Standardized speckle measurement method matched to human speckle perception in laser projection systems

Equal-intensity beam splitter realization by wire grid polarizers for passive laser speckle reduction

A method to realize an equal-intensity beam splitter (EIBS) using wire grid polarizers (WGPs) is proposed. The EIBS consists of WGPs with predetermined orientations and high-reflectivity mirrors. We demonstrated the generation of three laser sub-beams (LSBs) with equivalent intensities using EIBS. The three LSBs were incoherent by introducing optical path differences larger than the laser coherence length. The LSBs were used to reduce speckle passively, where the objective speckle contrast was reduced from 0.82 to 0.5 when all three LSBs were used. The feasibility of EIBS in speckle reduction was studied using a simplified laser projection system. The structure of the EIBS implemented by WGPs is simpler than EIBSs obtained by other methods.

Color-speckle assessment in multi-primary laser-projection systems based on a 3D Jzazbz color space

We propose and demonstrate a color-speckle assessment method based on a three-dimensional J_za_zb_z color space, which is appropriate for both three-primary and multi-primary systems. In the proposed scheme, new physical quantities are defined to describe the color-speckle characteristics, which provides a general and intuitive color-speckle evaluation for different laser projectors. Experimental verification is also performed using three-primary and six-primary laser projectors. The simulation and measurement results are consistent.

Investigation of resolution and microphone size for measurements of airborne ultrasound

The growth of ultrasound technologies has entailed the presence of airborne ultrasound emissions in industry and public and private spaces. For occupational safety and health, procedural and technical recommendations are needed that allow measurements to be performed in the field within a short time and with little effort. This study aims to answer two research questions concerning the minimum spatial resolution and microphone size required to determine a spatial distribution of sound pressure levels with sufficient accuracy within a given measurement uncertainty. The investigation focused on high spatial resolution datasets of the airborne ultrasound field of an ultrasonic welding machine acquired using a laboratory measuring system. Based on these datasets, datasets with lower resolutions were generated and measurements with microphones of different sizes were simulated. Subsequently, four different methods of parameterization were applied to the datasets to characterize the airborne ultrasound field with respect to the structure and sound pressure levels. Comparing the parameters of datasets with high precision (high resolution and small microphone size) to those of datasets with reduced precision (lower resolution and larger microphone size) allowed a quantitative assessment of the influence of the reduction in precision. Corresponding recommendations on resolution and microphone size are given.

High speckle reduction rate in a laser projection system

In this paper, we demonstrate a design for a laser picoprojector with a high rate of reduction in the speckle contrast value. The method utilizes a holographic diffuser, deformable mirror, and fast electrically tunable (FET) lens. Then we compare the reduction rate of the speckle contrast value for holographic diffusers with different divergence angles and an FET lens with different optical power ranges combined with a deformable mirror. The increase in reduction rate of speckle contrast value means that the minimum value of the speckle can be reached faster. There reduction rate increases from 4.76 to 5.74 for the 5×5 holographic diffuser, 3.45 to 5.66 for the 10×10 holographic diffuser, and 1.52 to 4.58 for the 30×30 holographic diffuser.

Enhancing laser speckle reduction by decreasing the pitch of a chiral nematic liquid crystal diffuser

The artefact known as speckle can plague numerous imaging applications where the narrow linewidth of laser light is required, which includes laser projection and medical imaging. Here, we report on the use of thin-film chiral nematic liquid crystal (LC) devices that can be used to mitigate the influence of speckle when subjected to an applied electric field. Results are presented which show that the speckle contrast (a quantitative measure of the presence of speckle) can be significantly reduced by decreasing the pitch of the chiral nematic LC from 2700 to 244 nm. Further reduction in the speckle contrast can be observed by operating the diffuser technology at a temperature close to the chiral nematic to isotropic transition. At such temperatures, we observe a simultaneous improvement in the transmission of light through the device and a decrease in the electric field amplitude required for the minimum speckle contrast value. We conclude by presenting a laser projected image of the 1951 USAF target with and without the LC device to demonstrate the visual improvement as a result of the speckle reduction.

Micro-refractive optical elements fabricated by multi-exposure lithography for laser speckle reduction

Laser speckle reduction with a macro refractive optical element (mROE) is restricted by the limited entrance facet size of light pipe. Here, we have fabricated a micro-ROE (μROE) that incorporates three-dimensional micro-optical structures. The μROE with 2 × 2 duplicated multi-level cells is made of SU-8 photoresist with the help of multi-exposure lithography process. When the μROE works together with the mROE, objective speckle contrast is reduced to 0.2, where the light source is a low-coherence multimode laser diode. In principle, more speckle reduction can be obtained by fabricating μROEs with more cells and larger height differences among the cells.

