Introduction to Microdisplays

Abstract

This chapter defines the term “microdisplay” and then overviews the origins, history, and applications of microdisplays. It briefly describes and compares various microdisplay technologies that are, and have been, prevalent. Subsequent chapters go into greater detail on selected technologies.

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Introduction

The term “microdisplay ” is generally accepted as referring to that class of miniature, high-information-content, electronic information display panels that are intended to be viewed indirectly by means of an optical system that provides a (typically highly) magnified image of the panel. This is in contrast to most other, even miniature, display panels that are intended to be viewed directly and without optical magnification although often with significant levels of optical enhancement. To be specific, then, the term “microdisplay” refers to the panel or display component alone. Additional electronic, mechanical, and optical components are usually required in order to complete the microdisplay module or system. Figure 1 is a photograph of a microelectromechanical system (MEMS)-based microdisplay, while Fig. 2 is a photograph of a polymer organic light-emitting diode (P-OLED) on a CMOS microdisplay that gives a good impression of the small size of these components.

Fig. 1

Photograph of a DLP microdisplay unit

Fig. 2

Photograph of a P-OLED microdisplay unit

In this section, we deal with only those microdisplay devices that contain a 2-D Cartesian array of pixels arranged in rows and columns. 2-D microdisplay-based systems are sometimes in competition with systems that contain (a) an image engine with a 1-D pixel array, such as the grating light valve, that produces one line of an image at a time and is quickly scanned in 1-D in the direction orthogonal to the 1-D array in order to produce all of the lines required for a 2-D image or (b) a single spot producing engine (such as a laser) that is scanned in 2-D and modulated synchronously to produce a 2-D image. These 1-D and 2-D scanning systems and their component devices will not be described here.

History

The earliest microdisplays were miniature CRTs that enabled the development of CRT-based HMDs for military purposes as described much later in Rash (1999). Most other early modulating microdisplay technologies were developed alongside or were derivatives of a class of devices called spatial light modulators (Casasent 1977; Efron 1994; Fisher and Lee 1987) (SLMs) also known as light valves. By way of example, one of the first reports on the DMD (Pape and Hornbeck 1983) (at the time called the deformable mirror device and only later to become the digital micro-mirror device) classed it as a SLM – not a microdisplay.

A spatial light modulator is a device that can impress a 2-D pattern on to an optical wave front. This would include a 35 mm transparency placed in a slide projector. SLMs are generally considered to be programmable or addressable and split into two subclasses – optically addressable SLMs (OASLMs) and electrically (or electronically) addressable SLMs (EASLMs). Light-modulating microdisplays comprise a subset of EASLMs.

SLMs were developed primarily to exploit the parallelism of optics in a field called optical information processing or optical computing at a time before the longevity of Moore’s Law and microelectronics was fully appreciated. SLMs were very expensive in the mid-1980s, and, amusingly, commercially available handheld TVs were sometimes cannibalized and their LCD screens (typically 1 or 2 in. in size) used as low-cost medium-quality SLMs (see, e.g., Casasent and Xia 1986).

Classification of Microdisplays

One method of classification of microdisplays relates to the means of light output within each pixel, namely, emissive or modulating. An emissive microdisplay is a process in which an electrical signal applied to the optical layer of a pixel (typically a current passed through it) causes the emission of light by electroluminescence or some other means. A modulating microdisplay is a process in which an external illumination source causes light of equal intensity to fall on the optical layer of all pixels; then an electrical signal applied to the optical layer of each pixel (typically a voltage or electric field applied across it) causes either a direct change in the amplitude of the output light or a change in some other property (such as direction or polarization state) of the output light that is converted to amplitude modulation further downstream. Modulating microdisplays can be reflective or transmissive depending on whether a combination of reflective electrode and opaque substrate, or transparent electrode and substrate, is used. Examples of emissive, reflective, and transmissive microdisplay pixels are shown in schematic cross section in Figs. 3, 4, and 5, respectively.

Fig. 3

Schematic cross section of a pixel of an emissive microdisplay panel

Fig. 4

Schematic cross section of a pixel of a reflective-modulating microdisplay panel

Fig. 5

Schematic cross section of a pixel of a transmissive-modulating microdisplay panel

Generic Microdisplay Architecture

Viewed notionally from above, a microdisplay is a miniature version of, but in other ways very similar to, most electronic information display panels, comprising a 2-D Cartesian array of pixels. A generic schematic is shown in Fig. 6. The pixels may be square or rectangular and may contain sub-pixels (e.g., a color pixel usually contains three sub-pixels, namely, red, green, and blue).

Fig. 6

Schematic plan view of a typical microdisplay

Overview of Microdisplay Technologies

Viewed notionally in cross section, a microdisplay has two main functional layers. The substrate, or backplane, offers mechanical rigidity and support. It also contains the electronics and handles all of the necessary processing, sorting, storing, and transmission of data. The vast majority of microdisplay technologies use almost standard mainstream-integrated circuit technology for the substrate, namely, complementary metal-oxide semiconductor (CMOS) . The primary alteration is that of a custom finish to the top metal surface (mirror-quality metal layer on a highly polished planar dielectric) in order to create a high-quality reflective substrate. A few microdisplay technologies use a miniaturized version of the transmissive polycrystalline silicon TFT-on-glass active-matrix LCD technology that is common in high-performance small-to-medium-area active-matrix LCDs.

The overlying electrooptical layer, or front plane, carries out, on a pixel-by-pixel basis, conversion of the electrical stimulus to an optical format that achieves or enables, in conjunction with downstream components, the formation of the image. Widely used modulating front-plane technologies for microdisplays include many configurations of nematic LC, surface-stabilized ferroelectric LC, and MEMs including the digital micro-mirror device (DMD) . Historically, magneto-optic and many other approaches have been used. Emissive front-plane technologies include various types of OLED and gallium nitride (GaN) micro-LED.

Qualitative Technology Comparison

The primary characteristics of the mainstream 2-D microdisplay backplane technologies already mentioned (along with, for comparison purposes, α-Si and organic TFT) are summarized qualitatively in Table 1, while those of the main front-plane technologies are summarized qualitatively in Table 2.

Table 1 Qualitative comparison of microdisplay and other backplane technologies

Table 2 Qualitative comparison of microdisplay electrooptical or front-plane technologies

Applications of Microdisplays

Microdisplays are used in a number of major display applications within two main classes of application. These are projection systems and near-to-eye (NTE) systems .

Projection systems create a highly magnified real image of the microdisplay on a flat surface where it can be seen by one or more viewers simultaneously, as illustrated schematically in Fig. 7. Types of projection systems include – in order from larger to smaller – digital cinema, lecture theater, home cinema, PC and laptop (data projector), portable projector, and pico-projector. The last category is a novel class of projector that emerged in the late 2000s. Pico-projectors are unique in that they are pocket sized and battery powered, thus setting tight constraints on size, weight, and power consumption. Pico-projectors may be stand-alone units designed to connect to a base unit such as an iPad or a mobile phone; alternatively a pico-projector function may be integrated into a digital camera, camcorder, or cell phone.

Fig. 7

Simplified schematic of a projection optical system

Near-to-eye systems create a highly magnified virtual image that can be seen by an individual viewer placing their eye close to (within the eye box of) the magnifying lens or lens group, as illustrated schematically in Fig. 8. NTE systems split into those, called electronic viewfinders, which are integrated into handheld devices such as cameras and camcorders and those which are integrated into hands-free consumer, professional, commercial, and military/security systems such as helmet-mounted displays (HMDs), head-worn displays (HWDs), and video glasses.

Fig. 8

Simplified schematic of a near-to-eye optical system