Introduction
 

Nowadays the LCD is fairly considered as the most widely spread Flat Display technology despite all well-known principal drawbacks (many of them unavoidable) adherent to this technology:

• necessity to use expensive substrates and circuitry during manufacturing;
• relatively slow response time;
• low efficiency of backlight utilization because of use of polarizers;
• necessity to use backlight in most applications, thus power consumption and stray light problems;
• viewing angle nonuniformity;
• visibility problems in bright ambient light;
• relatively narrow range of working temperatures.
   

Despite all the drawbacks, present trends in portable displays [1] still promise leading place of the LCD in many industrial applications leaving behind other competing technologies like OLED, Plasma, FED, and other displays in sense of universality, competitiveness, and range of applications.

Common sense suggests that inevitably new cost-effective Emissive Flat Display technology free of at least several principal LCD’s drawbacks and more cost-effective than all mentioned above technologies should emerge. That is why many companies are looking for new approaches to penetrate to the Flat Display Market to compete with LCDs.

One of such approaches in our opinion can be the Scattering Fiber Display (the SFD) described as a concept below.

 

 
Description of the SFD
 

Simplest, basic embodiment of the SFD is shown on Fig. 1. A 4x4 embodiment is presented for purpose of simplicity, though other resolutions (up to 1600x1200 and higher) are possible.
 

Figure 1. Schematic view of the Scattering Fiber Display

The SFD in its basic embodiment is an emissive display and consists from the optical Fibers with transparent coating placed over a Conductive Plate (non-expensive glass or plastic covered with conductive layer), the transparent Row Wires are situated over the Fibers, the light emitting devices, like e.g. LEDs of several colors (e.g. Red, Green, and Blue - RGB) are arranged in an LED Array and radiate into (coupled with) corresponding Fibers. The Row Wires and the LED Array are controlled by a Driving Circuit. Rows (R1-R4) are associated with the Row Wires, while Columns (C1-C4) are associated with corresponding LED groups in the LED Array. The Fibers (Fig. 2a, b) comprise Electrically Activated Scattering Centers.
 

Figure 2. Light passing through fiber when drive voltage (a) is zero and (b) exceeds threshold of activation of the Scattering Centers in the Fiber

 
To increase light emitting efficacy the Fiber’s (Fig. 1) opposite to the viewer side can be covered with reflective layer. In this case we have principally one-sided display. Otherwise, if the Conductive Plate is also made transparent, the SFD can produce visible image on both opposite sides of the SFD simultaneously, what can have a lot of applications, e.g. in signage area. If viewed from side of the Conductive Plate no visible grid is another advantage of the SFD (paper-like display). Another way to improve paper-like perception of the SFD (especially in mobile applications) is to make distance between the fibers as small as possible.

The Fibers as well as the Conductive Plate can be made flexible thus the whole display according to this invention can be flexible and even rolled up in at least one direction. It is also possible to use other than rectangular form-factors of the display due to flexibility of the Fibers.

It is also possible to use more than three primary colors in the LED Array (or arrays as described below) of the SFD. For example, adding white (or of other color(s)) LEDs to the LED Array allows creating displays with wider gamut.

As for material of the Fiber, it can be e.g. optical polymer containing dispersed liquid crystal droplets (serving as mentioned above Electrically Activated Scattering Centers) with electrically controllable scattering properties, what is known in the art [2], though this approach is considered by us as a least effective one for many reasons. One of the principal drawbacks of this “LC droplets” approach is substantial stray light in the Fiber due to essential scattering properties of the LC droplets if even no electrical field is applied, what may lead to low contrast of such display.

That is why it is important to research and implement other than liquid crystal approaches of electrically managed scattering in the Fiber. The fact is that until now the researchers have been mostly concentrated on how to minimize scattering in a fiber, not how to maximize it, though nowadays it doesn't seem to be a challenge to develop effective electrically activated scattering mechanisms (including transient ones) in either glass or polymer fiber (and mentioned LC droplets in polymer is just one of many possible approaches).
In any case, today's level of technology allows building a prototype of the SFD with quite reasonable investment already. While the rough, demonstrational and quick-to-built prototype still may use polymer Fiber with LC droplets as the scattering centers, future industrial samples of the SFD will most plausibly use more effective approaches in this direction.
 

