Led based projectors have numerous advantages compared to traditional projectors, such as: compact, larger color gamut, longer lifetime, lower supply voltage, etc. As LED's can switch rapidly, there is the possibility to pulse. However, there is also an important disadvantage. The optical power per unit of etendue of a LED is significantly lower than e.g. an UHP-lamp (approximately 50 times). This problem can be remedied partly by pulsing of the LED’s. If one drives a LED with a pulsed current source, the peak luminance can be higher, albeit that the average luminance will not increase. By pulsing X LED's alternately, their increased flux can be added up in time and will generate a higher average flux within the same etendue. This can be carried out in a number of different configurations. The first configuration uses moving components where a number of LED's (e.g. 8) are mounted on a carrousel and consecutively the pulsed LED is brought in the light path of the projector to fill up the time with its peak flux. An alternative without moving components can be reached with 2 LED's which are combined with a PBS. By alternately pulsing the LED's with 50% duty cycle and changing the polarisation of one LED with a switchable retarder, one can combine the flux of both LED's in the same etendue. Because of its fast switching time ferro-electric retarders are used here. This can be extended further to 4,8,16... LED's, at the price of a larger and more complicated optical architecture.
In terms of the Etendue, projection is a very demanding application for the light source. In order to reach sufficient brightness, the light from the lamp has to be collected into the small Etendue of nowadays' displays. This can only be achieved with lamps that provide a high luminance out of a short arc. For a long time now, Xenon-lamps are used as the light source for large-screen cinema projectors. While Xenon-lamps can offer the required high luminance, they suffer from a very low efficiency leading to the typical multi-kW power ratings of cinema projectors. Modern portable projectors show almost the same performance with much lower wattage UHP-lamps. Therefore, in applications with smaller screen sizes, UHP-lamps are nowadays most commonly used. In this article a comparison of these two different lamp technologies is given. The light-technical properties of these lamp types are compared and advantages as well as disadvantages of the lamps will be discussed for the case of projection.
Xenon produces a brilliant white flash of light when it is excited electrically. The characteristics Xenon brings to short arc, high-pressure xenon lamps are substantial, particularly in display systems. The broadband and relatively flat profile of the xenon emission in the visible spectrum generates superior metameric matching to life-like colors, whereby the visible spectrums of other lamps generally contain spikes requiring additional adjustment. Xenon-powered lamps generate a native color point of 5900K to 6200K - very near the optimal D65 point - increasing efficiency and minimizing the need for filtering. Filtering often results in an undesirable loss of luminous efficacy. Instant turn-on/turn-off is possible since the xenon is in its gaseous state at the operating pressures of the lamp. This is an improvement over most ultra-high pressure mercury lamps requiring warm-up times. The DC drive of the lamp provides a compact arc near one of the electrodes that produces a very small volume of light that is ideal for coupling to elliptical reflectors. The light output can be dimmed by controlling the applied electrical power. Xenon-powered short arc, high-pressure lamps operate safely within the specified parameters, and incorporating a reflector within the lamp body provides alignment-free replacement. The xenon lamps also eliminate the use of mercury, an important benefit in today's environmentally-conscious industry.
The rapid development of high power visible LEDs has prompted an increase in interest in the projection display industry. Although the power level has not achieved sufficient level for large projection displays, it is sufficient for smaller displays. It is also expected that in the near future, there will be significant improvements in the LED output and the market may open up. In this paper, a LED based illumination engine is presented which is suitable for projection display using an array of LEDs whose output is coupled into a single light pipe with minimum loss of brightness. The basic building block in the array consists of an LED chip mounted on a heat conductive substrate, a concave reflector, and a tapered light pipe. The concave reflector can be spherical, toroidal, elliptical, or dual paraboloidal. The LED light is focused at the input of the tapered light pipe by the concave reflector. The output dimension of the light pipe is made so that it has the same cross-sectional size of the building block, and therefore, the array of tapered light pipes are packed tightly at the output end without lost space in between. This will allow the most efficient way to combine the light output without loss in etendue. Various configuration of this system will be shown below that is applicable to projections displays.
We discuss a compact RGB source with ouput power of several watts per color consisting of a red (638 nm) diode and OPS lasers with blue (460 nm) and green (530) nm output. Suitability for display applications is shown by replacing the lamp of a standard Rear Projection TV.
With the pressure to reduce cost for mass-market introduction of microdisplay-based rear projection television (MD-RPTV), the image panel and the related optical components have to be reduced in size and novel optical arrangements have to be created to achieve the target price. One major issue always had been the need for more light. Traditional reflector systems, including elliptical and parabolic reflectors, perform well in most cases, but are inefficient for smaller etendue values corresponding to smaller image panels. The common remedy is to make lamps with shorter and shorter arcs to increase the coupling efficiency, but the corresponding lifetime of the lamps are reduced and most of the time, these short arc lamps can only operate at low power, thus limiting the total output of the illuminating system. This paper summarizes the progress in the last few years related to the dual Paraboloid reflector (DPR) system and the associated components including polarization recovery systems and light pipe based projection engines.
