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In addition to difficulties with the light source and focusing lens, we also had trouble with speeding up image acquisition time. We originally were sending frames to the projector from a standard MATLAB script. This was very slow. Using the Psychtoolbox plugin allowed us to take advantage of frame buffering . This sped up the rate to between 15 and 50 frames per second (fps). The frames shown in the video below change at a rate in this range.

Random frames

Illustration of random frames displayed as MATLAB movie

To further decrease capture time we tried exploring tighter control of the projector's image display capabilities.Although the video frame rate is 60 Hz, which is the maximum rate at which the projector can receive images, the mirrors on theprojector's digital micromirror array (DMD) flip at a rate of at least 1000 Hz. Possibly, the mirror flip rate may even be asfast as 9800 Hz . So, we wanted to use every mirror flip as a random frame for our CS measurements,rather than just using each video frame to take one measurement. The result would be such that for each video frame, we would obtain between 16 and 164 measurements. Ultimately, if for a 1-megapixel image we required 100,000 frames, then this could give an image capture time of 10 to 50 seconds.

In order to exploit each individual mirror flip, we needed to understand pulse width modulation (PWM). This is the scheme thatthe projector uses to determine mirror flipping. Under normal operation conditions, PWM is used to simulate grayscale values ofimage frames. For example, consider the image in the figure below. In this image, the grayscale of the right half is true while the grayscale of the left half is anillusion: the left half in fact contains only black and white dots, no grey. A projector creates the grayscale illusion in asimilar manner. By rapidly toggling its mirrors between "on" to "off" positions, it tricks the eye into perceiving a seriesof black and white squares as grey. The "on"/"off" pattern is determined according to PWM.

Grayscale image

Mirror flickering on a DMD creates the illusion of grayscale through PWM

Our investigation of PWM schemes led us to understand thatbasic PWM works as illustrated in the example of the figure below. First, the time allotted for a video frame (1/60th of a second) is divided into blocks. There is one blockfor each bit of a pixel's grayscale value. If grayscale values can range between 0 and 31, then 6 blocks are created. For thelength of each block, the mirror will either be turned to its "on" position if the bit associated with the block is a 1, orits "off" position if the bit associated with the block is a 0. This is an acceptable method for creating a grayscale effectusing only black and white projections. However, the long duration of blocks associated with higher-order bits can produceeffects that are jarring to the eye.

Basic pwm

An example of how basic PWM works

A smoother version of PWM is used in most projectors, to reduce the jarring effects seen with basic PWM . The methodology of smooth PWM is illustrated in the example below. This modulation scheme devotes the same amountof time to each bit as basic PWM. However, instead of laying the blocks for each bit end-to-end, it breaks them up into smallerpackets and mixes them up. Each block from basic PWM becomes a series of blocks in smooth PWM. These smaller blocks are spreadout over the entire time for a frame, alternating with blocks that represent the other bits of the desired grayscale value.

Smooth pwm

An example of how smooth PWM works

After completing textbook research of PWM, we attempted to observe PWM patterns in the lab. We sent the projector frames with known grayscale values and captured themirror-flipping with a visible-range photodiode. However, the mirror-flipping patterns we observed with this approach did not resemble anything like what we had expected. They looked something like the plot shown in the figure below. At first we thought the problem must be on the photodiode end. Yet, the specification sheet for the photodiode listed its response time in the nanosecond range. So, we took another look at the documentation of PWM available on TI's website, and emailed some TI representatives to see if they could give us some insight on the problem. From these sources, we learned that the PWM scheme employed by current DLP projectors is very much more complex than that described in the examples of basic and smooth PWM above. Although smooth PWM may be a part of what current projectors use, the actual mirror flipping patterns are determined based on an algorithm that takes into account a large variety of projector settings. We were told that, in fact, TI considers their mirror-flipping algorithms proprietary, as these codes contribute to the projector's image quality.

Observed pwm

Mirror flipping for the grayscale value 67

Due to the multitude of dead ends we ran into trying to create a projector-based version of a compressive sensing NIR camera, we turned to working with a stand-alone DMD board for the last few months of our project.

Questions & Answers

Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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