With AR eyeglasses, the screen of a mobile device can be transitioned to the lens of a pair of eyeglasses. The problem with that is that even though the technology has been around for a while, the glasses have been bulky and cumbersome.
I interviewed Dr. Peter Weigand, CEO of TriLite Technologies (Vienna, Austria) about a new approach that can lead to glasses that look and feel normal.
He explained that there have basically been three waves of light engine technologies.
LCoS is a panel-based technology that requires illumination optics. The problem with it is that you have to make sure that the illumination is nice and homogeneous and smooth. Then you project the input image into the waveguide. It’s not very efficient and since it has more optical elements, it is very bulky.
MicroLED displays are a semiconductor-based technology, which is much better than a reflective display because it’s self-emitting. But still, achieving the brightness levels required for outdoor applications is a challenge. And since it’s a two-dimensional display, when you go to higher resolutions and higher fields of view, it hardly scales.
The technology with the highest level of miniaturization is laser beam scanning. A laser beam scanner has three key elements: an RGB laser module, which is the light source, consisting of three separately mounted lasers — a red, a green, and a blue. They are followed by optics that collimate the three laser beams into one. Finally, there are MEMS mirrors, which generate the image scans that arrive at the eyeglass display. This can be done with two mirrors, one for the x-axis and one for the y. However, according to Weigand, the latest generation of TriLite scanners, Trixel®3, uses a single two-dimensional MEMS mirror that moves in both the x and y directions in order to achieve a smaller and lighter package.
A Tiny Light Engine
The image to be displayed is created with electronics that modulate the lasers. That image has to then be coupled to an optical waveguide to be sent to the display. Usually, for that, a laser scanner uses so-called relay optics, which is basically a rather large optical element. However, TriLite couples the laser beam scanner directly into the input coupler of the waveguide. That’s what enables the small size (<1 cm3, 1.5 grams) of the display engine, which includes the three lasers, the MEMS mirror, and the collimating optics.
One of the main reasons they are able to make their scanners so small is that their design philosophy has been to perform many of the scanning functions using software rather than hardware. Another significant contributor to the small size is that they use a single two-axis MEMS mirror instead of one for each axis.
The optical input coupler is part of the waveguide. The coupler is a pattern of microstructure gratings on the surface that allow the light to enter. There are similar structures on the output side, where the light leaves the waveguide.
The optical system consists of a waveguide with input and output couplers. The image output from the scanner is directly coupled into the waveguide, which is a two-dimensional glass structure. It takes the very small input image — only a couple of millimeters wide — and opens it up. This increases the “eye box” — the area within which an effectively viewable image is formed by a visual display. This is very important because if your glasses shift a little, you still need to see the image. With a large eye box, you see the same image wherever your eyes are aimed, thus making the glasses fit more universally.
There are two important functions of the wave guide. It conveys the image to the lens and also combines the incoming light with the “digital light” so that you can simultaneously see both the digital image and the real-world scene coming through the eyeglass lens.
Aligning the Lasers
It is necessary to align the outputs of the three — RGB — lasers in order to get a perfect image, or the image would be blurred (see Figure 3). You can spend lots of effort, energy, and money to do the alignment mechanically. However, in order to minimize size, weight, and power consumption, TriLite uses software instead — proprietary multiparameter algorithms for laser and mirror control. This enables them to tolerate a certain degree of misalignment.
The images in Figure 4 illustrate two alternate scan patterns for a single image frame. On the left is a typical raster scan of the type used on a CRT — the pattern is painted in lines from left to right proceeding from top to bottom. On the right is a Lissajous scan, which through a complex series of motions, gradually paints the entire image instead of doing it line by line. This is much like a jpg image gradually coming into focus and produces a more comfortable image for the eye.
In order to produce a Lissajous trajectory, TriLite puts the MEMS mirror in resonance in both axes. The pattern is generated by a difference in frequency and phase between the two axes. The mirror is controlled to make sure it’s moving at the selected frequency and the laser is controlled to define exactly where both are aimed. They are then able to write the pixel information. If it’s a red pixel, for example, only the red laser is pulsed. The color information comes by controlling the laser diodes; the spatial information is constant and comes from the mirror.
Controlling the Lissajous is complex, for both timing and pulsing — the mirrors are moving very fast, and the laser beams are moving super-fast, so the pulses sent out from the laser driver have to be very short.
The system is comprised of three separate elements. The electronics, consisting of the MEMS controller and separate drivers for the mirrors and lasers, are mounted on a printed circuit board (See Figure 5). The power source and electronics are designed to be packaged into one of the temple pieces of the eyeglass frames. The scanner module itself, at less than 1 cm3, is small enough to mounted in the eyeglass hinge. Finally, the optical wave guide with its input and output couplers are part of the lens.
Because of the many parameters, TriLite calibrates each single device using machine learning (ML). They use experts who know exactly which knobs to turn to get the perfect image quality to produce the training data. They then import that data for use by the ML to subsequently do the calibration automatically. Basically, there is a target pattern and a real pattern, and they vary the patterns to make sure there is an optimal overlap.
In actual applications — use cases — there has to be some sort of input connection, for example, Bluetooth connected to a smart phone. You would then receive notifications as on a smart watch. But with smart glasses you don’t have to avert your gaze to receive it.
A few examples:
A bicycle rider could be instructed where to make the next turn on their route.
A skier could see their speed as they’re going down a slope.
A musician would not have to look away to see their sheet music.
In a hospital, where nurses have to shuffle through papers while working with a patient, smart glasses would allow them to work hands-free, thus making their job a little easier and reducing one possible source of contamination.
The TriLite engine would be ideal for the skier, who is faced with sunlight and bright snow because the brightness of the display can be tuned up to 15 lumens, and it also has high contrast ratio and a wide and accurate color gamut.
The next level of use case would be augmented reality. For example, you could see the map of your route as you’re riding your bike, or at a train station, you could see an arrow that shows you which train to get on.
Dr. Weigand believes that the small size, light weight, low power consumption, quality of performance, and design for manufacturability of the TriLite laser beam scanner will enable smart glasses to finally become a practical reality for the mass consumer market.
This article was written by Ed Brown, Editor of Sensor Technology. For more information, visit here .