We investigate the level of speckle that can be tolerated in a laser cinema projector. For this purpose, we equipped a movie theatre room with a prototype laser projector. A group of 186 participants was gathered to evaluate the speckle perception of several, short movie trailers in a subjective ‘Quality of Experience’ experiment. This study is important as the introduction of lasers in projection systems has been hampered by the presence of speckle in projected images. We identify a speckle disturbance threshold by statistically analyzing the observers’ responses for different values of the amount of speckle, which was monitored using a well-defined speckle measurement method. The analysis shows that the speckle perception of a human observer is not only dependent on the objectively measured amount of speckle, but it is also strongly influenced by the image content. As is also discussed in [Verschaffelt et al., Scientific Reports 5, art. nr. 14105, 2015] we find that, for moving images, the speckle becomes disturbing if the speckle contrast becomes larger than 6.9% for the red, 6.0% for the green, and 4.8% for the blue primary colors of the projector, whereas for still images the speckle detection threshold is about 3%. As we could not independently tune the speckle contrast of each of the primary colors, this speckle disturbance limit seems to be determined by the 6.9% speckle contrast of the red color as this primary color contains the largest amount of speckle. The speckle disturbance limit for movies thus turns out to be substantially larger than that for still images, and hence is easier to attain.
Secondary optics that allow for the integration of a light-emitting diode (LED)-based luminescent light source into various étendue-limited applications—such as projection systems—are investigated. Using both simulations and experiments, we have shown that the optical efficacy of the luminescent light source can be increased using a collimator. A thorough analysis of the influence of the collimator’s refractive index on the optical outcoupling and luminance is investigated and it is shown that it is most optimal to use a refractive index of 1.5. The optimal shape of the collimator is equal to that of a compound parabolic concentrator. Experimental results show that by using a collimator, we can improve the amount of outcoupled light with a factor of 1.8 up to 2.1 depending on the used optical configuration of the LED-based luminescent light source.
Light pipes are key optical components used in projection systems to transport and homogenize light from the source towards the light valve. They can provide a uniform light distribution at their output as a result of multiple internal reflections. In laser projection systems, such light pipes are useful in combination with a laser-light module consisting of one or more single-mode lasers and a rotating diffuser. The partially coherent light emanating from the rotating diffuser is transported and homogenized towards the end of the light pipe. Consequently, propagation through the light pipe will also modify the coherence properties of the laser light. In this paper, a computationally efficient simulation model is presented to propagate partially coherent light through a homogenizing rectangular light pipe. The resulting coherence function clearly differs from that of free-space propagation over the same optical path length. The implications of these results on, for example, the appearance of speckle are discussed in further detail. The simulation results are experimentally verified using a reversing wavefront Michelson interferometer. The approach described in this paper can be extended further to investigate other types of light pipes, such as tapered light pipes or even more complex ones.
The introduction of lasers for projection applications is hampered by the emergence of speckle. In order to evaluate the
speckle distorted image quality, it is important to devise an objective way to measure the amount of speckle. Mathematically,
speckle can be described by its speckle contrast value C, which is given by the ratio between the standard deviation
of the intensity fluctuations and the mean intensity. Because the measured speckle contrast strongly depends on the parameters
of the measurement setup, in this paper we propose a standardized procedure to measure the amount of speckle
in laser based projection systems. To obtain such a procedure, the influence of relevant measurement set-up parameters is
investigated. The resulting measurement procedure consists of a single digital image sensor in combination with a camera
lens. The parameters of the camera lens are chosen such that the measured speckle contrast values correspond with the
subjective speckle perception of a human observer, independent of the projector's speckle reduction mechanism(s). Finally,
the speckle measurement procedure was performed with different cameras and the results were compared.
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.