Augmented Reality (AR) and Mixed Reality (MR) technologies are rapidly transforming how we interact with the digital world. Unlike Virtual Reality (VR), which immerses users in entirely synthetic environments, AR/MR devices overlay digital information onto our real-world view. This “see-through” capability is a defining feature, and it hinges on a critical component: waveguides.
While VR headsets typically employ opaque displays that fill the user’s entire field of vision, AR/MR smart glasses and headsets utilize transparent displays. These devices project images onto a clear surface, allowing users to simultaneously perceive both the virtual augmentations and their physical surroundings. The magic behind this seamless integration lies in optical waveguides.
Understanding Waveguides and Their Role in AR/MR
Waveguides, at their core, are thin, transparent materials – typically glass or plastic – engineered to manipulate light in precise ways. This technology isn’t new; waveguides are already integral to various applications, from fiber optic cables that power our internet to LED backlights in screens and even holographic displays. In essence, a waveguide directs electromagnetic waves, including light, along specific paths and patterns.
In the context of near-eye displays (NEDs) for AR/MR, optical waveguides are the unsung heroes. They skillfully bend and combine light from a micro-display to project virtual images directly into the user’s eye. This creates the illusion of digital content seamlessly overlaid onto the real world. The key mechanism enabling this is total internal reflection (TIR). Within the waveguide, light is trapped and propagates by repeatedly bouncing off the internal surfaces with minimal loss, akin to light traveling through a fiber optic cable. This internal bouncing continues until the light reaches a specific point where it is “extracted” and directed towards the eye.
Illustration of Total Internal Reflection in a Waveguide for AR Devices. Light enters through an input coupler, reflects internally within the waveguide layer, and exits through an output coupler, demonstrating the core principle of waveguide operation.
Unlike VR headsets where projectors can be placed directly in front of the eyes, AR/MR devices necessitate this “see-through” functionality. As explained in research, “The imaging system cannot block the front view, which therefore requires one or several additional optical elements to form an ‘optical combiner.’ The optical combiner reflects virtual images while transmitting external light to human eye, overlaying the virtual content on top of the real scene, for them to complement and ‘augment’ each other.”
Waveguide technology effectively solves this challenge in NEDs. An image projector, cleverly positioned outside the direct line of sight, projects images into a small area of the display lens. The waveguide then acts as a transparent “periscope,” propagating this image across the lens to an extraction point directly in front of the user’s eye. Think of it as a conduit for light, guiding the virtual image to the precise location needed for augmented viewing. Essentially, a waveguide functions as “a transparent periscope with a single entrance pupil and often many exit pupils.”
VR vs. AR Optics: A comparative diagram showing how VR devices (left) use direct displays while AR devices (right) utilize transparent waveguides to merge virtual images with the real world, illustrating the key difference in optical design.
The Growing Adoption of Waveguide Technology
For AR/MR to truly penetrate the consumer market, devices must deliver exceptional visual quality – high resolution and a wide field of view (FOV) – all within a compact, lightweight, and comfortable design. Among various optical combiner technologies, waveguides stand out as the only solution capable of achieving this delicate balance for AR applications in a small form factor. This is why waveguides have become a cornerstone component in leading AR devices like HoloLens and Magic Leap.
The AR/VR market is experiencing significant growth, projected to rise from US$ 17.67 billion in 2020 to over $26 billion by 2028, with head-mounted displays (HMDs), including smart glasses, commanding a substantial 65% market share. Applications across diverse sectors like healthcare, construction, education, and navigation are fueling this expansion, further driving innovation and development in optical waveguide technology.
Exploring Different Waveguide Structures
Waveguides used in AR/MR are typically constructed from thin glass substrates, ranging from sub-nanometer to nanometer thicknesses. By varying the waveguide and coupler design, and incorporating surface gratings or coatings, engineers can create a vast array of waveguide structures tailored to specific applications. The core components of a waveguide combiner are the input and output couplers. These can be realized using various optical elements: “simple prisms, micro-prism arrays, embedded mirror arrays, surface relief gratings, thin or thick analog holographic gratings, metasurfaces, or resonant waveguide gratings,” as well as beam splitters and free-form optics, each offering unique advantages and limitations. For enhanced color and wider FOV, multiple waveguide combiners can be stacked together.
Multi-Layer Waveguide Schematic: Illustration of a stacked waveguide system where each layer handles a portion of the light spectrum (red, green, blue) to create full-color AR displays with improved field of view.