Way of operation of the SFD
 

In basic embodiment of the SFD in case if no drive voltage (V_dr) is applied to the Row wires (Fig. 1), the Fibers are transparent (no scattering) all over their length and there is practically no radiation beyond the Fibers 1 due to complete internal reflection (Fig. 2a). It is necessary to note that in transparent (not scattering) condition of the Fiber some constant Voltage between the Row Wires (or additional plain transparent electrode above them) and the Conductive Plate can be applied to keep the Fiber in transparent stage. In this case applying V_dr is actually a way to change electric field created by mentioned constant Voltage to switch the Fiber into scattering stage

If the drive voltage is applied to e.g. Row Wire R1 (Fig. 1), then the Electrically Activated Scattering Centers (Fig. 2b) in the Fibers below the Row Wire are activated by electric field and due to this the Fiber regions below the Row Wire start to radiate beyond the fiber. Intensity and color of this radiation is managed by currents (either current intensity or its duration) applied to corresponding RGB LEDs of the LED Array (Fig. 1). It is important to note that the drive voltage in basic embodiment of the SFD can have just two values – On (scattering activated) and Off (no scattering) so switching between the two scattering stages can be relatively fast helping to reduce response time of the SFD (and response time of the LEDs is very fast itself).

Thus successively applying drive voltage to the Row Wires (line-by-line principle) and proper currents to corresponding RGB LEDs in the LED Array an arbitrary picture can be produced on the Display.
 

 
The SFD with more than one LED Arrays
 

The SFD with two LED Arrays (we conditionally call them Lower and Upper ones) is shown on Fig. 3.
 

Figure 3. Schematic view of the SFD with two, upper and lower, RGB LED Arrays

The main difference of this embodiment from the basic embodiment is that in order to decrease stray light due to parasite scattering in the Fibers (especially in closer to the LED Array areas of the Fibers), two similar LED arrays, are situated on both ends of the Fibers as shown on Fig. 3, so that the activated scattering spots are illuminated by LEDs from closer LED array. Thus rows R1, R2 are illuminated by LEDs from upper LED Array, while rows R3, R4 are illuminated by LEDs from lower LED Array.  Scanning of the rows may start from the Display center in interlaced (upper-lower) manner to decrease stray light in the most important, central part of the display, though other than this algorithm can be used as well. In the rest work of this embodiment is similar to that of the basic embodiment of the SFD.

Of course, this approach adds to cost of the display, but this can be validated where contrast requirements are especially strict, or when using LEDs of different colors in different LED Arrays allows producing specific gamut (in this case the scattering spots are illuminated by LEDs of both LED Arrays).
 

 
The SFD as a photo- or videodetector
 

It is obvious that in case if RGB LED’s are used as, or supplemented (or substituted in pure video-detecting applications) by the photodetectors (like, for example, it is shown on Figure 4), the SFD can work as a photo- or video detector since activated scattering spots in the Fibers send fraction of incident light into the Fiber (in both directions) what can be detected by the photodetectors.
 

Figure 4. The SFD with a Photodetector Array

This approach can also be used for measuring incident light in different parts of the SFD during its operation as a display (during special incident light measuring cycles) to adjust brightness of different parts of the SFD depending on the incident light. If a prism is inserted between the Fibers’ endings and the Photodetector Array (for splitting light spectrum) then RGB Photodetectors could be used to detect color of the electrically activated scattering spots as well. This approach could also be used for automatic calibration of the SFD, e.g. in different lighting conditions or to compensate aging effects with time.
 

The SFD as a look-through display, eye-to-eye videoconferencing

If the distance between the Fibers (Fig. 1) is compared or more than diameter of the Fiber then (under condition that the conductive plate is transparent) the display is transparent (look-through) with sufficiently low distortion of coming through light. This can be used in eye-to-eye videoconferencing with use of plain notebook or desktop displays (a video-camera should be placed just behind the display, close to it's back surface) without application-specific appliances/constructions (like Exovision's solution or [3]). Of course, the camera behind the SFD should be temporarily stopped every time when the Rows over the camera's eye are activated (quite a small fraction of time). Using this eye-to-eye capability can be especially useful in videoconferencing in mobile devices since it adds a very specific atmosphere of personal contact between the calling parties AND saves valuable (and limited) real estate in the mobile device.

This approach can also be used in e.g. on-shield displays in cars (as well as in planes, premises, etc.) under condition that side of the fibers looking outside of e.g. the car is covered with non-transparent material to prevent light-radiation outside of the car.

Another way to make the SFD transparent is to make the Fibers flat with rectangular-like cross-section with minimal gap between the fibers (so minimal light distortion), though in this case the display will definitely radiate in both directions. Advantage of such approach can be more uniform image (less grainy), what can be important for mobile devices (eye-to-eye video-conferencing, single display in clamshell phones, etc.).

