In this work, the reverse replication of circular micro grating structures on glass substrates is implemented using an ultra-violet curable resin and a polydimethylsiloxane (PDMS) mold which has the same structure as the original circular grating master. Two different techniques (“double PDMS replication” and “polymer- PDMS replication”) are employed to fabricate those reversed circular micro grating structures. Surface profiling measurements show that in case of the polymer-PDMS replication the dimensions of the resulting circular grating structures closely approximate those of the master, while the grating height is slightly decreased in case of the double PDMS replication technique, mainly due to the use of the releasing agent. For both methods, the grating slopes of the circular gratings are almost unchanged, leading to the desired optical performance. The two techniques are quite useful for more accurate reverse replications of micro optical and photonic structures.
In this article we report the fabrication of large arrays of micro-optical gratings using soft embossing with elastic Polydimethylsiloxane (PDMS) molds and ultra-violet (UV) curable resins. Three different kinds of resins are used to replicate the master gratings in a process akin to a roll to roll process. The optical surface profiling measurements show that the dimensions of the replicated gratings closely approximate those of the master gratings. Optical diffractions of these gratings are also measured and analyzed.
Well-being applications demand unobtrusive treatment methods in order to reach user acceptance. In the field of light therapy this needs to be carefully addressed because, in most cases, light treatment system size has to be significant with respect to human body scale. At the same time we observe the push to make wearable devices that deliver the treatment on the go. Once scaled up, standard flexible electronics (FPC) fail to conform to body curvatures leading to decrease in comfort. A solution to this problem demands new or modified methods for fabrication of the electronic circuits that fulfill the conformability demand (flexing, but also stretching). Application of Stretchable Molded Interconnect (SMI) technology, that attempts to address these demands, will be discussed. The unique property of SMI is that its manufacturing draws mainly from standard PCB and FCB technologies to inherit the reliability and conductivity. At the same time, however, it allows soft, flexible and stretchable circuits with biomimetic haptics and high optical efficiency. In this work a demonstrator device for blue light therapy of RSI is presented that illustrates the strengths as well as challenges ahead of conformable light circuits. We report system electro-optical efficiency, possible irradiance levels within skin thermal comfort and efficiency under cyclic, tensile stretching deformation.
Disparity estimation is a highly complex and time consuming process in multiview video encoders. Since multiple views taken from a two-dimensional camera array need to be coded at every time instance, the complexity of the encoder plays an important role besides its rate-distortion performance. In previous papers we have introduced a new frame type called the D (derived) frame that exploits the strong geometrical correspondence between views, thereby reducing the complexity of the encoder. By employing the D frames instead of some of the P frames in the prediction structure, significant complexity gain can be achieved if the threshold value, which is a keystone element to adjust the complexity at the cost of quality and/or bit-rate, is selected wisely. A new adaptive method to calculate the threshold value automatically from existing information during the encoding process is presented. In this method, the threshold values are generated for each block of each D frame to increase the accuracy. The algorithm is applied to several image sets and 20.6% complexity gain is achieved using the automatically generated threshold values without compromising quality or bit-rate.
Disparity estimation is a highly complex and time consuming process in multi-view video encoders. Since multiple views taken from a 2D camerea array need to be coded at every time instance, the complexity of the encoder plays an important role besides its rate-distortion performance. In previous papers we have introduced a new frame type called D frame that exploits the stron geometrical correspondence between views, thereby reducing the complexity of the encoder. By employing D frames instead of some of the P frames in the prediction structure, significant compexity gain can be achieved if the trhreshold value which is a keystone element to adjust the complexity at the cost of quality and/or bit-rate is selected wisely. In this work, a new adaptive method to calculate the threshold value automatically from existing information during the encoding process is presented. In this method, the threshold values are generated for each block of each D frame to increase the accuracy. The algorithm is applied to several image sets and 20.6% complexity gain is achieved using the automatically generated threshold values without compromising qaulity or bit-rate.
