Research Journal

Wednesday, 20 July 2011

In our Wednesday pizza lunch, Sam Goldwasser, the "Laser Guru," gave a talk on the different types of lasers that exist today. The four main types that Mr. Goldwasser presented to us were the gas laser, solid-state laser, semiconductor laser, and dye laser. A gas laser works by electric current being discharged through a gas serving as the gain medium (the substance in a laser where light is amplified). The excitation of the electrons in the gas molecules from a lower to a higher energy state and their subsequent transition back down to the ground state gives rise to the production of light. Helium-neon lasers are the most common of the gas lasers and are frequently used in holography due to their long coherence length (the distance away from the laser at which interference will be strongest). Solid-state lasers use crystalline or glass rods that are implanted with ions at the necessary energy states, so the gain medium for these lasers is a solid. A semi-conductor laser allows electric current to pass through it in one direction, leading to the recombination of electrons and electron holes (the absence of an electron from its ground state when it's in the excited state) which results in spontaneous emission (the transition of an electron from an excited state to its ground state, thereby releasing a photon). A dye laser uses an organic dye for the gain medium, which allows these lasers to be tuned to a broad range of wavelengths.

In the afternoon, I spoke with Dave Battin, a laser hobbyist and holographer. He showed me a number of reflection and transmission holograms that he had created as art pieces. They were very impressive, having great clarity and rich color. To achieve those results, he went through numerous exposures for each hologram and made minor adjustments as needed in the positioning of his experimental setup. Dave will be coming back to the LTC to help me set up my equipment on an optical table and to help me make my first hologram in the coming days.

Monday, 18 July 2011

Today Dr. Cohen and I discussed in more detail the method by which I will power the laser diode. I connected a voltmeter to the DC power supply and used the coarse and fine adjustment knobs to obtain a measured voltage of 3 volts. Although I have used a DC power supply in the physics courses I have taken and I feel comfortable adjusting it, using it to power the laser diode might not be the best option. Dr. Cohen suggested that after connecting the laser diode to the positive and negative terminals on the DC power supply, when turning it on there could be a voltage spike before it settles at 3 volts and that possible voltage spike could burn out the laser diode instantly. When trying to test whether there is a voltage spike when turning on the DC power supply, the voltmeter couldn't measure the voltage fast enough, so it couldn't be determined. Dr. Cohen then suggested connecting an oscilloscope to the DC power supply, but he later said that the oscilloscope would have to trigger at the same instant as the DC power supply switched on, so that method wouldn't work either. Finally, we came to the conclusion that I shouldn't use the DC power supply to power the laser diode due to the risk of burning it out. As an alternative to the DC power supply and to purchasing the battery holder, I will simply take two D-size batteries and wire them in series. The battery holder's job is essentially to wire the batteries in series and provide an easy method to connect the laser diode. By using batteries, I would be able to safely connect the laser diode and not worry about burning it out.

Friday, 15 July 2011

Today Dr. Cohen and I had a discussion about some of the components of my experimental setup. Since the battery holder for the laser diode couldn't be found, Dr. Cohen suggested that I could simply cut the connector off the end of the wiring on the laser diode and attach each wire to a DC power supply which will serve as the power source. The laser diode is supposed to be powered by two D-size batteries, which are each 1.5 volts and connected in series, giving a total voltage of 3 volts. The DC power supply can then be set to 3 volts to power the laser diode, which should solve the problem of the missing battery holder. The power supply should be adjusted very carefully to avoid putting too much power through the laser diode, which could burn it out very rapidly, since it was designed for a low voltage. For the reticle mask, a slightly different shape may be used. The exact shape of the reticle on a holographic weapon sight will be very difficult to produce, so probably a small circle will be the shape of the reticle. A small circle can be punched through a thin sheet of metal for the laser diode to shine through to create the reticle and this reticle mask can be made in the machine shop.

