Flat Panel TV and Projector Technology

Plasma TV – Panasonic TH-42PZ800U

Display technologies have come a long way since those early black and white cathode ray tubes (CRT). This two-part post will review the latest display technologies and techniques and their advantages and disadvantages.

This first post with provide an overview of both flat panel and projector technologies with a detailed review of flat panels The second post examines the current crop of projector techniques.

So, with the rapidly evolving display developments, I thought that a quick primer would be in order, so that the relative advantages and disadvantages of the technologies and techniques used in today’s flat (or curved) displays and projectors might be helpful for those about to take the plunge.

A Little Walk Down Memory Lane:

• 1897 Karl Ferdinand Braun builds the first Cathode-Ray tube (valve)
• 1907 Henry Joseph Round discovers electroluminescence
• 1925-1928 John Logie Baird demonstrates first TV pictures
• 1954 Color TV images appear on CRT’s
• 1961-1962 Robert Biard and Gary Pittman patented the light emitting diode (LED)
• 1964 The first working LCD and Plasma panels appear.
• 1986 Alexey Ekimov and Louis Brus coined the term Quantum Dot during their research
• 1987 Eastman Kodak invent organic light emitting diodes – OLED
• 1987 Digital Light Processing DLP – Texas Instruments
• 1990 Full color plasma displays become available
• 2003 OLED displays appear
• 2008 AMOLED Active matrix organic light-emitting diode – developed by
• 2017 Quantum-Dot – Samsung

The three most popular methods of displaying images are:

1. Televisions and monitors – direct view
2. Projectors – including rear projection TV’s (no longer manufactured for residential use)
3. Lasers

These three categories contain many technologies that perform very differently and provide quite varying visual performances.

Televisions and Monitors

This category can be broken up into two basic types:

1. Cathode-Ray Tube – CRT (valve). (No longer manufactured for residential use)
2. Flat (or curved) panels to include:
   • Plasma (no longer manufactured)
   • LCD – Liquid Crystal Display
   • LED LCD – LED backlit LCD
   • QLED LCD – Quantum dot LED LCD display
   • LCoS – Liquid Crystal on Silicon
   • OLED – Organic Light Emitting Diodes
   • AMOLED – Active-Matrix Organic Light-Emitting Diode
   • mLED – micro Light Emitting Diodes
   • SED – Surface conduction Electron emitter Display – experimental
   • FED – Field Emission Display – experimental

Modern residential displays tend to be limited to the following technologies:

1. LED/LCD – Samsung, Sony, LG, Toshiba, Et Al.
2. QLED – Samsung, Sony
3. OLED – LG, Sony

Projectors

1. LCD – Liquid Crystal Display – three panel
2. DLP – Digital Light Processing or DMD, Digital Micromirror Device
   • Single panel
   • Three panel
3. LCoS – Liquid Crystal on Silicon – three panel
   • SXRD – Silicon X-tal Reflective Display (Sony)
   • D-ILA – Direct drive Image Light Amplifier (JVC)
4. Laser – these will not be reviewed as they are not used in home theaters, mainly being deployed in theatrical and special effects environments

Modern residential projectors are limited to:

1. LCD – Epson, Optoma, Et Al.
2. LCoS – JVC, Sony
3. DLP – Texas Instruments – BenQ, JVC, Optoma, InFocus, Epson, Christie

Let’s take a more detailed look at these more popular technologies, see how they work and what their relative advantages and disadvantages are.

Flat & Curved Display Panels

For many years the only two contenders for the flat panel market were the LCD and Plasma display. Plasma is no longer made, so LCD technology now rules the roost. Depending upon the manufacturer they are often referred to as LED or QLED displays due to their use of LED’s for the backlight source. The main alternative to the LCD/LED display is the much more expensive OLED. OLED displays are fundamentally different as each pixel provides its own illumination, a bit like Plasma did, not requiring a back light from either an light emitting diode (LED) or cold cathode fluorescent lamp (CCFL) source.

For more information on curved display technology and techniques see this Samsung link.

Plasma Displays

Mainly manufactured by Panasonic and Pioneer, I still use a Panasonic TH-42PZ800U in my family room, these displays were once the pinnacle of flat panel TV performance. Providing outstanding contrast and black levels, respectable brightness and a very accurate color gamut. However, they are much more complex than modern LCD and LED displays and more expensive to produce, so have now fallen out of favor.