Optical Dual Laser Based Sensor Denoising for OnlineMetal Sheet Flatness Measurement Using Hermite Interpolation

Flatness sensors are required for quality control of metal sheets obtained from steel coils by roller leveling and cutting systems. This article presents an innovative system for real-time robust surface estimation of flattened metal sheets composed of two line lasers and a conventional 2D camera. Laser plane triangulation is used for surface height retrieval along virtual surface fibers. The dual laser allows instantaneous robust and quick estimation of the fiber height derivatives. Hermite cubic interpolation along the fibers allows real-time surface estimation and high frequency noise removal. Noise sources are the vibrations induced in the sheet by its movements during the process and some mechanical events, such as cutting into separate pieces. The system is validated on synthetic surfaces that simulate the most critical noise sources and on real data obtained from the installation of the sensor in an actual steel mill. In the comparison with conventional filtering methods, we achieve at least a 41% of improvement in the accuracy of the surface reconstruction.

Weak-scattering static diffuser by fast pumping dispersed-nanoparticles in a long distance using microfluidic flows for efficient laser speckle reduction

Changing diffusers are most commonly used for laser speckle reduction. Here, a weak-scattering static diffuser (WSSD) is proposed and demonstrated by fast pumping poly(methyl methacrylate) nanoparticles in a long distance using microfluidic flow. Experimental results show that this proof-of-concept device can effectively reduce speckle, where the lowest speckle contrast ratio is 0.04. Comparing with vibrating/rotating diffusers driven by mechanical motors for speckle reduction, the WSSD is static. Moreover, the WSSD can suppress speckle with weaker light scatterings (scattering angle equals to 5.8° at the lowest speckle contrast ratio value), making it superior to vibrating/rotating diffusers driven by micro actuators. Because of these promising advantages, the WSSD has prospects of wide range of applications, such as in laser projection displays.

Effect of incidence/observation angles and angular diversity on speckle reduction by wavelength diversity in laser projection systems

The speckle reduction for laser projectors has been vigorously studied because speckle causes a serious deterioration in image quality. Most speckle reduction methods can be categorized into wavelength diversity, angular diversity and polarization diversity, which are usually treated independently. In this paper, it is shown that the effect of wavelength diversity and angular diversity on speckle reduction is not independent, and that the effect of wavelength also depends on incidence and observation angles on screen. The speckle reduction effect by wavelength diversity is smaller when the angular diversity is larger. Also, the speckle reduction effect is investigated on various screens including matte and silver screens, and it is shown that the effect of wavelength diversity is larger on matte screen than on silver screen.

Combination of micro-scanning mirrors and multi-mode fibers for speckle reduction in high lumen laser projector applications

In high lumen laser projectors, it is required to use laser diodes coupled to multi-mode fibers (MMFs) to obtain a high power illumination module. In this paper, we have fabricated an electromagnetic micro-scanning mirror (EM-MSM), and we have firstly demonstrated a speckle reduction method by the combination of the EM-MSM and the MMF. With the help of a condenser lens, laser beams modulated and reflected from the EM-MSM are coupled into the MMF within its acceptance angle. Because the fast scanning behavior of the EM-MSM results in the phase modulation and mode coupling among the MMF guided modes, the light intensity field distributions at the exit aperture of the MMF are changing. During the charge-coupled device (CCD) integration time, the random speckle patterns are integrated and homogenized by the CCD camera, and hence speckle is reduced. By driving the EM-MSM in raster scan, the lowest compound speckle contrast ratio at 0.0794 is obtained, where the EM-MSM half scanning angles are 0.4 ° and the optical power loss is lower than 4.5%. The demonstrated technique is compact and can endure the high power of the laser module; thus, it has a promising potential in high lumen laser projector applications.

Effect of laser speckle on light from laser diode-pumped phosphor-converted light sources

Laser diode (LD) pumped white light sources are being developed as an alternative to light-emitting diode-pumped sources for high efficiency and/or high brightness applications. While several performance metrics of laser-pumped phosphor-converted light sources have been investigated, the effect of laser speckle has not been sufficiently explored. This paper describes our experimental studies on how laser speckle affects the behavior of light from laser-excited phosphor lamps. A single LD pumping a phosphor plate was the geometry explored in this work. Overall, our findings are that the down-converted light did not exhibit any speckle, whereas speckle was present in the residual pump light but much reduced from that in direct laser light. Furthermore, a thicker coating of small-grained phosphors served to effectively reduce speckle through static pump light diffusion in the phosphor coating. Our investigations showed that speckle is not of concern in illumination from LD-pumped phosphor-converted light sources.