The UV stability of empty liquid crystal (LC) cells incorporating commercial polyimide (PI) and silicon-dioxide (SiO2) alignment layers under nitrogen environment and the vacuum-filled LC cells were studied. Experimental results show that the molecular alignment of PI cells is degraded after 10 hours of UV (l~365 nm) illumination at intensity I~350 mW/cm2. Two commercial TFT-grade LC mixtures (low birefringence MLC-9200-000 and high birefringence TL-216) exhibit a longer lifetime in SiO2 cells than in PI cells. Moreover, MLC-9200-000 has a much longer lifetime than TL-216. To lengthen the lifetime of a LCD projector, UV transparent PI layers or inorganic SiO2 layers, high optical density UV filter, longer cutoff-wavelength blue filter, and low birefringence LC materials should be considered.
Contrast limits are investigated for MacNeille PBS based LCOS projection systems that use retarder stack filters (RSF). The two contributing factors are considered separately; namely the color management system and the panel port. To enhance performance of the former, skew ray compensated RSFs are introduced. For the latter, a general methodology is presented to optimize contrast by compensating the LCOS panel. It is shown that the orientation of the LCOS panel and compensator, relative to the MacNeille PBS, is critical. The significant impact of AR coating performance on system contrast is also revealed. A high contrast architecture will be presented by way of example.
An optical design of digital light processing projector accommodating three pieces of micro digital mirror array display panels is presented. The system is configured by the combination of a color separation unit and a color re-combination prism, which combines light signals from individual display panels and then directs the synthesized full color images projected onto the screen. The color separation unit comprises two dichroic filters set at 40 degrees with respect to the optical axis of the system and two reflecting mirrors with relay lens for guiding light. The color re-combination unit is contructed in the form of a modified x-prism for better achievement of image contrast, color saturation, and light efficiency. The present system out-performs systems based on the configuration of Philips prism in less occurrence of degraded image quality associated with teh thermal influence of the optical engine, short optical path in prism and back focal length of the projection lens, and the physical size and weight of the unit. Comparisons in overall optical characteristics of the proposed system and those designed by means of various modules of single prism, which operates color separation and color combination processes along same optical path through the prism block will also be presented.
A series of measurement techniques (including measurement of the image on the display) are employed to determine the severity of motion errors, in order to gauge their effects on subjective evaluation of image quality and interpretability, and to identify and eliminate sources of motion errors. High performance display systems are increasingly being used for playback of moving imagery, making the accurate depiction of motion more important than ever. Higher definition formats drive systems harder, and source image motion can result in gross motion errors in the image seen on the display. Contributing factors can be choice of content, compression and transmission, graphics engine, and physical display device. Such motion errors can significantly impact perceived motion imagery quality and the perceived quality of the content, the server, and the display device. The NIST Motion Image Quality project is concerned with: (1) perceived image quality, and (2) motion image interpretability. The project uses both subjective analysis and quantitative measures of image characteristics. In the course of producing and collecting video content for subjective analysis, the project has encountered combinations of source and display system that produce severe motion artifacts that clearly affect image quality, and appear to impact interpretability as well. The apparent severity of the effects varies somewhat by the display technology used (DMD, LCD, CRT), which may indicate differences in the ways the display systems react to incoming signals.
The accurate measurement of spatial resolution can be critical in the characterizations of projection display systems. Techniques can be used to determine resolution of projection systems by measuring the contrast of alternating grille patterns or fully modulated sine waves of various spatial frequencies. Unfortunately, the measurement of the contrast of these patterns may be influenced by stray light, either from ambient and reflected light in the environment, or from veiling glare (light scattering) in the lens of the light-measuring device. Such stray-light corruption can lead to large errors in contrast determination, providing an inaccurate and misleading characterization of the projector. For large-area measurements, various tools and methodologies have been employed, including the use of frustums and masks, to minimize such unwanted effects and provide a more accurate measurement. With some modifications, these same tools may be used for small-area measurements with similar results. Results will be shown comparing resolution determinations using different test methodologies with and without stray light compensation.
An approach to increase the coupling-out efficiency of organic light-emitting diodes (OLEDs) was studied. The approach, especially, is suitable for liquid crystal display (LCD) backlight applications. We demonstrated that the anisotropic scattering polarizer (ASP) laminated to the glass substrate surface increased the optical efficiency of OLED for LCD backlight applications. The ASP was properly prepared by drawing the liquid crystal polymer (LCP) dispersed poly(carbonate) film. The ASP could extract the emitted light trapped in substrate of OLED involved in polarization selectivity. The light extraction ability and the polarization selectivity were changed by the thickness of electron transporting layer (ETL) of OLED. The optical efficiency for 60 nm ETL thickness device through absorbing polarizer was not improved significantly by the lamination of ASP. Alternatively, the low efficiency of 120 nm ETL thickness device was effectively improved by the lamination of ASP, which was due to the extraction of waveguided light as a substantially polarized emission. Although the degree of increase for 120 nm ETL thickness device by the lamination of ASP was substantially large, the final optical efficiency was less than or comparable to that for 60 nm ETL thickness device. However, in the case of 90 nm ETL thickness device, the lamination of ASP increased the optical efficiency by the factor of 1.2-1.3 against 60 nm ETL thickness device.