We present two concepts to create multiview 3-D displays with high spatial and angular resolution at a reasonable system cost. We investigate a rear projection approach using only one projector with a digital micromirror device as a light modulator. The first concept is based on time sequentially illuminating the entire light modulator from different directions. Each illumination direction corresponds to a different viewing zone. We design an illumination system that generates all distinct illumination beams, and a lens system integrated into the projection screen to enlarge the viewing zones. A second concept is based on a projection screen architecture that steers images into different horizontal directions and does not require a directional illumination of the light modulator. In this way, the entire acceptance étendue of the projection system can be used for every image. This is achieved by horizontally moving a double-sided lenticular sheet with respect to a sheet of microlenses with a square footprint. Both concepts are investigated with advanced optical simulations.
Micromirrors are a typical example of Micro-Electromechanical Systems (MEMS) with many applications including
optical scanners, optical switching, projection displays, etc. We have succeeded in producing MEMS micromirrors in a
SiGe structural layer, which can be used to realize CMOS-integrated MEMS structures. Several pixel designs were
simulated using COMSOL multiphysics and subsequently verified in hardware. They differ in mirror size, hinge length
and number of attracting electrodes (two or four). One particular mirror design enables variable Pulse Width Modulation
(PWM) addressing. In this design, the mirror switches between two extreme states with a variable duty cycle determined
by two generic high voltage signals and two CMOS-compatible pixel-specific DC voltages applied to the four attracting
electrodes. The processed arrays were subjected to Laser Doppler Vibrometer (LDV) measurements in order to verify the
simulation results. The simulated and measured pull-in voltages are compared for 8, 10 and 15μm mirrors. The
agreement between simulation and measurement lies within the expectations, which is an encouraging result for future
Proc. SPIE. 7690, Three-Dimensional Imaging, Visualization, and Display 2010 and Display Technologies and Applications for Defense, Security, and Avionics IV
KEYWORDS: Light emitting diodes, Modulation, Visualization, Liquid crystal on silicon, Projection systems, Digital micromirror devices, 3D displays, LED displays, 3D visualizations, 3D image processing
LED-based projection systems have several interesting features: extended color-gamut, long lifetime, robustness
and a fast turn-on time. However, the possibility to develop compact projectors remains the most important
driving force to investigate LED projection. This is related to the limited light output of LED projectors
that is a consequence of the relative low luminance of LEDs, compared to high intensity discharge lamps. We
have investigated several LED projection architectures for the development of new 3D visualization displays.
Polarization-based stereoscopic projection displays are often implemented using two identical projectors with
passive polarizers at the output of their projection lens. We have designed and built a prototype of a stereoscopic
projection system that incorporates the functionality of both projectors. The system uses high-resolution liquidcrystal-
on-silicon light valves and an illumination system with LEDs. The possibility to add an extra LED
illumination channel was also investigated for this optical configuration. Multiview projection displays allow the
visualization of 3D images for multiple viewers without the need to wear special eyeglasses. Systems with large
number of viewing zones have already been demonstrated. Such systems often use multiple projection engines.
We have investigated a projection architecture that uses only one digital micromirror device and a LED-based
illumination system to create multiple viewing zones. The system is based on the time-sequential modulation
of the different images for each viewing zone and a special projection screen with micro-optical features. We
analyzed the limitations of a LED-based illumination for the investigated stereoscopic and multiview projection
systems and discuss the potential of a laser-based illumination.
We present compact illumination engines for DMD projection systems making use of light emitting diodes (LEDs) as light sources. The impact of uniformization optics and color-combining dichroic filters is investigated with respect to the color uniformity on the screen. PhlatLight LEDs are considered as light sources because of their superior luminance levels. Also PhotonVacuum optics are used to collimate and transform the emitted LED light distribution. The optical engines are simulated with advanced non-sequential ray tracing software. They are evaluated on the basis of étendue efficiency, compactness and color uniformity of the projected images. Color plots are used as tools to investigate the simulated color gradients in the image. To validate our simulation models, we have built a compact prototype LED projector. Its color-related specifications are compared with the simulated values.
We present two multiview rear projection concepts that use only one projector with a digital micromirror device
light modulator. The first concept is based on time sequentially illuminating the light modulator from different
directions. Each illumination direction reflects on the light modulator toward a different viewing zone. We
designed an illumination system that generates all distinct illumination beams and a lens system integrated
into the projection screen to enlarge the viewing angles. The latter is crucial since the viewing extent of the
viewing zones decreases inversely proportional to the size of the projected image. A second concept is based on
a specific projection screen architecture that steers images into different horizontal directions. In this way, the
entire acceptance ´etendue of the projection system can be used for every image. This is achieved by moving a
double-sided lenticular sheet horizontally with respect to a sheet of microlenses with a square footprint. Both
concepts are investigated with advanced optical simulations.
