Friday, March 21, 2008

The Display Inside The Display

When users talk about their recent experience with head-mounted displays, they often refer to image quality. "The colors were great", "the image was very bright" and so forth. Indeed, image quality (lack of smear, brightness, contrast, color gamut) scores very high on HMD requirement surveys, right next to high resolution and wide field of view.

It turns out that the micro-display technology used inside the HMD can critically impact the user experience, often more than other design decisions for a particular HMD.

Many HMDs today using LCOS (liquid crystal on silicon), which has similarities to LCD technology, with the exception the LCD is transmissive - that is, the light source is behind the display, where LCOS is reflective - which means the light source needs to project the front of the display. LCOS displays are a common choice because they come in higher resolution than comparable single-chip OLED (Organic LED) display, and are available from multiple vendors. Unfortunately, LCOS-based designs have some disadvantages:
- They require a light source which reduces the display contrast (the light is on even when a black picture is displayed) as well as substantially increases power consumption.
- They often have limited temperature range, or require somewhat exotic solutions like local LCOS heaters to warm up the display before initial use.
- They typically exhibit motion blur and smear, just like many of us experienced with LCD monitors. This is particularly relevant in HMDs since images inside an HMD constantly change to track user head movements.
- Depending on the specific design, users may experience color flashes or other artifacts during head movement.

OLEDs are also used in several HMDs. OLEDs are self-emitting, meaning that they do not require independent lighting. OLEDs, such as those available from eMagin, offer a larger color gamut, higher brightness, higher contrast and lower power consumption. OLEDs are also faster than LCOS and thus are free of LCOS artifacts. However, OLEDs typically come in lower resolutions that LCOS displays and are available from fewer vendors. To overcome the lower resolution of individual displays, vendors such as my company, use a tiled approach to combine multiple micro-displays into a larger, higher-resolution image.

So... look inside! Next time you look at an HMD, inquire what display technology is inside the display.

1 comment:

mertero said...

Hey Yuval,

Great to read this post. I'm all for OLEDs too you know ;-)