Diamond Pixels and Quantifying Image Quality

Karl Guttag recently posted some valuable details on TI’s DLP Diamond Pixel microdisplay structure seen in a wide variety of emerging “new era” and “pico” projectors.


While not yet detailed officially from TI, this article provides some valuable information in regards to how this design change affects the resulting projected image.
The field of display technology is an interesting one because the technology incorporates many different elements and disciplines (light sources, light modulators, computer graphics system, optics, etc) which are combined into very sophisticated systems directed at the human vision system (which of course is another very sophisticated system). The challenge for display developers comes from the fact that the display system should match/exceed the capabilities of the human vision system in order to create a “high quality” image and avoid perceived artifacts; things like pixel structure, motion blur, grayscale contouring.
A viewer can identify image quality, for example looking at two images, side-by-side, a viewer can select which one is “higher quality”, however it can be very difficult for viewers to identify which display characteristics have the most influence on their perceived quality of an image. The “resolution” of the display, which is directly linked with the pixel count (but not solely based on this number), is the key display characteristic that can be quantified with metrology tools and methods and one in which viewers are usually keenly aware of. So resolution/pixel count is a good metric for analysis, one in which Karl has provided, however resolution is not the only display metric and in fact there are many factors which influence the perceived quality of a display including both static and dynamic properties.
Here are a couple of references which describe how image quality is measured and how scientists and engineers have created measurement methods which can create quantifiable results for the comparing and analyzing displays. Click on an image and you’ll be able to get a full copy.

Image Quality Measurements: Necessity, Numbers and '...nesses'.	Measuring Images: differences, quality and appearance	Comparative study of the MTFA, ICS and SQRI image quality metrics for visual display systems
2009	2003	1991

Speckle reducer products

There aren’t many speckle-specific products out there. Yet here are two companies with speckle reducing products that are aimed and enabling broader laser applications, like laser display. Of course laser speckle is such a multi-faceted issue no one solution can completely eliminate speckle (“speckle-free” is a bit of a misnomer), however these two seem to have found a niche which is hopefully enabling new lasers applications and adding momentum to the inclusion of lasers in projectors and displays.

Optotune	Dyoptika

They also have some technical documentation, describing the fundamental principles and some measurement setups to quantify the results. Some Alces speckle expertise (and Alces' Speckle Camera) might compliment these nicely.

Optotune App Note	Dyoptika White Paper

Optotune also has a nice little demo video.

What is Speckle Contrast and how to analyze speckle statistics?

 

Speckle contrast is characteristic metric of a speckle pattern and it is often used as a figure of merit for the quality of the image in a laser display. For example a laser projector with low speckle-contrast is much more visually pleasing than a laser projection with high speckle-contrast. One way to understand "contrast" is to relate it to the signal-to-noise ratio of the system. In others words, speckle contrast can be inversely related to the signal-to-noise ratio of a laser projector so if the contrast is low the signal-to-noise ratio is high and vice versa. Consider the term "contrast" as it may relate to an audio signal: imaging listening to the radio and there's a lot of noise in the signal, the signal-to-noise ratio is low, meaning there's a lot of "contrast" in signal and the music is washed out by the noise.

The actual analytical expression for speckle contrast is:

which defines the speckle contrast as the ratio of the standard deviation and mean intensity of the observed pattern. To understand the meaning of this value it’s necessary to understand a bit about speckle statistics. When correctly measuring a  single, independent speckle pattern with a camera, the probability density function of the pixel values recorded by the image sensor (which measure the brightness variations of the speckle pattern) follow an Exponential distribution. Joseph Goodman is the pioneer which describes this fundamental theory in his book Speckle Phenomenon in Optics.

Shown below is a figure from Goodman’s book.