Disparity estimation can be used for eliminating redundancies between different views of an object or a scene recorded
by an array of cameras which are arranged both horizontally and vertically. However, estimation of the disparity vectors
is a highly time consuming process which takes most of the operation time of the multi-view video coding. Therefore,
either the amount of data that is to be processed or the complexity of the coding method needs to be decreased in order to
encode the multi-view video in a reasonable time. It is proven that the disparities of a point in the scene photographed by
cameras which are spaced equidistantly are equal. Since there is a strong geometrical correlation of the disparity vectors,
the disparity vector of a view can for most blocks be derived from the disparity vector of another view or views. A new
algorithm is presented that reduces the amount of processing time needed for calculating the disparity vectors of each
neighboring view except the principal ones. Different schemes are proposed for 3x3 views and they are applied to
several image sequences taken from a camera-array. The experimental results show that the proposed schemes yield
better results than the reference scheme while preserving the image quality and the amount of encoded data.
We present a single optical system that can simultaneously generate two linear polarized full-color images with
orthogonal state of polarization. The system architecture of the optical core is discussed. Four liquid crystal
on silicon panels are used to modulate both images. We also discuss the design of the illumination system
with light emitting diodes as light sources. The contrast of both images is simulated. A proof-of-concept
demonstrator is built and experimentally characterized. It is capable of two-dimensional and three-dimensional
image display. Three-dimensional images can be perceived, independent of the tilt angle of the viewer's head, by
wearing specific polarization sensitive eyeglasses and placing a quarter-wave retarder at the projector's output.
Important component specifications are overviewed to improve the performance of the demonstrator setup.
LED-based projectors have numerous advantages compared to traditional projectors. They are more compact, they exhibit a larger color gamut and a longer lifetime, the supply voltage is lower and they can even operate on batteries. LEDs can switch rapidly (possibility to pulse) and they have a high dimming ratio (contrast considerations). However, they have low optical power per étendue, although this is also improving consistently. With an efficient illumination engine design we can build an LED projector with a moderate light output and with superior properties. We present a relatively compact LED projector with two liquid crystal on silicon (LCOS) light valves (LVs). One of these LVs alternately modulates red and blue information, while the other permanently modulates green information to achieve a good color balance. Additionally, we apply some methods to increase the brightness on the screen. Our two-LCOS approach results in a compact, efficient LED projector that produces 171 lm projected D65 flux.
LED based projectors have numerous advantages compared to traditional projectors: they are more compact, exhibit a larger color gamut and a longer lifetime, the supply voltage is lower, the absence of ultra violet, infrared radiation and mercury vapour, etc. Furthermore LED's can switch on and off very rapidly (possibility to pulse them) and they have a high dimming ratio that can be used to improve the contrast. However, there is also an important disadvantage: the optical power per unit of etendue (luminance) of an LED is significantly lower than that of e.g. an UHP-lamp. Because of this and the etendue limitation of the projector (small light valve, f-number projection lens), the projected flux on the screen will not be high. Despite this shortcoming, LED's are still very interesting for low power applications because of their superior properties. However we have to collect the available light flux optimally and combine multiple LED's with high luminance within the available system etendue. In this paper we have studied collection optics that collect the LED flux with high optical efficiency and collimation and reshape the spot in a uniform illuminated rectangle with the sizes of the micro display. We have designed 'Gradually Tapered Light Pipes', 'Elliptical Reflectors' and 'Parabolic Reflectors'. Furthermore we have combined many of these LED/collector combinations to get a high luminance illumination engine for LED based projectors.
LED-based projectors have numerous advantages compared to traditional projectors, such as compactness, larger color gamut, longer lifetime, and lower supply voltage. As LEDs can switch rapidly, there is the possibility to pulse. However, there is also an important disadvantage. The optical power per unit of étendue of an LED is significantly lower than, e.g., an ultra-high-performance (UHP) lamp. This problem can be remedied partly by pulsing the LEDs. If one drives an LED with a pulsed current source, the peak luminance can be higher, albeit the average luminance will not increase. By pulsing two LEDs alternately (50% duty cycle), their increased flux can be added up in time and will generate a higher average flux within the same étendue. We combine the LEDs with a polarizing beam splitter (PBS) and change the polarization of one LED with a switchable retarder. The achieved substantial net gain after all losses is 36%.