Thursday, 14 July 2011

From what I had heard yesterday from Professor Metcalf, I did some reading on real-time holography. In real-time holography, the developing process that would be used to create a static hologram in transmission or reflection holography is shortened to a very small time interval and is repeated continuously, bringing you a continuous stream of holograms. This small time interval, on the order of a few seconds using current technology, isn't quite instantaneous yet, but can probably be improved to the point of video playback in the near future. Although video is only being played back at around 30 frames per second, it's fast enough for our eyes and brain to process as a fluid, continual series of still images. Decreasing the time delay in real-time holography to the same playback rate as video will give it the same properties as two-dimensional video except in three dimensions. This gives rise to the possibility of watching three-dimensional movies, giving viewers an unlimited number of vantage points in space. In addition, this technology could serve as an important tool in medicine, allowing doctors and patients to have live three-dimensional video conference calls over the internet, which gives doctors the ability to assess their patients from any angle.

Tuesday, 12 July 2011

With the materials for making holograms that are in the LTC, I'll still have to figure a few things out before I can make a hologram. If I decide to make a transmission hologram of just a simple object before moving on to a reticle similar to the one on a holographic weapon sight, then I'll need to figure out a method for powering the laser diode. The laser diode is supposed to be powered by two D batteries in a specific battery holder, but the battery holder is missing. Also, the laser diode seems to have a plug on the end of the wiring that appears to only fit certain battery holders, so I will need to acquire a battery holder from Radio Shack that was recommended by Integraf. As far as the developing kit is concerned, according to Integraf it may not work since it is a few years old, but I can still try to create a hologram using it. Basically, I should be able to make a transmission hologram of a simple object with only the materials in the LTC, but a new developing kit may be needed.

Monday, 11 July 2011

Using a guide from Integraf's website, I read about the method for producing a transmission hologram. The reticle image in a holographic weapon sight is created via a transmission hologram illuminated by a laser. Making a transmission hologram requires holographic plates, a 650 nm laser diode, a clamp to hold the laser diode (i.e. a clothespin), chemical developer and bleach, developer trays, and photoflo solution. There are two possible setups for making a transmission hologram, top-down and straight-on. The top-down setup involves placing the laser diode at a height above the object. The holographic plate should go directly behind the object. In the straight-on setup, the laser diode is placed on the same plane as the object and the holographic plate is placed to the side of the object at about a 45° angle. The holographic plate should always be perpendicular to the table and the table should be very stable, as to avoid shaking which can potentially prevent the proper development of the hologram. The laser diode should also be mounted securely to prevent shaking. Integraf recommends using a clothespin to clamp the laser diode on to something that will remain steady, such as a bowl of sand, salt, or sugar. After choosing a setup, the laser diode should be warmed-up for about 5 minutes prior to exposure, which only takes about 10 seconds. When the exposure is complete, the developing process is similar to developing photos. The holographic plate goes through a series of chemical baths and then finishes with an application of photoflo, which helps prevent smudging. To view the hologram created, simply shine the laser diode on to the holographic plate and you will see the virtual image of the object.

Friday, 8 July 2011

Today I went on a field trip with all of the students working in the physics department this summer to the American Museum of Natural History. Professor Simons accompanied us on the trip and brought us to his office in the Rose Center for Earth and Space. In the morning we toured several special exhibits, notably Journey to the Stars, a space show in an Imax theater that provides a background on the history of the universe; The World's Largest Dinosaurs, an exhibit presenting cutting-edge research on the sixty-foot long Mamenchisaurus dinosaur; and Brain: The Inside Story, an overview of the workings of the brain.

After lunch, we got to meet the REU students working at the museum and hear their PowerPoint presentations on their research interests. All of the REU students at the museum were doing research in astrophysics or earth and planetary science. It was nice to see other students' research in areas outside of optics. I thought the level of sophistication of some students' projects in astrophysics was quite impressive using a telescope with a primary mirror of only 1 meter or 1.5 meters in diameter. Their research projects were good reminders of the notion that good science can result from simple means, as seen in the 2010 Nobel Prize in physics, where researchers isolated graphene planes using adhesive tape.