Four primary types of waveguides are prevalent in modern AR/MR HMDs:
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Reflective Waveguides: These utilize molded plastic substrates and semi-reflective mirrors placed in front of the eye to guide light. Images from a micro-display are magnified and directed through the waveguide to the semi-reflective mirror, which then reflects the virtual image into the user’s eye, overlaying it on the real world. Google Glass and Epson Moverio devices are examples of devices employing reflective waveguide structures.
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Polarized Waveguides: Also known as transflective waveguides, these require multiple layers of coatings and polarized reflectors, precisely aligned and polished to efficiently guide light waves. Lumus notably used polarized waveguides in their AR products.
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Diffractive Waveguides: This is the most widely adopted waveguide structure for AR displays. Incident light enters the waveguide at a specific angle through slanted gratings called in-couplers. Light propagates through the waveguide and is extracted at the exit pupil via a second slanted grating, the out-coupler. These couplers are typically diffractive optical elements (DOEs) with slanted gratings. Vuzix Blade smart glasses, Microsoft HoloLens, and Magic Leap One are prominent examples using diffractive waveguide structures.
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Holographic Waveguides: Similar to diffractive waveguides, holographic waveguides utilize holographic optical elements (HOEs) as in- and out-couplers instead of DOEs. HOEs can reflect monochromatic or polychromatic (RGB) light waves and are fabricated using laser interference during the hologram recording process.
Waveguide Structure Types: Comparison of (a) Polarized, (b) Diffractive with surface gratings, and (c) Holographic (diffractive with volumetric holographic gratings) waveguide designs commonly used in Augmented Reality devices.
At the heart of diffractive waveguides lies the grating – a “periodic optical structure, whose periodicity can either be represented by the embossed peaks and valleys on the surface of the material, or by the ‘bright/dark’ fringes formed by laser interference in the holographic technology.”
Evaluating Waveguide Performance in AR/MR
The journey of light through an optical waveguide inevitably alters its properties. As light traverses the waveguide, reflecting multiple times within its structure, factors like incidence and reflection angles, and grating diffraction, impact optical efficiency. This can lead to a reduction in light energy reaching the eye, potentially resulting in diminished brightness, contrast, and clarity of the virtual image. Maintaining high image quality is paramount in see-through NEDs, where superimposed images must remain clear and visible across varying ambient lighting conditions. Diffractive waveguides, in particular, can present challenges with color tones in grayscale images, causing uneven brightness (luminance) and color (chromaticity) distribution.
Diffractive Waveguide Projection Issue: Image showing non-uniform brightness and color in a projection from a diffractive waveguide, particularly noticeable in mid-level gray tones, highlighting a challenge in waveguide performance.
Research has demonstrated that even minor surface roughness in optical waveguides can degrade display image quality, as quantified by modulation transfer function (MTF) analysis, depending on the angle of incident light.
Waveguide MTF Study: Images from a study analyzing the impact of incident angles on waveguide surface quality and resulting Modulation Transfer Function (MTF). (a, b) Incident angles at 65° and 75°. (c, d) Corresponding grayscale images. (e, f) MTF calculations in specific image areas.
To assess waveguide performance during development, a common method involves projecting light through the waveguide from a light source or picture generating unit (PGU). The output image is then analyzed for parameters like brightness (luminance), color (chromaticity), uniformity, and sharpness. Colorimetric imaging systems are invaluable tools for capturing precise measurements to guide optical design. Radiant’s ProMetric® I-Series Imaging Colorimeters, for example, can accurately measure luminance and chromaticity across the entire image, providing quantifiable data to evaluate uniformity and compare design iterations based on structural modifications to the waveguide.
Waveguide Measurement Concept: Diagram illustrating the process of projecting an image through a waveguide and measuring the output using a ProMetric Imaging Colorimeter, with analysis performed by TT-ARVR™ Software for quality assessment.
For deeper insights into quality testing for AR, VR, and MR devices (collectively XR), including waveguides, the webinar “Novel Solutions for XR Optical Testing: Displays, Waveguides, Near-IR, and Beyond” offers valuable guidance. This webinar, presented by Radiant in collaboration with Photonics Media, showcases advanced technologies that mimic human eye perception and are optimized for accuracy and efficiency in XR component development and production testing.
Watch the Webinar on Novel XR Testing Solutions for Displays and Waveguides