Plasma Screen Cross Section

A plasma display panel comprises millions of tiny compartments in between two panels of glass. These compartments, or ‘cells’, hold a mixture of noble gases and a tiny amount of another gas such as mercury vapor. When a high voltage is applied across the cell, the gas in the cells forms a plasma. Some of the electrons flowing through the cell strike mercury particles which release the excess energy as ultraviolet (UV) photons. These UV photons then strike phosphor that is painted on the inside of the cell causing it to light up. Depending on the phosphors used, different colors of visible light can be achieved.

Every pixel is composed of three separate subpixel cells, each with different colored phosphors. One subpixel has a red-light phosphor, one subpixel has a green-light phosphor and one subpixel has a blue-light phosphor. Plasma panels use pulse-width modulation (PWM) to control brightness. This is achieved by varying the pulses of current flowing through the different cells thousands of times per second, the control system increasing or decreasing the intensity of each subpixel color to create billions of different combinations of red, green and blue. Plasma displays use the same phosphors as CRTs, which accounts for the extremely accurate color reproduction.

These panels use a significant amount of power compared to modern flat panels and could get quite warm. There other disadvantage was that they would suffer from burn in if bright static images were left on display. Various techniques were deployed to randomly move the image to prevent this from occurring.

LED/LCD Technology

LCD technology is based upon the use of polarized light. That is light waves that only move in one direction as opposed to moving in all directions when they are released from a typical light source. They may be vertically or horizontally polarized by using polarizing filters. These filters only allow the light wave energy to pass through in one direction. In the case of an LCD panel, they use both vertical and horizontal polarization.

The LCD pixel is again created using three sub-pixels, one for each color, red, green and blue.  The three sub pixels making up one primary pixel. So, an HDTV that is 1920×1080 will have 2,073,600 red, green and blue pixels for a total of 6,220,800 sub pixels.

The original LCD panels are said to be passive, as they let light pass through them, therefore requiring a light source or backlight behind them to function. So how does the LCD panel control the light source that is behind it? A layer of liquid crystals is placed between vertically and a horizontally polarized filters. Being of opposite polarity, these filters would naturally block the passage of all light. The layer of LCD crystals cause the light entering from the rear filter to be rotated by them, and therefore the panel appears almost clear as the light now aligns with the second filter, and passes through it. LCD crystals have a unique property that enables them to twist when an electric field is applied across them. The amount of twist being proportional to the magnitude of the electrical field. When no field is applied they cause the incident light to rotate and the pixel is lit up. As the fields magnitude is increased, they twist more and more, causing less rotation of the incident light. This twisting gradually reduces the alignment of the polarized light with the second filter and causes the pixel to eventually go black. Thereby controlling the brightness, and color, of each pixel.

Simplified Cross Section of an LCD Panel

Manufacturers have their own physical configurations as to how each pixel is broken out into the three primary sub-pixel colors as shown below.

A Typical Sub Pixel Layout

Sub Pixel Layout 3

Sub Pixel Layout 2

Rear and Edge Lighting

The rear light source for an LCD display comes from either a cold cathode fluorescent lamp (CCFL) or light emitting diode array (LED). These sources may be mounted over the entire back of the display assembly (direct lighting) or around the edges of the display chassis (edge lighting).

Full LED Backlighting

Top & Bottom Edge LED Backlighting

Top & Side Edge LED Backlighting

Fluorescent Lamp (CCFL) Back Panel Lighting

Back lighting generally produces the most even light output over the display but can produce a lot of heat and requires a lot of devices. Edge illumination uses less devices but will often produce an irregular brightness over the displays which can usually be seen in darker images or a black screen in a darkened room. Manufacturers get up to all sort of tricks to spread out the rear or side light in a uniform manner using light guides and such.

Unfortunately, LCD crystal displays  do not block all the rear or edge light, so the contrast ratio is limited as they cannot produce absolute black.

To improve the contrast performance of these displays, and cut costs, manufactures apply  a number of techniques to control the back lighting. Such as varying the brightness of the lighting in various picture zones based upon that zones scene brightness. There can be anything from 6 to 300 zones, a technique called local or full array local dimming. How effective and visible these techniques are depends upon how well the manufacturer has implemented them.

