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Optical waveguides enable AR Glasses to overlay virtual images onto the real world, producing high-quality displays and immersive experiences that have applications over a wide range of fields. From healthcare to manufacturing, from entertainment to defense, these AR optical solutions enhance the way people interact with information.
But what is the science behind waveguides? Here we’ll examine just how these optics work to deliver immersive AR experiences. (Spoiler: it’s all based on total internal reflection.) We’ll also take some time to look at the types of optical waveguides for AR glasses that are available today.
How does AR work?
Augmented reality (AR) glasses are designed to combine the virtual and real world by superimposing computer-generated images and videos on real-world visuals. Waveguides enable the glasses to blend virtual content directly into the user’s field of view, enabling them to engage with digital content even as they see the physical world around them.
This is very different than virtual reality (VR) displays, which use a display screen located in front of the user’s eye to block out the real world. The display of AR glasses is typically positioned on the forehead or near the temple, and specialized optics magnify an image and alter its path before transmitting it to the eye.
There are two key components of AR glasses: the display and the lens, and these work together with the human eye to create a hybrid experience.
A waveguide is a thin, transparent optic made of highly polished glass. The virtual image is reflected multiple times within the waveguide before it is directed, off-axis, to the human eye. Meanwhile, real-world visuals are also transmitted through the highly transparent guide.
Total Internal Reflection refers to the way light ‘bounces around’ within a substrate. This happens when the substrate is of higher density than the medium it is embedded in, and the angle of incidence is greater than a critical angle. Total internal reflection (TIR) is important for more than just augmented reality— we also see it at work in light rods, prism, and the optical fibers that power high-speed internet.
There are two primary types of optical waveguides: geometric and diffracted. Both work with the same two basic components— a display and an optical lens. Both also follow the same basic principles, with light directed through the lens at pre-determined angles that lead to TIR.
Geometric optical waveguides will provide good imaging even while minimizing light loss. It is based on mirrors, which guide light into the lens. The optical signal is reflected by a semi-reflective mirror that is also transparent to light and images from the real world. The manufacture of a geometric optical waveguide is complex, as it requires special coatings and adhesion processes.
Diffracted Optical Waveguides
An alternative format is the diffracted optical waveguide. Here, light is transmitted through a grating structure. As it passes through the grating, the light can be directed and the period and depth can be adjusted. Either a surface relief waveguide or a volume holographic waveguide may be used.
Waveguides aren’t the only optical solution for AR glasses, although they are considered the most promising. Two other options are prisms and free-form reflective surfaces. Each method has its own benefits and downsides.
AR Prisms
In a prism-based system, semi-transparent, semi-reflective optical prisms are used to project images to the eye, blending virtual content with the real world. Prisms are a robust solution, but their field of view is very limited.
Free-Form Surfaces
Using reflective surfaces to project images is another option, one for which there are numerous variants. The Birdbath is one— in this configuration, a polarizing beam splitter is used to reflect light from a free form surface to the eye. AR glasses based on free form surfaces are lightweight and less costly to manufacture than the alternatives, but the many reflections and transmissions in the system decrease transmittance, darkening the display.
We’ve looked at how optical waveguides power augmented reality, making possible immersive experiences that blend elements of both the real and virtual worlds. Waveguides may be of many different types; two important categories are geometric and diffracted. Both use total internal reflection for optimal light control, and both have applications across a wide range of fields. We’ve also looked at other options for AR glasses, including prisms and free-form surfaces
Shanghai Optics produces state of the art optical waveguides as well prisms and free form reflective surfaces for use in AR glasses. For more information, or to place an order, contact us today.