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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

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
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
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
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.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Nir single pixel camera. OpenStax CNX. Apr 29, 2008 Download for free at http://cnx.org/content/col10525/1.1
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