Virtual reality. Augmented reality. Mixed reality. What's the difference, and how do optical coatings fit in? In this application note, we'll explore the specific optical coating needs of this emerging technology, and share how AccuCoat is responding to its challenges.
VR vs AR vs MR – What’s what?
As our lives become increasingly digital, technology is keeping pace to blur the lines between the digital and physical world even further. Virtual reality is finally within reach, thanks to advances in smartphones, graphics cards, software, and engineering. It has also spawned offshoots in the form of augmented and mixed reality.
Virtual reality (VR) immerses the user in another world, tricking the brain into perceiving the viewed world is real. The viewer wears a heads up display (HUD) or head mounted display (HMD) to block the external world and place the user in a virtual environment (VE). This illusion is tenuous, as aberrations, lag, or reduced field of view caused by the headset can be easily detected by the human brain.
Augmented reality (AR) blends the digital world with the real one through overlays of images or information on our view of the external world. This may come in the form of information or a simulated screen produced via glasses worn by the user, or it can be delivered by viewing the world through a smartphone camera and seeing computer-generated items overlaid on screen.
Mixed reality (MR) takes this one step further, allowing the user to interact with the software-generated images. These objects are anchored in space, and change in size and perspective as the viewer moves. In the hybrid world of mixed reality, synthetic objects react to a user’s actions or physical objects in the background in real time.
Applications of these three technologies include video games, entertainment, psychology research, clinical therapies, engineering, and design. They also have tremendous potential for the training of medical professionals, first responders, pilots, and military personnel. In industry, AR could allow information to be delivered hands-free to workers performing critical functions.
Challenges in Hardware
One of the biggest hurdles the field faces is in hardware design, as this is critical to optimizing the user experience. HUDs must deliver realistic visuals at an accessible price, while still being lightweight and comfortable enough for prolonged wear. The human eye has a 180° field of view, 90-110° of which can be replicated by current VR systems, depending on model. What does this mean for the optics used? They need to be plastic or polymer to reduce weight, with coatings that are intelligently engineered to minimize cost while still delivering high performance, often at high cone half angle (CHA) to maintain the needed field of view.
Virtual reality headsets work on the same principle as 3D glasses, combining two slightly different images to give the appearance of depth. Lenses placed between the display and each eye angle the 2D images relative to each other to create a stereoscopic effect. Advanced headsets include head-tracking systems to detect when the user is “looking” somewhere else in the virtual environment and adjust the image. Optical head-tracking systems may use lasers or other light sources to determine a user’s position or movement via reflections from the environment, depending on the approach used. AR glasses, in contrast, often use prisms or beamsplitters to combine the computer-generated images with transmitted light from the real world. Both types of headsets require anti-reflective coatings that preserve color balance and maximize transmission.
At AccuCoat, our breadth of coating technologies allows us to serve these varied optical coating needs, offering our customers a single source for all of their optical coatings – in a particular headset design, or across product platforms. Our proprietary low-temperature coating processes for plastics and extensive experience with different substrates allows us to create coatings that stretch the performance limits of traditional plastic optics. By truly understanding our processes, we can hit new specs on the first run, and deliver dependable performance in volume.
Optical Coatings for VR/AR/MR
In order to maximize delivery of light to the eye, it is important that all optical surfaces be coated to reduce reflectivity for visible light, typically from 400-700 nm. The optics may be flat or curved, and are often irregular in shape. Our multi-axis coating capability allows us to achieve uniform deposition across an optical surface and throughout a full coating run, even for unusually shaped parts.
The anti-reflective coatings used in VR and AR headsets often require large cone half angle (CHA); 0-30° from normal is typical, but up to 45° may be needed in some cases. This may require a tradeoff between reflectivity and CHA in the design when coating on plastic or polymer optics, particularly for broadband operation. If so, it can help to consider the light distribution itself. Where is the majority of your light – at what angles? One approach we can take is to design for a narrower CHA range, while predicting the performance at higher angles. We’ve also designed AR coatings for use off-axis (i.e. at 20-40°), spanning that central angle with the true CHA needed for operation.
Given that the required coatings are often the external surface in headsets, durability is extremely important, particularly in industrial or military applications. We’ve optimized our cleaning and deposition process to meet the most stringent environmental and durability testing requirements, including a variety of MIL-SPEC and ISO standards – so much so that we’re a preferred supplier for many military applications.
We’ve designed a variety of beamsplitters for headsets to divide the display light for routing to each eye. Our coating designs and materials offer low scatter and high efficiency to reduce the initial intensities needed and maximize battery life. Splitting ratios may vary from a few percent to 50/50 in VR and AR applications, and are typically specified for average polarization over the visible range.
The challenge with coating beamsplitters for imaging is maintaining neutral or equal transmission over the full wavelength range. In a typical beamsplitter coating design the relative efficiency changes as you move further away from the center wavelength of the coating. Any imbalance in the splitting ratio with wavelength causes the colors in a VR display to be skewed between the two channels. We therefore optimize our designs for uniform efficiency in each channel across the wavelength range at the required CHA, drawing on a variety of coating technologies to find the best fit for each application.
Hot mirrors are used in virtual reality systems to reflect the IR light used by eye or positional tracking systems while passing the visible light. These heads up display optics need to be lightweight, and thus are coated on plastic. Optical requirements often vary, but can include 80-90% reflection of 800-830 nm light or non-polarizing beamsplitting of the laser and visible, both of which we can coat on plastic substrates.
Mirrors are often used for routing of light within headsets, at angles ranging from 30° to 60°. These reflectors are generally coated with enhanced aluminum or silver, which offer excellent reflectivity, protection against oxidation, and moderate mechanical resistance. We maintain a large collection of tooling fixtures to accommodate the variety of substrates used for reflectors, from flats to curved surfaces used for focusing and beam shaping.
Now Make it Smaller
As with any new technology, as the performance hurdles are cleared, the next goal is to reduce size and weight. Components used in heads up displays for virtual reality are now getting smaller and smaller, increasing the challenges of cleaning & handling the optics involved. One solution is to start with optics molded with integrated handling pieces, allowing multiple surfaces to be coated on the same tiny optic, after which the handling piece can be cut off.
At AccuCoat, we have developed custom fixtures to hold smaller parts and adjusted our cleaning and coating processes to navigate the fine features involved. This is particularly important for curved and irregularly shaped optics, which is why we have invested significant development efforts into achieving excellent coating uniformity on every run, from quick turn prototyping to volume production.
A Realistic Coating Partner
As with many industries, the needs of VR, AR, and mixed reality technology are varied and constantly evolving. Though we never know what the next project will bring us, AccuCoat has the experience, processes, and equipment to adapt and deliver high quality solutions with dependable performance. Call us today to begin discussing the needs of your next virtual reality, augmented reality, or mixed reality system design with a partner that understands the unique demands of this emerging technology.