In this paper, we report a high efficiency organic light-emitting device (OLED) with a high electron mobility electron transport layer (ETL) material and high efficiency blue dopant material. Typically, the mobility of a hole transport layer (HTL) material is much higher than that of an electron transport layer (ETL) material. Here we used the bis(10-hydroxyben-zo[h]quinolinato)beryllium (Bebq2) as the ETL material that exhibits superior electron mobility. It effectively reduced the driving voltage and increased the power efficiency. The blue dopant material was 4,4'-bis[2-(4-
(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) in the 9,10-bis(2’-naphthyl) anthracene (BNA), that was the host material of the emitting layer (EML). At 100 cd/m2, the current efficiencies with different dopant concentration can be as high as 19.2 cd/A with the CIE coordinate at (0.154, 0.238). Driving voltage at 20mA/cm2 was 4.94 V of this device. With increasing the doping concentration, the drive voltage variation did not vary much and was within 0.6 V at 20 mA/cm2. The emitting mechanism in such a device may be mainly due to energy transfer rather than carrier trapping. CIE coordinate of such a device shifted toward blue with increasing current density due to intense light emission from the host material of the EML. The highest efficiency was achieved when doping concentration is 3%.
Proc. SPIE 5740, Fabrication of organic light-emitting diodes using ITO anodes grown on polyethersulfone (PES) substrates by pulse-laser deposition, 0000 (10 April 2005); https://doi.org/10.1117/12.590113
Organic light-emitting diode (OLED) has a good candidate for next generation flat panel display (FPD). However, it is very difficult to fabricate high performance OLEDs on plastic substrate because its mechanical and thermal properties are very poor. Before the ITO deposition, we used a new plasma treatment for good contact with ITO and PES. PES substrate is stayed in vacuum above 12 hours to reduce humidity and unknown chemical gas.
We successfully fabricate OLED on PES substrate using PLD-ITO anodes. We can observe more uniform and bright emission image from the OLED and fix the optimum conditions for fabrication process for OLED. Maximum electro luminescence (EL) and current density at a 100 cd/m2 are 2500 cd/m2, 2mA/cm2, respectively and external quantum efficiency of OLED is about a 2.0%.
In this paper, we demonstrate simulation results of a top-emission organic light-emitting device (TOLED) with a passivation layer and a dielectric layer. Passivation layer is usually composed of silicon dioxide (SiO2) and/or silicon nitride (Si3N4) to protect organic layers from oxygen and moisture. Dielectric layer is a high refractive index thin film for enhancing the external quantum efficiency. The TOLED device has a microcavity structure which comprised of an opaque and high reflective anode and a thin semitransparent cathode. When varying dielectric layer thickness, the output intensity changes and the spectrum peak shifts. The peaks oscillate as a function of the dielectric thickness and the period is around hundreds of nanometers depending on the refractive index of the dielectric layer. When adding the passivation layer, which is on the order of micrometers, more than single peak are observed. With a simple model, we found that the frequency difference between two peaks corresponds to the free spectral range of the fabry-perot cavity formed by passivation layer. When a passivation layers is added on the TOLED, the microcavity effect results in the presence of multi-peaks. It limits the view angle and decreases the color purity.
A novel 5-in. R.G.B densely packed poly-crystal MgAl2O4 (magnesium-aluminum-oxide) phosphor screens with multi-layer interface filter have been developed. The filter is a multi-layer alternate high and low index of refraction film which was fabricated by depositing high index of refraction of titanium oxide and low index of refraction of silicon oxide. The 5-in.R.G. B densely packed poly-crystal phosphor screens with the filter have 60% more luminance than those of common R.G. B poly-crystal phosphor screens, and the area of their chromaticity is almost 10% larger than that of common poly-crystal phosphor screen. The 5-in.R.G. B densely packed poly-crystal phosphor screens with multi-layer interface filter are fabricated by using a centrifugal sedimentation method. The resolution of densely packed poly-crystal phosphor screen with multi-layer interface filter is higher than that of conventional poly-crystal phosphor screen. An experimental 52-inch TV set adopting the set of 5-inch densely packed poly-crystal projection CRT with multi-layer interface filter has very high resolution of 2000 TV lines and 1800 cd/m2 luminance, and it is easy to realize the HDTV display.
An electron beam pumped vertical cavity laser, or an “eVCSEL”, has been developed as a low-cost light source for LCOS and DLP based consumer television. 1000 lumens directed towards the spatial light modulator requires a total power of 144 watts for lasers in the three primary colors. This power surplus allows for high screen brightness for rear projection televisions of diagonals greater than 50 inches and eliminates the need for high gain screens with the benefit of larger viewing angles. Because of the high saturation of laser light, a color gamut approaching that of the human visual system is possible, creating superior image reproduction.