Microdisplay

A microdisplay is a very small display panel, usually about one inch or less along the diagonal, built with a very high pixel density. Because the panel itself is tiny, it is almost always paired with magnifying optics that enlarge the image and place it close to the eye. This makes microdisplays the core image source in most head-mounted displays, camera electronic viewfinders, and pico projectors. In a wearable near-eye display the panel sits within a few centimeters of the eye, so a small, dense, bright panel matters far more than physical screen size.

Most microdisplays are built on a silicon backplane rather than the glass substrate used in direct-view televisions and phone screens. Using a silicon wafer and standard semiconductor lithography lets the pixels and their drive circuits be packed extremely tightly, which is how panels under an inch reach thousands of pixels per inch. Pixel pitch, the distance between neighboring pixels, is often only a few micrometers, far smaller than the pixel pitch of a direct-view panel.

Why head-mounted displays need them

A head-mounted display has to put a sharp, wide image in front of each eye while staying light enough to wear. A large panel cannot sit directly against the eye because the eye cannot focus on something that close, and a big panel would make the headset heavy and bulky. The standard solution is a small, high-density microdisplay viewed through a lens or other magnifying optics that form a virtual image the eye can focus on. The smaller and denser the panel, the more compact the optics can be, which keeps the headset light and helps widen the field of view.

Pixel density is the other reason. When a panel is magnified to fill much of the field of view, any gaps between pixels are magnified too, producing the visible grid known as the screen-door effect. A microdisplay with thousands of pixels per inch shrinks those gaps below what the eye can resolve, so the magnified image looks continuous. Brightness matters as well, especially for see-through augmented reality, where the image has to compete with daylight after passing through optics that throw away most of the light.

Display technologies

Several different technologies are used to build microdisplays. They split into two broad groups. Emissive panels such as Micro-OLED and microLED generate their own light at each pixel, so they need no separate lamp. Light-modulating panels such as liquid crystal on silicon (LCoS), the digital micromirror device, and transmissive LCD do not make light themselves; they steer or gate light from an external source.

Liquid crystal on silicon (LCoS)

Liquid crystal on silicon is a reflective technology. A thin liquid crystal layer sits on top of a silicon backplane whose surface is a grid of reflective aluminum electrodes, one per pixel. Light from an external source passes through the liquid crystal, reflects off the mirror underneath, and passes back through the crystal a second time, so the panel works in double pass. The voltage on each electrode twists the liquid crystal and changes the polarization of the light, which a polarizer then turns into bright or dark pixels. Because LCoS only modulates light and does not emit it, it always needs a separate illumination module and polarizing optics.

LCoS panels can be made very small while reaching very high pixel density and high optical efficiency, which is why they are widely used in pico projectors and in augmented reality light engines that feed waveguides. Google Glass used a near-eye LCoS display. Because the light source is separate, an LCoS engine can be paired with a bright LED or laser source to drive a lossy waveguide, but the extra illumination and polarization optics add bulk compared with an emissive panel.

Micro-OLED

Micro-OLED, also written as micro-OLED or OLED-on-silicon, places OLED emitters directly on a single-crystal silicon CMOS backplane instead of a glass substrate. Each pixel makes its own light, so there is no backlight and the panel can be very thin. Micro-OLED inherits the strengths of OLED: per-pixel emission gives true blacks and very high contrast, often above 100,000 to 1, with microsecond-level response times. Building on silicon pushes pixel density very high, commonly in the range of 3,500 to 4,500 pixels per inch, which all but removes the screen-door effect in a headset.

Micro-OLED is the dominant high-resolution panel in current VR and mixed-reality headsets. The Apple Vision Pro uses two micro-OLED panels carrying about 3800 by 3000 pixels each, with the silicon backplane made by TSMC and the OLED frontplane by Sony, running at a default 90 Hz with a 96 Hz mode for 24 fps video. Peak brightness is over 5,000 nits at the panel, though a large part of that is lost in the lenses. The main limit of micro-OLED is brightness: tandem (stacked) designs reach roughly 5,000 to 10,000 nits peak, which is plenty for closed VR but can fall short for see-through augmented reality in daylight. eMagin, a long-time maker of OLED microdisplays, was acquired by Samsung Display in 2023 for about 218 million dollars.

MicroLED

MicroLED microdisplays use arrays of tiny inorganic light-emitting diodes, typically gallium nitride (GaN), built on a silicon backplane. Like micro-OLED they are self-emissive, but because the emitter is an inorganic LED rather than an organic film, they can be far brighter and are more robust against burn-in and heat. That brightness is their headline advantage and the reason they are the emerging choice for see-through AR smart glasses, where most of the light is thrown away in the optics.

The brightness figures are striking. JBD's Phoenix series microLED microdisplay reached a white-balanced brightness of about 2 million nits, with single-color demonstrations reaching even higher. Such extreme brightness is needed because a diffractive waveguide may pass only a tiny fraction of the light. The main challenge is full color: efficient, high-yield red, green, and blue in one tiny panel is hard, so several shipping smart glasses still combine separate monochrome microLED panels or use a single green panel. The RayNeo X3 Pro, for example, uses an optical engine built from three monochrome microLEDs. Single-chip full-color panels are appearing: a demonstrated 0.49-inch full-HD (1920 by 1080) microLED reached about 4,536 pixels per inch at around 3,000 nits.

