Designing MEMS-based pico projection with DLP
Over the last few years, millions of products incorporating pico projection have shipped, and developers are constantly innovating new applications for this rapidly growing display category. Ideal applications for pico projection include near eye display, interactive digital signage, ultra short throw (UST) TV, standalone portable projectors and embedded projection in smartphones, tablets and laptops.
By Carlos Lopez, Strategic Marketing Manager, Pico Products, Texas Instruments DLP
DLP (Digital Light Processing) is committed to helping engineers think bigger, innovate into new categories, and keep pushing for more imaginative use-cases - for example, a design for a thermostat using a display powered by gesture recognition or interactive touch.
We want to make it as easy as possible for these new uses to come to life, so we have laid out some simple design considerations for developing using pico technology.
DLP MEMS technology shown enabling a near eye display application
After a developer formulates an idea on how to use pico technology in their application, they are faced with several factors to consider, including selection of display technology, light source, optics and software. A well-chosen combination of these variables can result in an end product with optimal power and light efficiency, capable of delivering large, bright, high-quality images.
Designers are faced with many imaging technology options, the most important being selection of a device that most efficiently utilises light. There are two different optical path architectures in the marketplace: transmissive and reflective. Reflective technologies utilise an array of microscopic mirrors to create the image without fundamentally altering the light, in turn maximising light efficiency (see diagram below).
Reflective MEMS micromirror array
In contrast, other technologies employ transmissive or a hybrid of transmissive and reflective systems, requiring polarisation of light to control the intensity of each pixel. Transmissive methods incur significant light loss, thus reducing optical efficiency. When considering the selection of a display technology, it is important to efficiently capture light without worrying about polarisation, resulting in higher brightness at lower system power. Reflective displays with micromirrors, such as DLP technology, provide this advantage.
Switching speed is a third consideration. The developer should look for a technology that can switch as quickly as possible, as this will allow the design to instantaneously 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 point of reference, Texas Instruments DLP Pico devices can switch each pixel/micromirror up to 3,000 times per second.
When considering light sources, there are three primary options: lamps, LEDs and lasers.
A typical pico display system
Lamps are commonly used in conference room and home theatre projectors, where high lumen levels (over 2000L) are required.
For pico projectors, the most common light sources used are red, green and blue LEDs, whose benefit is that they provide the best trade-off between cost, size, brightness (lumens per watt) and reliability.
Laser illumination has the benefit of high flux density (in lumens) from a small size, as well as highly-saturated colours. It is also an attractive option for pico applications requiring hundreds of lumens and where the cost of lasers can be accommodated.
Creating an optical engine design involves numerous trade-offs, each of which has an effect on size, cost and optical efficiency. There is an existing, mature network of Optical Engine Manufacturers (OEMs) that can supply fully-tested, off-the-shelf (OTS) designs for most pico projection applications - the fastest way to get to market.
However, if there isn’t a design that meets a developer’s needs, OEMs are fully capable of creating semi-custom or custom designs.
Algorithms to increase battery life, image quality and brightness
For most pico projectors, achieving efficiencies for battery operation is critical. An important aspect of managing power comes through utilising algorithms to analyse the image on a frame-by-frame basis, adjusting the intensity of each LED per 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 savings in power consumption of up to 50% without compromising image quality or brightness, and in many cases actually improves both. There are numerous resources available online to explore and learn more about proprietary algorithm offerings.
MEMS shown enabling embedded smartphone projection
Furthermore, adding an ambient light sensor to a pico projector enables algorithms to adjust image brightness to suit varying ambient light conditions. This further maximises battery life and optimises the viewing experience.
Special considerations for emergent product categories
For each emerging product category using pico technology, there will be unique design considerations. For example:
- In near eye displays - whether in the form of glasses, goggles, visors, augmented reality helmets or other, MEMS switching speed and display contrast are far more important than light source brightness;
- For resilient digital signage that can be projected onto storefront windows, having a long-lasting and bright light source is just as important as the projection quality enabled by the imaging technology and more important than power optimisation;
- In order to develop competitive ultra-short throw (UST) TV devices, consideration needs to be given to high definition resolution, battery conservation and light engine size.
By exploring these guidelines and pragmatically selecting image technology, light sources, optics and software implementation, we hope that developers will feel inspired to create innovative and exciting world-class applications incorporating DLP Pico projection.