the screen. Depending on the television, it can be all around the screen or only on the sides or the bottom. This allows the screen to be very thin. However, it can cause some spots on the screen to be brighter than others, like the edges. This problem is called flashlighting or clouding. It can be seen when watching a dark scene in a dark environment.

LCD screens work by applying a voltage to twist LCD crystals in the display to allow or shutter light but they vary in the the way they twist the crystal.
LCD/LED & OLED/AMOLED all types of displays uses a layer of TFT (thin-film transistors) to activate the sub-pixels. This is also called active matrix.

Backlight – Polarizer – Polarizing Agent / Liquid Crystal /w TFT matrix – RGB filter/subpixel - Polarizer

So depending on this LCD have mainly 3 types:

  • TN
  • VA
  • IPS

LCD/LED = LCD (TN, VA, IPS) + Backlit (CCFL, LED)

TN (Twisted Nematic)

  • Cheapest
  • High refresh Rate
  • High Speed gaming
  • Poor Color Reproduction
  • Poor viewing angle specially vertically
  • Modern TN panel can have same horizontal viewing angle as IPS but they lack vertical viewing angle

VA (Vertical Aligned)

  • MVA – Multi Domain VA, PVA – Patterned
  • Better refresh rate than IPS
  • Better color reproduction Than TN
  • Compromise between IPS & TN
  • The mid range option

IPS (In-plane Switching)

  • Best Color Reproduction
  • Widest Viewing Angle
  • Expensive
  • Low refresh rate
  • Variants found (S-IPS) , PLS (Samsung)
  • Suitable for Color sensitive Task

Problems : All LCD’s suffer from some extent of motion blur because LCD subpixels need some time to activate (turn on/off, twist/untwist)
Response Time is related to how subpixels turn on or off which causes motion blurs
Input Lag – From graphics card to actually show the display signal
Other problems includes - Local Dimming, Viewing angle due to LCD filter, Flickering for backlit

OLED (Organic LED) Link to heading

Due to problems in manufacturing solid state LED’s; higher end displays use OLED. This types of organic LED can be individually lit and as they have their own light so OLED doesn’t need any backlight.
Depending on the tft array’s OLED are of two types:

OLED = Active Matrix, Passive Matrix
AMOLED = OLED + thin-film transistors (TFTs) /w Active Matrix

  • The best viewing angle ever as their is no RGB filter like LCD
  • Low Brightness but not on all displays
  • Image Retention when static images are displayed for longer period of time
  • PMOLED : Size restriction as it uses passive matrix
QLED (Quantam Dot LED) Link to heading

QLED is a marketing name used by Samsung to describe their newer LED TVs. They use traditional LCD panels lit using LEDs. Between the LCD layer and the backlight, a filter with energy reactive nano-particles filters the light to produce more pure and saturated colors. Both the color volume and color gamut of QLED TVs are the best you can find today.
LG and Sony (as of 2019) being the only makers of OLED TVs (Sony OLED TVs use LG OLED panels)

Color gamuts

Color gamut refers to the various levels of colors that can potentially be displayed by a device There are actually two types of color gamuts, additive and subtractive. Additive refers to the color that is generated by mixing together colored light to generate a final color. This is the style used by computers, televisions and other devices. It is more often referred to as RGB based on the red, green and blue light used to generate the colors.

Substractive is used in books, photos, printers

To quantify the various color gamuts in terms of their relative range of color of narrowest to widest would be
CIE 1976 < sRGB < AdobeRGB < NTSC

The average computer monitor will display around 70 to 75% of the NTSC color gamut. 72% of NTSC is roughly equivalent to 100% of the sRGB color gamut The CRTs used in most old tube televisions and color monitors also produced roughly a 70% color gamut.

An LCD monitor’s backlight is the key factor in determining its overall color gamut. The most common backlight used in an LCD is a CCFL. These can generally produce around the 75% NTSC color gamut. Improved CCFL lights can be used to generate roughly 100% NTSC. Newer LED backlighting has been able to actually generate greater than 100% NTSC color gamuts. Having said that, most LCDs use a less expensive LED system that produces a lower level of potential color gamut that is closer to generic CCFL.

Here is a quick list of the common ranges for different levels of displays:
Average LCD: 70 to 75% of NTSC
Professional non-Wide Gamut LCD: 80 to 90% of NTSC
Wide Gamut CCFL LCD: 92 to 100% of NTSC
Wide Gamut LED LCD: 100%+ of NTSC

Online sources https://www.rtings.com/tv/learn/lcd-vs-led-vs-plasma/how-they-work https://www.eizoglobal.com/library/basics/lcd_monitor_color_gamut/