 

From this figure we can begin to understand what "speckle contrast" really means. The figure shows four cases which represent four different speckle contrasts. The first case, N=1, represents a single "pure" speckle pattern with a speckle contrast of 100%. A physical interpretation of this plot would be illuminating a polarization-maintaining screen with a single laser and then correctly measuring the brightness variation across that screen. In this case the standard deviation is equal to the mean which produces a 100% speckle contrast. The second case, N=2, represents the summation of two independent speckle patterns, perhaps created by measuring the reflected illumination of laser light off a polarization scattering screen i.e. a white projector screen. Here the theoretical speckle contrast is reduced to 70.7% (a factor of 1/√2). Continuing to overlap independent speckle patterns further reduces the speckle contrast by factors of 1/√N. Thus with 10 independent speckle patterns the speckle contrast is reduced to 31.6%.  As multiple independent speckle patterns are summed the probability density function follows a Gamma distribution which represents the sum of independent exponentially distributed random variables.

Understanding the fundamental statistical characteristics of speckle patterns is key to developing efficient and practical speckle reduction methods. In up coming articles Alces will dive into more details on generating independent speckle patterns and how to measure probability density function of a speckle pattern. For more information on speckle statistics or speckle contrast don’t hesitate to get in touch with us directly. Alces is developing a wide range of useful resources around speckle to pave the way for emerging high-performance laser displays.

3D depth capture and Structured Light

In 2009 Alces learned of a class of technology often referred to as 3D depth capture, 3D scanning, range imaging, or 3D depth sensing. While there are a variety of techniques, the fundamental concept behind these technologies is to acquire 3D information through a non-contact optical system. The animated image above shows a basic approach referred to as "Structured Light." A basic structured light system consists of two parts: a camera and projector. By projecting a series of known pattern onto an object and measuring the deformation of those patterns with a camera, the XYZ position of each pixel can be determined through triangulation calculations. These calculations are performed in parallel for each pixel within the camera producing a depth map or "point-cloud" consisting of hundreds of thousands of points at 0-60fps. The highly-successful PrimeSense/Microsoft Kinect platform uses this basic principle to create new and intuitive user interfaces for computer applications.

New technical articles added on Laser Projectors and Speckle

If you'd like to brush up on your knowledge of Laser Projectors and Laser Speckle check out these four technical publications I've added to our Tech Articles library. We'll continue to add more so stay in touch.

Laser Engines and Laser Speckle	Speckle contrast reduction in laser projection displays	Laser projector speckle measurements	Effective speckle reduction in laser projection displays
2011	2002	2009	2008

TI white paper: Laser Power Handling for DMDs

Technical documentation doesn’t present the same kind of splashy headlines that make it jump to the front-page of a website but, from an engineer’s standpoint, it’s always interesting to see new publications within the field. TI recently released this white paper, inconspicuously placed within the Technical Documents section of TI’s website describing the thermal effects of using lasers with a DLP microdisplay. This is the type of document you’d like to reference if you’re building a laser-based DLP projector so you don’t destroy the MEMS mirrors with too much laser power. TI provides a very logical, step-by-step approach to the analysis which provides some very valuable information for those working with laser projectors. From Alces’ perspective there is a couple of interesting details presented within the paper:


Points of interest

• “Laser applications use continuous and pulsed mode operation. One of the many advantages of pulsed operation is that during the pulse very high peak powers can be reached with relatively low average power consumption.”
• “…it is desirable to keep pixel surface temperature below a critical temperature of 150˚C.” This value includes the ambient temperature, so TI gives the example: “typical consumer projection systems can easily reach 50-55˚C…” which means that the temperature rise associated with laser illumination should not exceed 95-100˚C. From a MEMS perspective, the 150˚C critical temperature is most likely from the sensitivity of the aluminum (or aluminum alloy) to high temperatures. As thin-film aluminum is heated the induced stress can cause the aluminum surface to degrade through a process called “hillocking”. Another possible source for a 150˚C limitation may also be from TI’s anti-stiction film which is in place to prevent the landing springs from sticking to the anchor points.
  
• TI describes the average areal optical power specification as 25 W/cm² at 8% absorbed (92% aluminum reflectivity). It’s unclear what this value means because TI goes on to describe if a short-pulse, high peak-power, laser source is used, and even if that average power was ~25W/cm² , then the temperature rise would still exceed the critical temperature.
  