Thursday, 7 July 2011

After browsing through Integraf's website, I learned about the two major types of holograms: reflection holograms and transmission holograms. A reflection hologram is viewed by shining white light from the viewer's side at a certain distance and angle away from the hologram, with the resulting image being made up of light reflected by the hologram. Although the hologram of an eagle as seen on all Visa credit cards is seen with reflected light, it is actually a transmission hologram that takes on the characteristics of a mirror due to a thin layer of aluminum behind it. A transmission hologram is viewed by shining light from a laser (usually the same wavelength light as used to produce the hologram) that is behind the hologram, thereby illuminating the hologram and creating the image. Using a hologram kit from Integraf, I can make either a reflection or a transmission hologram.

Tuesday, 5 July 2011

After a long weekend for the Fourth of July holiday, we regrouped at the LTC to pick up where we had left off. During lunch, Professor Metcalf gave a presentation on laser cooling. Professor Metcalf spoke about the most common method of laser cooling, known as Doppler cooling. In Doppler cooling, light that has a frequency slightly below the frequency of an electron's transition to an excited state is shined from opposing directions at a cluster of atoms. The atoms absorb more photons from the laser pointing in the opposite direction of their motion, losing momentum equal to the momentum of the photon. The atoms are now in the excited state and if the atoms spontaneously emit photons, they will be moved in a random direction due to the momentum of the photons. This results in the atoms traveling at lower velocities, thereby decreasing the average velocity and kinetic energy of the atoms, which means a lower temperature. The random movement of atoms caused by the spontaneous emission of photons counteracts the cooling of the atoms, which places a limit on this method of cooling known as the Doppler temperature. The lunch seminar was overall a very interesting introduction to an area of physics that I have not yet had the opportunity to study.

Friday, 1 July 2011

Today Dr. Noe showed me an interesting article and video on touchable holograms. When trying to touch a hologram, which is made of light, you'll notice that your hand simply goes straight through it. Researchers at the University of Tokyo have developed holograms that can be touched and felt with your hands. You can use your hands to bounce a ball, feel raindrops falling from a virtual sky, and feel a small animal crawling on your hand. The holograms being used in this technology are computer-generated, which isn't the groundbreaking part of the invention. By using the same sensor technology in the Nintendo Wiimote, used for playing video games on the Nintendo Wii video gaming system, the computer controlling the hologram is able to sense where your hand is in relation to the hologram. The feeling of touch is created by ultra-sonic wave emitters controlled by the computer controlling the hologram. These ultra-sonic wave emitters shoot pulses of air on your hand as you touch the hologram and the ultra-sonic waves are responsible for the feeling of an object really being there. The researchers mention that these touchable holograms can be used to replace light switches in hospitals to help prevent the spread of disease on a commonly touched area. What these researchers have done is very impressive and I expect that their work will lead to applications in unexpected areas.

Thursday, 30 June 2011

Today another student was scavenging for materials around the LTC and found a box full of materials for producing holograms made by Integraf. There were more than thirty holographic plates, a developing kit, a laser diode, a booklet on holography, and a cylindrical hologram of a Volkswagen buggy. I contacted Integraf and I was told that the developing kit was a little bit outdated and isn't guaranteed to work since it's several years old and it has been replaced with a better kit, but it might be worth a try to develop the cylindrical hologram. Using the materials found in the LTC to attempt to develop this hologram could be used as a first step toward replicating an EOtech holographic sight. Producing a hologram of a reticle will be a challenge, so I need to start off with something easier first. The battery holder for the laser diode is missing, which is a key component of this setup because the laser diode is powered by battery.