Poor Backlight Dispersion

Uneven Backlight Dispersion

Good Backlight Dispersion

OLED Technology

The most fundamental difference between LED and OLED technology is that each OLED pixel generates its own light, and when it is not powered produces no light, so is totally black. Each pixel is made from a material that glows when you apply electricity to it, an effect known as electroluminance. The specific materials used in TV’s are organic compounds. (Although light emitting polymers (LEP’s) also have the same properties). Each color requiring a particular compound. Note: The compound colors used are not necessary red, green and blue. Unfortunately, blue organic compounds have a relatively short life so other approaches are taken by manufactures to create the required red green and blue light, including appropriate color filters. These OLED pixels give off light that is proportional to the current that is applied to them. No current no light, lots of current, lots of light.

While the ‘black’ pixel is something that an LCD panel cannot produce, the light output from OLED panels is less than that of an LCD panel. Despite that, OLED panels produce exceptionally high contrast ratios, theoretically infinite, with brightness levels and a chrominance range that allow them to support 4K HDR with a color gamut that exceeds DCI P3. LCD’s have difficulty achieving such wide color gamut’s.

A typical OLED is composed of a layer of organic materials, consisting of a conductive and an emissive layer that are sandwiched between, or printed on, two electrodes, the anode and cathode.  The anode is typically transparent in nature to let the light be emitted from the electroluminance process.

Simplified Cross Section Of An OLED

Quantum Dot Displays – QLED

Quantum dots are semiconductor nanostructures that are so minute they have different optical and electrical properties than larger particles.

With electricity or light is applied to them, quantum dots will emit light of specific frequencies. These frequencies can be altered precisely by changing the size, shape, or material of the quantum dot. It is this property that makes quantum dots useful in display devices.

Simplified Cross Section of a Photo-Luminescent Quantum Dot Display

Simplified Cross Section of a Electro-Luminescent Quantum Dot Display

Quantum Dots Under High Magnification

Quantum dots are both electro-luminescent and photo-luminescent, allowing their incorporation into this new emissive display TV architecture. They are more effective than white light sources as they produce monochromatic light naturally, and hence more saturated colors.

A quantum dot TV uses its photo-luminescent properties and is basically a backlit LED LCD TV with enhanced color. The backlight system, as before, being a bank of LEDs that are mounted either behind or around the edges of the screen. The LED light is diffused and aimed through a polarized filter by a light-guide plate. The photons then hit a liquid crystal layer that either allows the light to pass through or block it. This light then strikes a layer of quantum dots lighting them up. A quantum dot TV performs in a very similar way to an LED/LCD display, but uses a blue backlight that upon striking the photo-luminescent quantum dot layer creates the required red and green light. We now have the three required colors. These colors are far more pure than those of a typical LED/LCD display, so the final colors are easier to filter, producing more saturated colors with a wider gamut.

The major benefits for using quantum dots in TV’s are the enhanced colors that they can produce, which can be fine-tuned to ensure the exact light required is emitted. This precision enables accurate colors and whiter whites. Quantum dots also create higher color saturation than conventional LCD displays.

Televisions using quantum dot technology have a wider color gamut, which is required for 4K televisions that must process a lot more color information than HD televisions. Quantum dot TVs are also generally less expensive than OLED’s.

See here for a detailed description from Samsung on Quantum Dots and here for Samsung’s learning center technical papers.

Flat Panel Display Advantages and Disadvantages – (best first)

• Brightness
  1. LED/LCD
  2. QLED
  3. OLED
• Black Level
  1. OLED
  2. QLED
  3. LED/LCD
• Contrast Ratio
  1. OLED
  2. QLED
  3. LED/LCD
• Resolution – all similar
• Refresh rate and motion blur – all similar
• HDR
  1. OLED
  2. QLED
  3. LED/LCD
• Color Gamut
  1. OLED
  2. QLED
  3. LED/LCD
• Viewing Angle
  1. OLED
  2. QLED
  3. LED/LCD
• Uniformity
  1. OLED
  2. QLED
  3. LED/LCD
• Power consumption
  1. LED/LCD
  2. QLED
  3. OLED
• Lifespan – too soon to say
• Burn in – no data
• Price
  1. LED/LCD
  2. QLED
  3. OLED
• Picture quality
  1. OLED
  2. QLED
  3. LED/LCD

Part two of this post will examine the current crop of projector technologies and techniques.

Click on the following links for more product information and to see some of the latest designs.

• LG displays
• Sony displays and projectors
• Samsung displays
• JVC Projectors
• BENQ projectors
• Epson Projectors