Also a mirror can be placed behind such display, not in front of it like in known Philips displays, what has advantages before the Philips' solution because in case of the SFD we have rather a display than rather a mirror.
 
 

The SFD as a reflective display
 

The SFD with reflective capabilities is shown on Fig. 5.
 

 

Figure 5. Schematic view of the SFD with reflective capabilities

The main difference of this embodiment from the basic embodiment is presence of the Column Wires connected to the Driving Circuit instead of the Conductive Plate (Fig. 1), what allows scanning the SFD’s surface in pixel-by-pixel manner rather than in line-by-line manner as described in previous embodiments. This allows using this display in reflective mode as well (either purely reflective or combined reflective-emissive mode), similar to way described in [2]. Magnitude (or/and duration) of drive voltage applied between corresponding Row Wires and Column Wires (Fig. 5) defines intensity of scattered light in the region of intersection of corresponding Row Wires and Column Wires. Thus if the surface of the Column Wires is made light-absorbing, amount of reflected light (due to scattering) will be proportional to incident light intensity and to magnitude of applied drive voltage. In this case reflected light will be monochrome what can be acceptable in many applications.

In order to provide colored reflected light, colored filters should be placed over the fibers, in this case in emissive mode each Fiber 1 should be illuminated by single LED of specified primary color close to color of corresponding filter.

So as follows from the above, the SFD according to this embodiment can be used in either reflective or emissive modes or both simultaneously, the last can be especially useful in fast-changing light conditions.
 
 

The SFD as a back display for a keypad
 

Having ability to be done thin (less than 1 mm) and at low-cost the SFD can be effectively used as a back display for a keypad with transparent buttons or whole key module as e.g. described in [4] and here. This approach can allow creating application/language/orientation adjustable keypads for mobile phones as well as for many other application-specific devices (like e.g. medical equipment, mission critical equipment, avionic products, etc.). Even more, keypad’s content can be dynamically adjusted by the Operator via wireless connection what opens new market opportunities for service providers. One of possible embodiments of such approach is shown on Figure 6.
 
 

 

Figure 6. The SFD as a back display for mobile phone keypad
 

It seems to be worth to notice that in this embodiment a single SFD display can be simultaneously used as both a back display (under the keypad) and a main display (beyond the keypad) what allows substantial cost saving of the mobile phone since same RGB LED array and driving circuit can be used for both purposes (main display and keypad back display). Since the SFD can be done flexible the back display for a keypad and the main display can be positioned in different levels, even not parallel to each other if necessary.
 
 

Conclusion

As it follows from the above description,  the recognizable advantages of the SFD before LCD's are as following:

• VERY cheap, no expensive circuitry, no expensive substrate, circuitry;
• Easy manufacturing (due to no need of very specific substrates as well);
• One colored pixel per dot  on the SFD surface (not three as in the LCD and other displays);
• VERY effective from point of power consumption (just a part can be operated if needed);
• No need in backlight;
• Can be done very thin (about or less than 1mm);
• Bright enough for good visibility in the bright sun;
• High resolution, up to 300 dpi and higher can be achieved (good for mobile devices and wearable applications);
• No visible grid;
• Looking-through capability, eye-to-eye videoconferencing on mass computers and mobile devices;
• Can be done flexible at least in one direction (even rolled up if necessary);
• More than three primary colors per dot can be cost-effectively used for wider and more color-reach gamut;
• Can be done big (as big as modern plasma displays) cost effectively;
• Wide angle of visibility (up to 180 degrees);
• Can be visible from both sides (if the Conductive Ground Plate is transparent);
• High durability (less breaking parts like glass substrate, simpler driving circuit);
• Low-cost automatic aging/ambient light/temperature correction capability (if e.g. using photodetectors as described above);
• Other than rectangular form-factors can be easily realized (due to flexibility of the fibers and simplicity of the wiring);
• Wider range of working temperatures if other than “LC in polymer” scattering mechanisms are used;
• Wider range of applications.
   
   

References

1. Chi Kwong Chow. “Trends in Portable Displays”. Information Display Magazine, Vol 21, No. 1, p. 18-20 (2005).
 
2. Miki, Yuichiro. “Display element and display device”. United States Patent Application 20040245915, published on December 9, 2004.

3. Smoot, Lanny S. "Teleconferencing terminal with camera behind display screen". United States Patent 4,928,301, issued on May 22, 1990.