Wednesday, 29 June 2011

During today's pizza lunch, the students in the Laser Teaching Center presented their own research interests and pursuits thus far. I presented a short PowerPoint on the inner workings of an EOtech Holographic Weapon Sight. I explained the optics behind the device and its many advantages over more traditional targeting aides, in addition to the ways to spot a counterfeit. I concluded with the possible method by which my holographic sight works. Dr. Cohen and I believe that there is actually no real element of holography in the counterfeit type holographic sight at all. We believe that there is a laser diode which faces the user and shines through a reticle mask to produce the reticle image, which is enlarged by a magnifying lens. The reticle image is simply being shined onto a piece of glass in between the front and rear windows, which when viewed, gives it the appearance of a true holographic reticle because the reticle appears as if it's an illuminated hologram floating in between the front and rear windows. Dr. Cohen and I are still unsure of the mechanism that controls the color change of the reticle from red to green, so we will need to continue to explore possible methods by which that occurs.

After the pizza lunch, Professor Metcalf suggested that another possible optical device that I can explore is a heads-up display (HUD). Although there is no holography present in a heads-up display, it might be worth exploring. A HUD displays information on a transparent screen, allowing the user to be able to focus their attention more on their surroundings and less on instrument panels or other more traditional information displays. Professor Metcalf was inspired by the way in which the reticle of an EOtech holographic sight is superimposed on the field of view. A HUD usually contains three basic elements: a combiner, a projection unit, and a computer. The combiner is essentially the transparent viewing screen that the information will be displayed on. Combiners are either concave or flat and are specially coated to reflect the projected light, which is monochromatic (light of a single color), and allow all other wavelengths of light to pass through. The projection unit usually uses a light emitting diode (LED) to project monochromatic light on to the combiner. The computer is responsible for controlling what information is to be displayed and is usually connected to instruments or sensors which gather data to display on the HUD. The applications of HUDs include displaying information for pilots flying military or commercial aircraft, drivers in cars, or any other task where being able to have a full field of view while reading data gathered from sensors is crucial.

Monday, 27 June 2011

With the start of the Simons Summer Research Program, we welcomed two high school students to the Laser Teaching Center. With the addition of the two new students, bringing the total to seven students, the LTC is now up to full capacity for the summer. As current undergraduates, we look forward to helping them learn about optics through our own experiences that we have had thus far. Two summers ago when I was in the same position as our two high school students, I was involved in a summer pre-college program at Carnegie Mellon University. Living on campus full-time, I was in the Advanced Placement/Early Action program, which was a program for high school students to take two undergraduate courses from CMU in the span of six weeks. I had decided to enroll in Experimental Physics and Introduction to Nanoscience and Nanotechnology. Both courses kept me very busy, both in and out of the classroom. In Experimental Physics, I had the opportunity of performing seven multi-part experiments covering a wide range of areas in physics. One of these experiments was in optics. The experiment covered single-slit diffraction, double-slit interference, multi-slit interference, lenses, and laser beam expanders. Detailed accounts of our experimental setups, observations, data, and write-ups needed to be completed in separate lab notebooks for each lab. This was one of my first experiences using lab equipment and my first experience working in a university lab. After taking this course, I had confirmed my interest in physics and I have been pursuing that interest ever since then.

Friday, 24 June 2011

This morning I had my website biography reviewed by the other students in the Laser Teaching Center, in addition to Dr. Noe. My typed biography was projected on the screen in the conference room for everyone to read and afterward give their opinions or comments. Dr. Noe liked the logical progression of my biography, as it started off describing some of my personal interests and transitioned into how I became interested in physics, finally ending with my experience in the field of optics. The brief description of my personal interests in the first paragraph provides a good basis for conversation with another person who looks at my website and later meets me in person. There were several areas where Dr. Noe and the other students helped to make certain phrases more accurate and succinct. I believe that having other sets of eyes to proofread your writing helps to catch little mistakes that you might miss on your own. You yourself know exactly the ideas you are trying to convey in writing, but readers need to be able to understand your ideas in their written form. I'll need to make some adjustments to my biography before I can post it on my website.

