Improvement of greyscale rendering in a DLP system using weighted subframe LED illumination

Problem Definition:

DLP (Digital light processing) is one of the dominant microdisplay technologies and is used in many beamers as well as professional projectors. The system is based on a digital micromirror device (DMD), a microdisplay with movable micromirrors made in a MEMS technology. Texas Instruments is the sole producer of these microdisplay chips. 
DMD chips do not require polarised light and are very fast, fast enough to use the so-called 'colour sequential' technique to produce full colour images. This implies that the basic colour components (red, green and blue) of the projected images are shown consecutively, using a rotating colour wheel to change the illumination colour between colour subframes. This results in a low-cost system with only one microdisplay. Competing technologies such as '3-LCD' and LCOS need 3 microdisplays for a full colour projector. 
The DLP system also has limitations: the mirrors can only produce white or black pixels on the projection screen. In order to generate grey levels, a pulse width modulation technique is used. Every colour subframe is divided in several grey level bitplanes. In order to produce 256 different grey levels, 255 subframes are needed (because every subframe has the same light intensity), and the resulting grey levels are linearly distributed. For high quality images, more grey levels are needed and they should be distributed according to a gamma or Dicom distribution. However, the DMD chips are already operated at their maximum switching frequency. 
The introduction of LEDs as projection light sources makes it possible to change the illumination intensity for every subframe. Hence, with 10 subframes, 2^10 different grey levels can be generated. This would tremendously increase the number of grey levels that can be produced with a given number of subframes. Alternatively, it could allow faster colour sequential operation, reducing the so-called colour-breakup or rainbow effect.


PowerLED mounted on heat sink ; Blue LED seen through integrating rod.

Objectives:

The student will study the feasibility of such a weighted subframe LED illumination system for a DLP projector. This includes the design and realisation of a power-LED driving circuit capable of rapid current level changes, while maintaining a high energetic efficiency. Commercial LED drivers are not suited for such applications and an original circuit will have to be invented.  The specified number of current levels is eight, but the feasibility of having ten levels will also be assessed. Once the driving circuit is finished and its operation verified, it will be incorporated in a DLP test setup that is available in the research group B-Phot in the department Tona of VUB. This will imply the realisation of imagery prepared with the correct bitplane information, so that 2^n grey levels can be demonstrated with only n subframes.  The complete system will be thoroughly characterised, including the maximum achievable lumen output, the grey level distribution function and the energetic efficiency.

Keywords:

LED driving electronica / electronics, video signal processing, displays

Location:

Ardoyen CMST  + at home + VUB (final phase) .