The Evolution of Display Technologies Continued

Stage 4: DLP: Many moving mirrors

DLP or Digital Light Process works opposite to the principle behind LCDs. LCD displays work by alternately blocking light and letting it through. DLP works by creating images through multiple reflections of light. These mirrors can switch between ON (where light is fully reflected) and OFF (where light is fully deflected) states. And, by switching between light and dark states at great speeds, these mirrors are able to reproduce the gray scale with staggering accuracy.

DLP techonology was originally developed in 1987 as an alternative to LCD technology. The mirrors in the DLP unit can be quite small (5 microns square or so) and therefore can have a higher pixel density on a display than LCD displays. Hence DLP technology started to appear mostly in projectors (like the one used in this class) as a better and brighter alternative to LCD projectors. But now, LED technology has likely made DLP technology obsolete

The Digital Micromirror Device (DMD): The functional core of DLP technology is the DMD semiconductor which is operationally defined as a binary spatial light modulator, which just means its a fast ON/OFF switch.

DMDs contain millions of microscopic mirrors--one for each pixel in their display--that can be directed to tilt 10 back and forth on their axes. These micromirrors are made of aluminum for maximum reflectivity and durability. In the first generation of devices measure just 16 x 16 microns or .000256 square mm of reflecting area. Thus the display is made up literally of millions of tiny mirrors. The actual number of micromirrors present on a given DMD is equal to the number of pixels on your TV's screen. A display with a resolution of 1280 x 1024 contains a DMD consisting of 1,310,720 micromirrors. This "one pixel, one mirror" setup lends itself to exceptionally precise digital imaging.

The Mirror As Switch: Since each micromirror on a DMD chip operates independent of its neighbors on the DMD grid, each aluminum mirror can reflect light in one of two directions--either 10 degrees toward or 10 degrees away from a lens. The light that gets reflected is directed towards an optical lens, which projects that light onto a screen in the form of a lit pixel.

A light absorber absorbs the light that is deflected away from this lens, so no light reaches the screen at that particular pixel, producing a dark, square pixel image. In short, each micromirror either illuminates a pixel (i.e., switches it ON by moving +10 degrees) or darkens a pixel (i.e., switches it OFF by moving -10 degrees ). Since your eye can only process image information at rate of about 30 milliseconds, then fast switching between light and dark would produce some value of gray, according to your eye-brain.

To produce color, DLPs utilize a color wheel composed of red, green, and blue filters, which direct individual pulses of colored light toward the DMD for gray-scale processing. This means that, at any given instant, only one of the primary light colors is hitting the DMD, but when the filter system spins fast enough (~120 times per second), the colors appear blended into a full-color digital image. DLPs display color images as rapid sequences of red, green, and blue light signals, which the brain effectively integrates into one signal. For 8 bits this is 256 x 256 x 256 shades of RGB = 16.8 million different colors.

Stage 5: The development of vertical or stacked pixels

One facet of all the previous technologies lies in their horizontal arrangement of individual R, G, and B pixels (or sub-pixels). To attain the goal of cramming more pixels into the same amount of physical area, requires moving to some vertical arrangement where the individual RGB elements are stacked on top of one another.

One key to making the vertical pixel rely's on the fact that photons of different energy (e.g. wavelength, color) can penetrate silicon to different levels of depth. Photons with the shortest wavelength have the shortest penetration lengths (due to more interactions with the atoms). So in principle, one can make this kind of pixel device:

The layers are positioned to take advantage of the fact that silicon absorbs different wavelengths of light to different depths. The bottom layer records red, the middle layer records green, and the top layer records blue. Each stack of pixels directly records all of the light at each point in the image.
Until recently, this scheme was difficult to engineer on a commercial scale due to various processing problems and the need for the thicknesses of the respective silicon layers to be very precise. This limitations are now over come, and if you can afford one, this is by far the the best digital slr camera out there

Stage 6: Using Organic Stacked pixels

The latest revolution in display technology involves Organic Light Emitting Diodes (OLEDs).
Samsung 40 Inch OLED Monitor

The tremendous advantage of OLED pixels is that they can be affixed to flexible surfaces (e.g. your clothing). The disadvantage is that organic material decays and therefore the display surface does have to be periodically refreshed.

An OLED consists of the following parts:

Substrate (clear plastic, glass, foil) - The substrate supports the OLED.