In the afternoon, as a group we worked on trying to couple the multi-mode and single-mode optical fibers. A multi-mode optical fiber has a larger core size (about 100 micrometer diameter) than a single-mode optical fiber (about 10 micrometer diameter). The larger core size gives the multi-mode optical fiber a greater capacity to capture light due to the larger acceptance cone. Multi-mode optical fibers can only be used for communication over short distances due to modal dispersion, which is the spreading of the light being transmitted through the fiber and it is caused by the differences in the propagation velocity of the modes of light. A single-mode optical fiber is designed to only transmit one ray of light, so its core and acceptance cone are very small. Single-mode optical fibers still are subject to modal dispersion that is caused by multiple spatial modes. The amount of modal dispersion that occurs in a single-mode optical fiber is far less than that of a multi-mode optical fiber, so single-mode optical fibers are generally used for much longer range communication. A Helium-Neon laser shined its beam on to one mirror, which then reflected on to another mirror and into a fiber optic collimator (a small lens that focuses the beam into one end of the optical fiber). By adjusting the mirrors' vertical and horizontal components independently from one another, the beam was shining directly on the collimator and light could be seen from the other end of the optical fiber. This process was the same for both optical fibers, but adjusting the mirrors to be precise enough to get a large transmittance through the single-mode optical fiber was very difficult. Using a photodetector, the efficiency (amount of current detected from the Helium-Neon laser divided by the amount of current detected from the end of the multi-mode optical fiber) of the multi-mode optical fiber was 70%.

Thursday, 23 June 2011

Using what I learned from yesterday's lesson in html editing, I worked on improving my journal entries to make them more readable. I turned Greek letter spellings into the actual Greek letters themselves, added exponents to variables and trigonometric functions, italicized key terms the first time they appeared, and created large, bold headings for the date of each entry. I also needed to separate the paragraphs in my latest journal entries because the plain text appears as just one giant paragraph on the website until it's properly formatted in the Pico text editor.

The other significant part of my website that I worked on was my biography. I've never wrote an autobiography, so it felt very awkward to write about myself because I'm not accustomed to being the focus of my writing. I read some past students' biographies in order to get a better sense of the approach that I should take when it comes to writing my own biography. I plan to have a first draft complete by the end of the day for revision tomorrow.

Wednesday, 22 June 2011

Finally I'm able to make these journal postings on my own! After a thorough lesson, I learned some important basic Linux commands. The directories are hierarchically organized, taking on the appearance of a pyramid. Being able to navigate through the different directories is vital. Since I'm running a Windows Operating System on my laptop, I needed to install PuTTY, a free SSH client, so I could remotely log in to the Laser Teaching Center terminal. Then I added some journal entries on here, using the Pico text editor, to keep my website up to date. The next step is to make my journals more readable and aesthetically pleasing by adding Greek letters, exponents, italics, and headings in the appropriate places.

Monday, 20 June 2011

After having written in the third person perspective, Dr. Noe would like me to write more in the first person in my journal entries, which I will start doing right now. In the morning, I got a review of complex numbers, numbers with both a real (x-axis) and imaginary (y-axis) part. I was reacquainted with the notion of the complex conjugate, which is a complex number possessing the same real part but an imaginary part of equal magnitude and opposite sign. The talk on complex numbers led into Euler's formula, e^iθ = cosθ + isinθ, where i is an imaginary number and the argument θ is in radians, which I had seen briefly in my physics courses this past year. Using these complex numbers, which I have had some experience with, it's possible to derive the intensity for Young's double-slit experiment. Without going through a lengthy derivation, the intensity is I = 4cos²(k(z₁-z₂)/2), where z is a complex number and k is a constant, or I = I₀cos²(kaθ), where I₀ is the intensity of a single wave, k is a constant, a is the slit separation, and θ is the angle in radians. In order to simplify a complex number z, the binomial approximation was used. The binomial approximation, (1+x)^α ≈ 1+αx, is most useful for approximately finding powers of numbers that are close to 1.