• This is somewhat of a side note but a table is presented which includes a “pixel size” of 5.4um, which is interesting because we were unaware of any DLP chip with pixel sizes less than 7.6um…

TI presented a good analysis of thermal effects associated with pulsed-laser sources on a DLP. At Alces, we’ve done some similar analysis as it relates to our 1D MEMS arrays. The different MEMS structure and characteristics of the laser display require some additional considerations which we’ll present in future, but overall thermal management plays a very important role in the design of a laser projector and requires making careful design choices.

Imaging Laser Speckle

In order for laser displays to reach a tipping point and become commercially viable there needs to be an answer for the question: “what about laser speckle?” For those unfamiliar, laser speckle is the shimmering, noise-like, variations in brightness we see when we look at the reflection of laser light on a rough surface (which is basically any surface used for projection display). Laser speckle is very distracting for display in the same way white noise is very distracting when listening to the radio, yet there is not a viable way to “tune out” speckle in displays.

Laser speckle is an interesting challenging facing laser displays because there is very little information on how to properly measure it. Being able to measure speckle is of course a prerequisite of determining the best way to reduce the speckle contrast. As the saying goes; “if you can measure it, you can improve it.” So from Alces’ perspective, if we can develop tools, practices, and understanding on how to measure speckle, we can provide resources on how to improve, i.e. lower the speckle in a wide variety of laser projection displays. We’ll be going into more details on our metrology tools in the future but first we wanted to show a basic example of imaging speckle and how the speckle contrast can be reduced by producing independent speckle patterns.

These four images were captured using Alces speckle metrology tools by imaging a monochrome, uniform field of green laser light (the images are in black and white as captured by the camera, and I’ve adjusted the brightness of the images slightly to make them easier to view). There are many important considerations to imaging speckle with a camera, which we’ll discuss later, but they key theme is that the camera should mimic the imaging characteristics of the human eye in order to create a measurement that matches the our own perception of speckle.

N=1

With an LED (rather than a laser) this image would appear as uniform bright field but because laser light is coherent (and LEDs are not), the reflection off the rough surface causes the beam to interfere with itself and create speckle. This is a “pure” speckle pattern, equivalent to a single, unique, speckle pattern (N=1) produced by illuminating a polarization-maintaining screen. The speckle contrast, which is measured as the relationship between the standard deviation and the mean, is 100% (we’ll present more details the definition of speckle contrast in upcoming posts). If this speckle contrast was produced by laser display, any video or image would be nearly indistinguishable from the speckle noise. This is of course the worst-case scenario.

N=2

While there are numerous ways to generate two “independent” speckle patterns (N=2), the most straightforward is to use a polarization-scattering screen e.g. a white wall. This, in effect, produces two independent speckle patterns which do not interfere with one another but rather add. This is the most basic form of speckle reduction. This additive process is the key to lowering the speckle contrast, and as you can see produces an image with roughly 70% speckle contrast. There is a well-established mathematical foundation which describes this additive process and how speckle contrast relates to the summing of independent speckle patterns, however we’ll save those details for later.

N=4

Creating four independent speckle patterns (N=4) reduce the contrast even further; this image shows a contrast of ~50%. Creating many independent speckle patterns is the fundamental challenge facing laser displays because there are only a limited number of variables which can be manipulated. We’ll discuss those variables in the future.

N=100

With a 100 unique, independent, speckle patterns (N=100) the ideally-uniform field begins to emerge. Fundamentally, speckle is a noise variable, it can’t be completely eliminated but the speckle contrast can be reduced below human perception. At 100 speckle patterns the speckle contrast is roughly 10% and approaching the human perception threshold.




If you’d like to learn more don’t hesitate to contact us by phone or email, we are always willing to “talk speckle.” And if this information was useful let us know with a comment or feedback. We see speckle as a very interesting technical challenge and are hoping to create valuable resources for those involved with laser displays.

Come see state-of-the-art display technology at Alces

Display technology comes in many shapes and sizes. At Alces, our display technology is embodied in our engineering platforms which not only have the ability to demonstrate the key display fundamentals but are versatile enough to be measured, upgraded, and improved as we advance our development programs.

The following images highlight four key pieces of technology in place at Alces for anyone to see.