Digital micromirror device (DLP)

The digital micromirror device (DMD) is the chip at the heart of Texas Instruments' digital light processing (DLP). It is a microelectromechanical (MEMS) array of up to several million microscopic aluminum mirrors, each about 16 micrometers across, sitting on a CMOS chip. Each mirror can tilt by roughly plus or minus 10 to 12 degrees into an on or off position thousands of times per second. In the on state a mirror reflects light from the source into the projection optics so the pixel looks bright; in the off state it sends the light into an absorber so the pixel looks dark. Shades of gray come from pulse-width modulation, switching each mirror on and off so fast that the eye averages the result; color usually comes from a spinning color wheel or from multiple chips. Like LCoS, the DMD is reflective and needs an external light source. DLP is most common in projectors, but the same chip can drive a near-eye AR engine.

Transmissive LCD

Transmissive LCD is the classic microdisplay used in many camera electronic viewfinders and some AR optical modules. Light from a backlight passes straight through a liquid crystal panel whose pixels gate how much light gets through. Its drawback at microdisplay scale is the aperture ratio: drive transistors and wiring block part of each pixel, so for the small high-density panels made on polysilicon or single-crystal silicon only about 30 to 50 percent of the area actually passes light. That lower efficiency makes a high resolution at small size harder than with reflective or emissive panels, though a very bright backlight can still produce a very bright image. Kopin's Brillian transmissive LCD microdisplays, aimed at high-brightness AR, reach contrast above 500 to 1 and brightness above 34,000 candela per square meter.

Feeding AR combiners

In see-through augmented reality the microdisplay does not sit directly in the line of sight. Instead a light engine built around the panel injects the image into a combiner that overlays it on the real world. Two combiner styles dominate.

A birdbath uses a beam splitter and a curved partial mirror. The panel, usually Micro-OLED or LCoS, sends light to the beam splitter, which directs it to the curved mirror; the mirror reflects a magnified image back through the beam splitter to the eye while real-world light also passes through. Birdbath optics are relatively efficient and give good image quality, which is why consumer viewer glasses such as those from Xreal use them, but the beam splitter dims the real world, so the user sees the outside scene as if through sunglasses.

A waveguide is a thin transparent plate that carries the image across the glasses by total internal reflection and releases it in front of the eye, usually through diffraction gratings. Waveguides allow slim, glasses-like designs but are optically lossy.

Brightness for see-through AR

See-through AR sets the hardest brightness target of any microdisplay use, because the virtual image has to be visible against real-world light. Outdoors on a sunny day, ambient luminance is around 3,000 nits, so an AR display is generally expected to deliver on the order of 3,000 nits to the eye to stay readable outside. The difficulty is that AR combiners waste most of the light. A diffractive waveguide can have an efficiency of only about 10 percent, and some systems pass under 1 percent, while partial-mirror combiners can reach roughly 50 percent. To put 3,000 nits at the eye through a low-efficiency diffractive waveguide, the panel itself may need to produce on the order of 2 to 3 million nits. That requirement is the main reason microLED, with its extreme brightness, is so attractive for AR smart glasses, while micro-OLED, capped near 10,000 nits, is better matched to closed VR or to dimmer indoor AR where only a few hundred to a thousand nits to the eye is needed.

Comparison of microdisplay technologies

Technology	Emits its own light?	Typical strengths	Typical limitations	Common uses
Micro-OLED (OLED on silicon)	Yes	Very high pixel density, true blacks and very high contrast, fast response, no backlight	Peak brightness capped near 5,000-10,000 nits, organic material can be more prone to burn-in	VR and mixed-reality headsets such as Apple Vision Pro, electronic viewfinders
MicroLED (inorganic LED on silicon)	Yes	Extremely high brightness (up to millions of nits), robust, long-lived	Full color in one panel is hard; often monochrome or multi-panel; costly and lower yield today	Emerging AR smart glasses
LCoS (liquid crystal on silicon)	No	Very high density and optical efficiency, mature, small panels	Needs a separate light source and polarizing optics; reflective double pass	Pico projectors, AR waveguide light engines, Google Glass
DLP / DMD (micromirror MEMS)	No	Fast switching, high reliability, good contrast	Needs a separate light source; color often via sequential color wheel	Projectors, some near-eye AR engines
Transmissive LCD	No	Mature and inexpensive, can be very bright with a strong backlight	Low aperture ratio (about 30-50 percent) limits resolution and efficiency at small size	Camera viewfinders, some AR optical modules

History

Microdisplays exist because of the need for electronic viewfinders in cameras, according to Doug Lanman. The same small, dense panels later became the enabling image source for head-mounted displays and near-eye displays, and the field now spans the reflective, emissive, and transmissive technologies described above.

A microdisplay can also serve as the image source in a small microlens-based light field display, and microdisplays are used to make monocles, including ones that use light field methods, because their very small pixel pitch suits light field display designs.

Companies

This is a list of companies that sell or have previously sold microdisplay products.

• eMagin (acquired by Samsung Display in 2023)
• Sony
• Microoled
• Kopin
• Dresden Microdisplay

See also

• Micro-OLED
• Near-eye display
• Head-mounted display
• Waveguide display