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Exploring near-eye display design

Posted: 24 Feb 2015 ?? ?Print Version ?Bookmark and Share

Keywords:virtual reality? augmented reality? near-eye display? DLP Pico chips? NED?

At present, there are a variety of virtual reality (VR) and augmented reality (AR) near-eye display (NED) solutions in development, and the viability of a visual experience that seamlessly blends digital content with the physical world continues to grow. Let's take an in-depth look at some of the top challenges for designing a compelling see-through near eye display solution that "seamlessly" blends the digital world with the physical world.

There are many situations where the technical refinement of a NED solution is not just nice-to-have, but important to the usability of the NED. Imagine a surgeon or EMT is wearing a NED solution as a supplemental tool during a medical procedure. In such an environment, a clean, unobtrusive experience is critical. Or consider a video game player, for whom a very low display lag is required in order to deliver a seamless, real-time experience.

In either case, a compelling visual experience depends on minimising latency (delay) of the image being displayed, maximising the optical contrast, and maximising the field of view (FOV) of the information being displayed.

Display latency: The key to creating a real-time experience
First considering system latency, there are many system-level components which contribute to the latency experienced by the user. For our purposes, we will focus on the portion associated with the display engine, which can be divided into two components:

Display (Pixel) Latency = Pixel Data Update Time + Pixel Switching Time

The first component, called "pixel data update time," is the time it takes for the display device to "load" a new data value into a display pixel. For many display engine architectures, this is one or more frame times, when measured from the input to the engine. Assuming a one frame delay, this is about 16.67ms for a 60Hz source, which is common because many modern display technologies include a frame memory for facilitating image processing. For some display engines, pixel data update time can be two or more frames.

The second component of the display latency is "pixel switching time," which is the time it takes for a pixel to switch from its current state (on or off) to the opposite state. The end of the pixel switching time is when the pixel has settled enough that the human observer can clearly perceive the new data.

The pixel data update time plus the pixel switching time sets the total display lag time as perceived by a human observer. A display latency time of 16.67ms is often considered very good, but some displays can have lag times of 60ms or more.

Texas Instruments DLP Pico chips are touted to have some of the fastest pixel speeds available and can flip each digital micromirror (pixel) thousands of times per second, thereby reducing the display latency, and thus supporting display frame rates up to 120Hz while maintaining high image quality.

Contrast: The key to visually blending digital content with the real world
In addition to delivering a low-latency, real-time experience, the ideal NED solution should deliver transparent content with high clarity so as to not obstruct the end user's view of the real world. For example, if the data to be displayed is only using 20 per cent of the display-device pixel array, the other 80 per cent should be practically invisible to the user, thereby blending the digital content with the real world.

It is important to note that within a see-through NED optical system, the image is not being displayed on a semi-transparent surface (i.e., eyeglass lenses). Displaying on a semi-transparent surface would not be effective since such a surface would, by definition, be very close to the user's eye, and the eye cannot comfortably focus on something so close. Rather than creating an image on a surface, the optical system forms an optical pupil and the human eye acts as the last element in the optical chain C thereby creating the final image on the eye's retina.

Figure 1: Block diagram of a DLP-based NED.

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