Efficient speckle-suppressed white light source by micro-vibrated and color-mixing techniques for lighting applications

In this paper, we have demonstrated an efficient speckle-suppressed white light source generation when a blue laser diode illuminates on a micro-vibrated phosphor paper. Both micro-vibrated and color-mixing techniques are used in this system. With only micro-vibrated technique, the speckle contrast can be reduced from about 50% to 7.4% for the scattering blue image for a 16-ms integration time. Together with color-mixing technique, mixing speckle contrast is defined for laser diode pumped phosphor and almost speckle-free result is achieved. For color temperature lower than 5000 K, almost speckle-free mixed white can be obtained even without vibration technique.

Speckle disturbance limit in laser-based cinema projection systems

In a multi-disciplinary effort, we investigate the level of speckle that can be tolerated in a laser cinema projector based on a quality of experience experiment with movie clips shown to a test audience in a real-life movie theatre setting. 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. The speckle disturbance limit for movies turns out to be substantially larger than that for still images, and hence is easier to attain.

Human speckle perception threshold for still images from a laser projection system

We study the perception of speckle by human observers in a laser projector based on a 40 persons survey. The speckle contrast is first objectively measured making use of a well-defined speckle measurement method. We statistically analyse the results of the user quality scores, revealing that the speckle perception is not only influenced by the speckle contrast settings of the projector, but it is also strongly influenced by the type of image shown. Based on the survey, we derive a speckle contrast threshold for which speckle can be seen, and separately we investigate a speckle disturbance limit that is tolerated by the majority of test persons.

Analysis of the speckle properties in a laser projection system based on a human eye model

In this paper, the properties of the speckle that is observed by humans in laser projection systems are theoretically analyzed. The speckle pattern on the fovea of the human retina is numerically simulated by introducing a chromatic human eye model. The results show that the speckle contrast experienced by humans is affected by the light intensity of the projected images and the wavelength of the laser source when considering the paracentral vision. Furthermore, the image quality is also affected by these two parameters. We believe that these results are useful for evaluating the speckle noise in laser projection systems.

Speckle noise reduction on a laser projection display via a broadband green light source

A broadband green light source was demonstrated using a tandem-poled lithium niobate (TPLN) crystal. The measured wavelength and temperature bandwidth were 6.5 nm and 100 °C, respectively, spectral bandwidth was 36 times broader than the periodically poled case. Although the conversion efficiency was smaller than in the periodic case, the TPLN device had a good figure of merit owing to the extremely large bandwidth for wavelength and temperature. The developed broadband green light source exhibited speckle noise approximately one-seventh of that in the conventional approach for a laser projection display.

A hybrid temporal and spatial speckle-suppression method for laser displays

We propose a system that reduces laser display speckles by vibrating the light pipe. A small displacement of the light pipe appears to allow the total reflection of the laser, thereby resulting in a homogenized speckle field that changes with time. In this case, the speckle interference generated by the pattern projected by the laser through the light pipe destroys the spatial homogenization of the laser beams when the light pipe is vibrated. Moreover, when the light pipe begins the sequential vibration, the phases and paths of the beams are changed after the beams traverse the light pipe. Consequently, temporal speckle wavefront superposition can homogenize the luminous intensity distribution of the speckle pattern. This process reduces the speckle contrast to less than 4% while maintaining a luminous intensity of greater than 70%.

Reduction of the recorded speckle noise in holographic 3D printer

A holographic 3D printer produces a high-quality 3D image reproduced by a full-color, full-parallax holographic stereogram with high-density light-ray recording. In order to produce a high-resolution holographic stereogram, we have to solve the problem of speckle noise in this system. For equalizing an intensity distribution inside the elementary hologram, the object beam is modulated by a diffuser. However the diffuser typically generates speckles, which is recorded in the holographic stereogram. It is localized behind the reconstructed image as a granularity noise. First we show the problems of some conventional ways for suppressing the granularity noise using a band-limited diffuser, and then we analyze an approach using a moving diffuser for the reduction of this noise. In the result, it is found that recording with a moving diffuser is effective for reducing the granularity noise at infinity of reconstructed image, although an alternative noise occurs. Moreover we propose a new method introducing multiple exposures to suppress the noise effectively.