How to design an efficient MeMS-based pico-projector
Over the last few years, millions of products incorporating pico projection have shipped, and developers are innovating new applications for this rapidly growing display category. Applications for pico projection include near eye display, interactive digital signage, head mounted display, ultra short throw TV, standalone portable projectors and embedded projection in smartphones, tablets and laptops as illustrated in Figure 1. New uses continue to emerge; for example, imagine a thermostat using an on-demand display with interactive touch.
Fig. 1: A smartphone embedding a DLP Pico MEMS projector
After a developer formulates a great idea on how to use pico technology in their application, they are faced with several factors to be considered. As noted in the block diagram of Figure 2, these include selection of the display technology, light source, optics and software. A well-chosen selection of these variables can result in an end product with optimal power and light efficiency capable of delivering bright, high quality images.
Fig. 2: A typical DLP display
So what are the considerations for designing a pico projector that will maximize power efficiency and yet deliver large, bright and crisp images? We will next address each one of these variables.
Designers are faced with imaging technology options. Selecting the imaging device that most efficiently utilizes light is most important. There are two different optical path architectures in the market place: transmissive and reflective. Most pico projectors use Texas Instruments’ DLP Pico technology, which is reflective. It utilizes an array of microscopic mirrors to create the image which utilizes reflection to maximize light efficiency – see Figure 3.
In contrast, other technologies employ transmissive or a hybrid of transmissive and reflective systems, requiring polarization of light to control the intensity of each pixel – both of which incur significance light loss thus reducing optical efficiency
Another consideration for the selection of the display technology is the ability to tilt the micromirrors. DLP technology uses a microelectromechanical system (MEMS) superstructure to tilt the micromirrors toward or away from the optical path to create each display pixel see Figure 4. Tilting mirrors allows the device to more efficiently capture light without worrying about polarization, resulting in higher brightness at lower system power.
Fig. 3: Typical DLP pico display system
Fig. 4: DLP MEMs mirror array
Switching speed is another consideration for the selection of the display technology. For this case, the developer should consider a technology that can switch as fast as possible as this will allow the design to quickly control the light path and colour sources for the system. The faster switching speed not only provides better colours but also better image quality as there is less motion blur, resulting in a better viewing experience. As a reference, DLP Pico devices can switch each pixel / micromirror at up to 3000 times per second – making it the fastest solution available.
When considering light sources, there are three primary options:
lamps, LEDs and lasers. Lamps are commonly used in conference room and home theater projectors, where high lumens levels (1000l to 2000l) are required.
For pico projectors, the most common light sources used are LEDs, specifically individual red, green and blue LEDs. The benefit of LEDs is that they provide the best tradeoff between cost, size, brightness (lumens per watt) and reliability.
Laser illumination has the benefit of high flux density (lumens) from a small volume, and highly saturated colors. Laser illumination is an attractive option for pico projector applications requiring 100s of lumens and where the cost of lasers can be accommodated.
Creating an optical Engine design involves making numerous trade-offs, each of which has an effect on size, cost, and optical efficiency. DLP has developed a mature network of Original Equipment Manufacturers (OEMs) that can supply fully tested, off-the-shelf (OTS) designs.
Using an OTS design is the fastest way to get to market. If there isn’t an OTS design that meets a developer’s needs, DLPs OEMs are fully capable of creating semi-custom or custom designs such as shown in Figure 5.
Fig. 5: A compact off-the-shelf DLP optical engine
Typical DLP optical engine
For most pico projectors, achieving efficiencies for battery operation is critical. An important part of managing power is by utilizing algorithms to analyze the image on a frame-by-frame basis. By doing so, the intensity of each LED can be optimized for each frame. For example, a blue sky will not need much red and green, while a red sunset won’t need much blue and green.
This can provide a savings in power consumption of up to 50% without compromising image quality or brightness – and in many cases actually improving both. TI’s DLP IntelliBright suite of algorithms can be tuned by the device manufacturer to intelligently provide the optimal brightness, power consumption and contrast according to the specific usage of the device.
Fig. 6: Effects of the content-adaptive illumination control algorithm
The first algorithm in the suite is called Content-Adaptive Illumination Control (CAIC). This algorithm operates by adjusting red, green and blue illumination strength on a frame-by-frame basis. The algorithm can be configured to maintain “constant image brightness” (which results in lower power consumption) or to maintain “constant illumination power” (which results in higher image brightness). This enables developers to select their desired amount of brightness boost versus power savings.
The second algorithm is Local Area Brightness Boost (LABB) which identifies ‘dark areas’ and ‘light areas’ within a frame. The gain is then adjusted for the darker parts of the image to give a more balanced realistic picture.
Furthermore, adding an ambient light sensor to a pico-projector enables the algorithms to adjust the image brightness to suit varying ambient light conditions. This further maximizes the battery life and optimizes the viewing experience.
Through careful selection of image technology, light source, optics and software implementation, developers can create innovative world-class applications incorporating pico projection.
Fig. 7: Effects of the local area brightness boost algorithm