Monochrome Laser Projector

Alces’ monochrome laser projector represents a pinnacle of Alces’ R&D efforts. The projector system is true demonstration of a scanned linear microdisplay in a laser projector. The 1D MEMS microdisplay and  board-level drive electronics are capable of streaming 256 x 3840 pixel (4K wide) video and the lab-style green laser is bright enough to meet DCI-brightness specifications. A small optical core built from off-the-shelf components includes a galvo-scanner and short focal-length lens to project a modest sized image roughly 15-feet away. The system does not have any speckle reduction methods in place but with the use of a floursescent screen the speckle contrast is attenuated enough to analyze and measure characteristic display parameters and goes relatively unnoticed during viewing.

RGB-LED “Look-into” Display

Alces’ first engineering platform, the “Look-into” display provided Alces with the first glimpse of imagery generated using Alces’ unique display fundamentals several years ago. The system incorporates Alces’ 1D MEMS microdisplay, a triad of RGB LEDs, a galvo-scanner and a collection of board-level electronics, and Alces’ unique optical core to create a full-color near-eye display. This system was first to demonstrate Alces’ approach for single-chip color (which is a column-sequential approach rather than the field-sequential approach used commonly today). It was also first to demonstrate the simplicity of the optical core and the versatility and performance of the MEMS microdisplay which has the capability to reach >120fps with 256x1000 RGB-pixel imagery. Replacing the eyepiece with a CMOS camera, the “Look-into” display transforms into a very powerful piece of engineering equipment. Using custom image analysis tools, Alces is able to evaluate and characterize MEMS and driver performance and explore new high frame-rate operating modes using quantitative measurements.

Laser Illumination

Direct-diode lasers are a low-etendue, high-brightness, small form-factor illumination source which is fundamental to Alces’ display technology. And because they play such a pivotal role, Alces’ has been diligent in investigating the key technical challenge; laser speckle, and establishing relationships with key lasers suppliers and supply chains in order to lead commercialization efforts. Alces has a number of laser sources for evaluation and is doing significant work on establishing metrology practices and tools to quantify and reduce speckle contrast in laser projectors.

 

MEMS Microdisplay

Many people are unfamiliar with details of MEMS technology but these small mechanical devices can do extraordinary things, one of which is to transform a simple beam of light into an image. Alces’ has spent several years optimizing the small, springy, and fast micro-ribbons within the MEMS microdisplay to achieve maximum performance, and has greatly simplified the core MEMS fabrication process improving it’s compatibility with traditional CMOS fabrication environments. Much of Alces’ work is now focused on demonstrating full-scale MEMS arrays and optimizing the drive signals for power consumptions and uniformity.

CLICK HERE to download a PDF copy of these slides

The Grating Light Valve

The Grating Light Valve was introduced at Stanford in the early 90’s by Alces’ founder and CEO Dave Bloom. The general concept behind the technology is to use a micro-opto-mechanical-system (MOEMS) device consisting of thousands of reflective micro-ribbons to create small sections of diffraction gratings in order to direct the light into different angles which is than discriminated/blocked to produce a 1D pixel pattern. Silicon Light Machines began commercializing the technology in the late 90’s which ended up being briefly licensed by Sony in 2000. Check out Silicon Light Machine’s site which has a good collection of white papers describing the technology.

While similar in form and function; a 1D MEMS microdisplay-based system used to modulate laser light, Alces’ display technology relies on several key different fundamental principles including the use of polarization-based optical components, a uniquely engineered MEMS micro-ribbons and novel electronic drive methods for color control. These features enable new opportunities and applications for this type of system and open up the possibilities for bringing lasers into low-cost and scalable projectors. More details to come on Alces’ display technology so check back soon.

Laser Projectors in the news

Lot’s of news around laser projectors this week as CES paves the way to new technologies.

• Laser projection is the cutting edge
• First demo of Barco’s revolutionary laser projector technology on a giant screen
• Projectors: Lamps, LEDs, Lasers and Hybrid Illumination
• The Age of the Pico Projector is Upon Us (For Real this Time)
• Soraa shows blue laser power for projectors
• Speckle reduction – an important part of the new laser projectors