Friday, 17 June 2011

After several days without a formal lecture, we finally sat down for a talk on fiber optics and Gaussian beams. We were presented with a fiber bundle and discussed the way in which it the many fibers inside it transmit light. Light enters one end of the optical fiber from an angle larger than the critical angle (determined by the difference in the refractive indexes between air and the fiber material), thereby causing total internal reflection within the optical fiber. Angles larger than the critical angle where light is able to be transmitted in an optical fiber are said to be in the acceptance cone. The light then continues down the length of the optical fiber through repeated total internal reflections. Shining a laser through one end of the fiber bundle results in the transmission of the laser light through the optical fibers and out the other end. The resulting light shined on a wall shows a large and clear speckle pattern (many light and dark spots), caused by destructive interference with the rough surface of the wall.

A Gaussian beam is a beam of light with an electric field and intensity that can be approximated with a Gaussian curve. As a Gaussian beam propagates through space, there will be a location along the beam where the spot size will be at a minimum (the waist) and the location near where this occurs appears as a hyperbola. The larger the beam size, the greater the convergence, so larger beams have a smaller waist. The distance along the direction of propagation from the waist to the point where the area of the beam's cross section is two times the size is known as the Rayleigh range. When a Gaussian beam travels through a focus, the phase shifts by a factor of pi, known as the Gouy Phase. This phase shift is due to the Heisenberg Uncertainty Principle, since the beam is confined at the focus, that leads to an uncertainty in the transverse momentum, causing the change in phase.

Thursday, 16 June 2011

Using the guide on how to spot a counterfeit written by EOtech, I was able to discover the numerous flaws of my replica holographic sight, which turns out to be a counterfeit by EOtech's standards. Firstly, obviously my replica sight lacks the official manufacturer and serial number label, but externally the dimensions seem to match its EOtech equivalent very accurately. The front and rear windows were very reflective and reflected lots of the ambient light from the fluorescent bulbs in the ceiling of the Laser Teaching Center. When observed closely, it's possible to see your reflection on the front and rear windows. There was also no optical component (a collimating reflector) at the top of the front window. The battery housing was one of the few components that matched the EOtech sight exactly. The reticle on my replica sight did seem to get out of focus very slightly when viewed toward the edge of the front window, but the distortion was very minimal. Pushing the NV button on my replica simply toggles the color of the reticle from red to green which is what would happen on a counterfeit. Changing the color of the reticle from red to green allows the reticle to be better seen in certain environments, which is actually much more useful for me since I don't happen to own a pair of multi-thousand dollar night vision goggles! The final and most convincing evidence that shows that my replica sight is a counterfeit is the presence of the laser diode that can be seen externally. When seen from a distance, what appears to be a small red dot can be seen through the rear window and it becomes even more apparent in a dark room. The reticle itself can't be seen anywhere except through the front window by the user, but even the presence of a small red dot through the rear window can easily reveal the user's location. From what was presented in the guide, it seems pretty conclusive that my replica is in fact a counterfeit. Since it's almost assuredly a counterfeit, it probably doesn't have the same holographic properties as its EOtech counterpart. Dr. Cohen mentioned that it is a good possibility that the laser diode is facing the user because it is simply shining through a reticle mask on to a screen, thereby giving it the appearance of a hologram. My replica holographic sight, which by now can be called a counterfeit, will need to be explored further in order to determine the true method by which it functions and how the manner in which it functions differs from an EOtech holographic sight.

