17.3 Projection systems and microdisplays

Projection systems and microdisplays are key technologies in modern imaging. From DLP to LCoS and laser projection, these systems create stunning visuals for cinema, home theaters, and more. They use tiny mirrors or liquid crystals to manipulate light and produce high-quality images.

Display characteristics like throw ratio, keystone correction, and brightness are crucial for optimal image quality. Specialized applications like head-up displays (HUDs) take projection tech further, overlaying info on windshields for pilots and drivers. These innovations are shaping the future of visual experiences.

Projection Technologies

Digital Light Processing (DLP)

• Uses a digital micromirror device (DMD) consisting of an array of microscopic mirrors that can be individually tilted to reflect light and create an image
• Each mirror represents a pixel in the projected image and can switch on and off rapidly (up to 10,000 times per second)
• Color is achieved by using a spinning color wheel with red, green, and blue filters or by using three separate DMDs for each color channel
• Advantages include high contrast ratio, fast response time, and good color reproduction (cinema projectors, home theater systems)

Liquid Crystal on Silicon (LCoS) and Micromirror Array

• LCoS combines liquid crystal display (LCD) technology with a reflective silicon backplane
• Liquid crystals are applied to a reflective silicon substrate, which is then addressed by an electronic circuit to control the orientation of the crystals and modulate the reflected light
• Micromirror array technology, such as that used in Texas Instruments' DLP systems, employs an array of tiny, individually addressable mirrors to reflect light and create an image
• Each mirror can be tilted to control the amount of light reflected towards the projection lens (digital cinema, high-end home theater systems)

Laser Projection

• Uses laser light sources (red, green, and blue) to create an image directly on the screen or through a scanning mirror system
• Laser light is highly directional, coherent, and monochromatic, resulting in a wide color gamut, high brightness, and excellent contrast
• Eliminates the need for a color wheel or separate light modulation devices
• Enables compact, efficient, and high-quality projection systems (large-venue displays, digital signage, automotive head-up displays)

Display Characteristics

Image Geometry and Keystone Correction

• Throw ratio is the distance between the projector and the screen divided by the width of the projected image, determining the size of the image at a given distance
• Short throw ratios (0.4:1) are used for close proximity projection, while long throw ratios (3:1) are suitable for larger venues
• Keystone correction compensates for the trapezoidal distortion that occurs when the projector is not perpendicular to the screen
• Can be done manually by adjusting the projector's angle or electronically through image processing

Brightness and Aspect Ratio

• Lumens measure the brightness of a projector, with higher lumen ratings indicating a brighter image (1,000 lumens for home theater, 5,000+ lumens for large venues)
• Aspect ratio is the width-to-height ratio of the projected image, with common ratios being 4:3 (standard), 16:9 (widescreen), and 21:9 (ultra-widescreen)
• Matching the aspect ratio of the content to the display ensures optimal image quality and prevents distortion or cropping

Specialized Applications

Head-Up Display (HUD)

• Projects important information directly into the user's line of sight, allowing them to view data without looking away from their primary focus
• Commonly used in aircraft to display flight data, navigation information, and targeting systems on the windshield or a transparent display
• Automotive HUDs project speed, navigation, and safety information onto the windshield or a separate combiner glass
• Augmented reality (AR) HUDs overlay digital content on the real-world view, enhancing situational awareness and providing interactive experiences (military, gaming, industrial applications)