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.
We developed a measurement method for the characteristics of microdisplays specifically aimed at vertically aligned nematic reflective cells. It allows determination of contrast ratio and cell gap, and gives good estimates for the pretilt angle and the elastic surface-coupling constant. The set-up consists of a laser source, high quality polarisers, a beamsplitter mirror, a quarter-wave plate and a sensitive photodiode. A model for the polarization changes in the light caused by each component allows the extraction of the initial phase retardation induced by the cell and gives a first estimate of the thickness. Simulation of the director configuration in liquid crystals is then used to enhance the accuracy by taking into account the properties of a real LC cell. Matching of the simulation and the measurements yields the required values together with a calibrated simulation model.
Microdisplays are considered to be an enabling technology for ultra-high resolution displays. Pixels can indeed be made very small using CMOS technology. Nevertheless, in multimillion pixel microdisplays, the die size becomes a limiting issue, because the optical field size of present day lithographic steppers, and consequently the resulting chip size, is limited to about 20x20 mm2. This paper shows how this limitation can be overcome using stitching techniques and discusses the resulting challenges in design as well as lithography. The partitioning of the schematics and the layout in stitchable modules is reviewed, as well as the definition of boundary conditions for these modules. The difficulties concerning design verification, and a virtual stitching procedure that can be helpful, are presented. From a technological point of view, the stepper job creation and the scribe line definition is described, as well as the positioning accuracy that is achieved. It is also shown that careful partitioning of the design can lead to a single stitchable mask set that allows the creation of microdisplays with several different resolutions. The paper is illustrated with results from the European Esprit project Mosarel, a project that has shown the feasibility of a 2560x2048 pixel microdisplay.
Microdisplay-based imaging system are quickly becoming very important. A light valve based on a GXGA microdisplay with 5 megapixels is targeted. The pixel pitch is 15 micrometers , leading to an active display area of 38.4 by 30.72 mm. Both the design and the fabrication of such a large die have presented several challenges. This paper describes the design of reticles for GXGA resolution with on-chip drivers. The chip size associated with this resolution e4xceeds the available reticle size. Stitching is used for the processing of large x-Si backplanes. The reticle set is also suitable for GXGA and XGAp resolutions with drivers. The front-end layers are implemented in a standard double metal 0.7micrometers CMOS technology extended with minimal adaptations. The back-end comprises advanced sub-halfmicron processing steps: two metallization layers with a 500nm-feature size for the light shielding function resp. mirroring function.
Recently LCOS microdisplays are becoming available for personal IT applications, despite some problems which are less critical in poly silicon or amorphous silicon based displays. The most common problems which must be encountered are the polarization of the pixels and the light shielding of the silicon substrate. In this paper a methods proposed which solves the light shielding and pixel flatness problem. A non-critical back-end processing which can be applied outside the silicon foundry has been developed. The effectiveness of the light shielding on a working demonstrator display is shown. To avoid light losses caused by a polarization filter, a polymer dispersed LC has been chosen. By decreasing the cell gap we made the PDLC voltage compatible with a standard 3 micrometers CMOS process and its response fast enough to be used for video applications. It is shown that this choice is very suited in direct view and portable applications. The realized prototype has 3 bit grey levels and is video compatible and can be used in a number of applications, such as personal viewers, PDAs and data displays.
Study of the possible use of a-SiN: H thin films for 2D direct x-ray sensor arrays lead to the development of a simple prototype. The sensor array is a 100 X 100 array of simple cross-over silicon rich a-SiN:H thin film diodes with sizes 200 X 200 micrometers formed on a 2 inch glass substrate. There are neither switching elements nor x-ray conversion layer involved which leads to extremely simple 2 mask processing for the whole array. Specific behavior of the a-SiN:H sensing diodes under x-ray irradiation requires special attention to be paid to the driving strategy and read out electronics. Experimental results obtained with the help of the prototype provide solid base for the discussion on both sensor properties and electronic components used for the sensor array control.