Anode (transparent) - The anode removes electrons (adds electron "holes") when a current flows through the device.

Organic layers - These layers are made of organic molecules or polymers.

Conducting layer - This layer is made of organic plastic molecules that transport "holes" from the anode.

Emissive layer - This layer is made of organic plastic molecules (different ones from the conducting layer) that transport electrons from the cathode; this is where light is made.

Cathode (may or may not be transparent depending on the type of OLED) - The cathode injects electrons when a current flows through the device.

Various designer polymers (polyaniline, plyfluorene) can be used in the organic layers. OLEDs emit light in a similar manner to regular LEDs, through a process called electrophosphorescence.

The basic process works like this:

A voltage is applied across the OLED

Electrical current flows from the cathode to the anode through the organic layers. The cathode injects electrons to the emissive layer; the anode injects holes into the conductive layer

At the boundary between the emissive and conductive layers, the electrons recombine with the holes and transition down atomic energy levels.

The OLED emits light

The color of the emitted light depends on the type of organic molecule in the emissive layer. Different molecules within the layer can give different colors and hence a vertically stacked arrangement of RGB can occur.

The intensity of the light is proportional to the amount of supplied current, so, like an LCD, intensity per pixel can be controlled.

OLEDs offer many advantages over both LCDs and LEDs:

The plastic, organic layers of an OLED are thinner, lighter and more flexible than the crystalline layers in an LED or LCD. Because the light-emitting layers of an OLED are lighter, the substrate of an OLED can be flexible instead of rigid. OLED substrates can be plastic rather than the glass used for LEDs and LCDs.

OLEDs are brighter than LEDs. Because the organic layers of an OLED are much thinner than the corresponding inorganic crystal layers of an LED, the conductive and emissive layers of an OLED can be multi-layered. Also, LEDs and LCDs require glass for support, and glass absorbs some light. OLEDs do not require glass.

Because OLEDs do not require backlighting, they consume much less power than LCDs (most of the LCD power goes to the backlighting). This is especially important for battery-operated devices such as cell phones.

OLEDs are easier to produce and can be made to larger sizes. Because OLEDs are essentially plastics, they can be made into large, thin sheets. It is much more difficult to grow and lay down so many liquid crystals.

Problems with OLED:

Lifetime - While red and green OLED films have longer lifetimes (46,000 to 230,000 hours), blue organics currently have much shorter lifetimes (up to around 14,000 hours). That's less than 2 years of continuous use.

Manufacturing - Manufacturing processes are expensive right now so these devices are not yet near consumer price points.
Within the last 18 months, some of these problems have been solved and now LG offers large scale displays which can easily be curved since the OLED substrate is flexible>

And of course its all about marketing

But are we reaching a pixel limit?

But this is bullshit. Your eye can see hundreds of millions of pixels:

The resolution of eye is about 0.3 arcminutes - angular diameter of the full moon is about 30 arcminutes of 100 eye resolution limits - so to you naked eye the moon is about a 100x100 pixel image or 10000 pixels,

But your eye can see easily a field of vision of 90 degrees by 180 degrees which is 90x180x60x60 = 58,320,000 sq arcminutes

at a resolution of 0.3 arcminutes per pixel this is then 58,320,000/(0.3*0.3) is about 580 Million Pixels

Stage 7:
Carbon Nanotube pixels
Field Emission Displays

The FED is a new type of flat-panel display in which electron emitters, arranged in a grid, function individually to generate colored light. It may be a breakthrough technology because it can accommodate a thin panel like liquid crystal display (LCD) and offers a wider field-of-view and higher image quality. Field-emission displays use much less power than plasma displays because they re intrinsically more efficient.

The key feature likes in the "cathode tips" which are carbon nanontubes that serve as the electron gun. In that since, FED works like a CRT with multiple electron guns at each pixel.
Potential advantages of FEDs:

The electron gun can be very well localized and the subsequent pixels can be made quite small. Therefore resolution elements can be increased per physical area. This has important applications to mobile devices which are likely the main market driver.

Power consumption is dependent on the actual display content.

Pixel response time (refresh rate) is much faster than TFT LCD response times.

Individual pixel brightnesses can be quite high.

Currently tho, there is no production scale involved in the creation of the necessary carbon nanotubes (CNT). Samsung appears to have proprietry leader in the production of CNTs and you may want to invest with them.