From projectors to heads-up displays – how DLP works
Pioneered by Texas Instruments, DLP is a MEMS-based form of optoelectronics that has shown itself to be incredibly versatile, with uptake from a broad range of different market sectors. In the following article we will look at the sizeable value proposition it presents.
DLP specifically deals with Digital Micromirror Devices (DMDs). These are arrays of millions of micro-scale mirrors which manipulate light according to electrical signals that are applied to them. Each mirror is incredibly small. By switching one way or the other, thousands of times a second, these mirrors can create dazzlingly high resolution images. Furthermore, because MEMS technology is employed, prolonged operation is assured while keeping the costs involved extremely low.
electron microscope.
While DLP technology is commonly seen in movie theatres, as we will see, its use goes far beyond just showing films. TVs, heads-up displays and even 3D printing are all incorporating it. And while today DLP is mostly known for its use in multimedia devices, there are applications emerging within industrial, medical and automotive too.
The DLP story
Invented in 1987, by Dr. Larry Hornbeck at Texas Instruments, the first DLP chips were relatively crude by today’s standards, but still unquestionably revolutionary for their time. The objective was to replace the analogue projection techniques (which were already starting to show their inadequacies) with a more sophisticated MEMS-based arrangement that would deliver high fidelity, greater reliability and all the benefits of digital functionality.
Each DMD pixel would be a multi-layered device consisting of an aluminium mirror mounted on hinges. These pixels would all rest upon a CMOS memory cell. By changing the value of the memory cell, electrostatic forces could alter the angle of the mounted mirror. The earliest DMD arrays were made up of just 848 pixels (29 x 29 resolution). But, while still quite basic, these initial prototypes demonstrated the potential of DLP technology to transform how displays and projectors worked. Instead of relying on analogue film projection, with all its inherent limitations, DLP-based projection units (in conjunction with a light source) could create digitally controlled image content.
After establishing a division focused on DMD research and development, to help improve performance characteristics and commercialize the technology, Texas Instruments introduced the first DLP products in the spring of 1996. DLP soon found its way into a variety of electronics goods, including some of the first commercially available HDTVs. Before DLP, projection-based flat screen TVs were bulky, had inconsistent brightness and were low resolution. The small form factor and strong performance of DLP helped create some of the first true HDTVs which not only had higher resolution but superior image quality to previous flat screen TV offerings.
Besides the home environment, movie theatres also benefited greatly from DLP-based projection systems. Instead of relying on analogue film, which degraded each time it was copied, DLP enabled a completely digital cinema process, from source to display. Audiences could finally see exactly what the filmmaker saw through the camera lens, with precise, high resolution detail and without visual artefacts from the analogue filmmaking and distribution process. Even for traditional film-based productions, digital distribution and display meant movies were more accurately projected and lasted longer than with analogue film reels, while costing much less in terms of their logistics. As DLP’s inventor, Dr. Hornbeck, has even been recognized for his contribution to the movie business, being presented with an Academy Award in 2015.
DLP technology was also responsible some of the world’s first digital projection systems. Due to the small size and low power consumption of DLP chips, mobile digital presentation systems with high resolution were made possible, which proved a positive boon to the corporate world. DLP-based projectors allowed digital presentations to be shown easily in meeting rooms and even on the go.
DLP technology
As already outlined, each micromirror on a DLP chip is mounted on a hinge. The micromirrors can support extremely fast switching speeds, so by using pulse width modulation to cycle the on/off time of the pixel, various grayscales can be created. To produce colour images, the light can be either run through an RGB colour wheel to a single DLP chip, or modulated using three separate RGB DLP chips.
via reflection.
A single-chip solution is the most compact and cost-efficient way to achieve colour images. In this system, a light source is run through a colour wheel which time-multiplexes it into red, green and blue light elements. The DLP chip modulates each colour accordingly. Because of the extremely high frequency of this process, the human eye perceives the resulting image as full colour. This single-chip approach is extremely cost effective and space efficient, and has allowed DLP projectors to be made in very small and affordable form factors.
light to create an RGB image.
A three-chip approach to DLP image projection is admittedly more costly and bulkier, but it produces much brighter images and significantly better colour performance. Here the light source is separated by prisms and dichroic filters into RGB light which is directed to the corresponding DLP chip. The DLP chip modulates the light and sends it on to be recombined by the same prismatic system and then projected onto the screen.
Projecting the future
While DLP has seen most traction as a technology for displaying PowerPoints and movies, its implementation goes much further than that and is continuing to grow. Modern DLP solutions includes chips which are smaller than a 5 Euro cent, allowing them to be incorporated into an ever-greater variety of devices – such as laptops or even smartphone handsets. Besides extremely small portable projectors, DLP can also be used in automotive heads-up displays, digital signage and augmented reality wearables. DLP can enable 3D scanning techniques by projecting a series of patterns onto an object and then analyzing the light distortion, machine vision can be used to generate a 3D model for analysis. In addition, DLP has great potential in 3D printing. In this application, the DMD array is used to selectively cure each layer of a photo-sensitive material such as liquid photopolymer resin. Compared to point-based exposure, or traditional single-point 3D printing techniques, this provides much faster build-times that are independent of layer complexity.
Digital lithography is another area where DLP technology has value. Similar to the 3D printing example, here the DMD array can provide a precise, high speed and high resolution light pattern to expose photoresist film or other photosensitive materials, without the need for contact masks. This reduces costs of material, improves production rates and allows for rapid design changes.
DLP can even be used for spectrography. Here different wavelengths of light are shone onto materials and the resulting pattern is sensed to analyze molecular content. Similar to the 3-chip DLP projection system, broadband light is separated into constituent wavelengths using a diffraction grating or prism. Subsets of the DMD array are assigned specific wavelengths and project the light on the material. This approach enables higher performance, lower cost and smaller form factor spectral analyzers.
While originally invented and used as a substitute for analogue film projectors, DLP technology has progressed far beyond those initial confines. From their humble, low resolution beginnings, the latest generation of DLP chips on the market have 500,000 mirrors which toggle thousands of times per second and can create 4K images. They are instrumental in today’s state-of-the-art technology, as well as helping to empower the devices of tomorrow.
To assist engineers in developing DLP-based systems, Mouser offers evaluation hardware and in-depth technical advice.
About the author:
Mark Patrick is Supplier Marketing Manager, EMEA at Mouser Electronics – www.mouser.co
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