Wednesday, 15 June 2011

Today a guest speaker, Rich Migliaccio, President of East Coast Optical Technologies, gave a presentation about his company during lunch. To start off, Mr. Migliaccio provided us with some background information that has brought him to the place that he is in today. After earning his bachelor's degree in Physics from Stony Brook and his master's degree in Physics from UMass Lowell, he worked for many years in the optical engineering industry, with military, industrial, and medical applications. Currently, East Coast Optical Technologies is a small consulting group that specializes in the design of advanced electro-optical equipment. Three of their most successful products include the RAM 2000, RAM GEN II, and the GammaCam. The RAM 2000 is security screening device that can detect harmful liquids and distinguish those from non-harmful liquids. An open container of a liquid is placed near a "sniffer" which "smells" the liquid and very quickly determines whether it is safe or not. The RAM GEN II takes after its predecessor, the RAM 2000, but instead of being housed in a large doorway, the RAM GEN II is designed for handheld use and it takes real- time, streaming samples of the vapors passing through it. The GammaCam is a portable device that is capable of imaging gamma rays against a physical background. The intensities of gamma radiation are also depicted in the images, which is very useful for locating deadly radioactive sources. Eleven of these devices are currently being used in Japan to assist with the rebuilding effort following the high magnitude earthquake. Mr. Migliaccio's work was very impressive, but his experience was also something for us to learn from. Having worked for both government and private industries, Mr. Migliaccio noted that government projects usually require doing something next to impossible and also money is no object, while in private industry products are designed to perfection and costs are minimized in all areas possible.

In the afternoon, I spoke with Dr. Cohen more about holographic sights. Dr. Cohen told me that he had thought about the internal layout of a real EOtech holosight and basically the laser diode was illuminating a hologram of the reticle, so a virtual image of the hologram of the reticle appears in the field of view at a virtual distance away from the user. I need to do some more reading on interference to be able to better understand how the hologram in a holographic sight is created.

Tuesday, 14 June 2011

Today I found an interesting guide written by EOtech on the i,/internet that could prove useful in my understanding of how a holographic sight works. The guide was about the various ways in which one could spot a counterfeit holographic sight. An obvious external indicator of a counterfeit is the lack of an official manufacturer and serial number label, but a label can easily be forged which brings us to other methods of determining a counterfeit from the real product. On a counterfeit, the front and rear windows are very reflective of ambient light, whereas on an EOtech sight there is almost no reflection at all. On an EOtech sight there is also a small optical component (most likely a collimating reflector to align the light from the laser diode on to a holographic grating) at the top of the front window, while a counterfeit does not have this small piece. The manner in which the batteries are housed also differs. An EOtech sight has a fully detachable battery compartment, as a counterfeit has simply a plastic covering that slides over a battery compartment that is integrated into the sight itself. In terms of the viewing quality of the two holographic sights, there is some slight but noticeable distortion of the reticle if the reticle is viewed near the edge of the front window. Also while looking through a holographic sight, pressing the NV (abbreviation for night vision) button on an EOtech sight will cause the reticle brightness to drop to a very dim level for compatibility with night vision goggles and on a counterfeit, pressing the NV button toggles the reticle color from red to green. Lastly and probably most puzzling, was the visibility of the laser diode in a counterfeit. From a distance away from a counterfeit, one can see the laser diode through the front window, thereby giving away one's location. The laser in an EOtech sight is completely discreet and cannot be seen from the outside. This seems to indicate a different internal parts configuration or possibly a discrepancy in the parts used in each sight. The next step is to determine the manner in which my replica holographic sight works, since it seems to work in a different way than an EOtech holographic sight.

Monday, 13 June 2011

Today I brought in my replica holographic sight for discussion with Dr. Noe and Dr. Cohen and they both found it to be very intriguing. The color of the reticle can be toggled from red to green, but the sight is only labeled with a sticker that reads: 0.08 mW 650 nm Class II Laser Product. Red lasers are typically in the 650 nanometer wavelength, so if there was a second green laser, which would typically be in the 500 nanometer wavelength, there would have to be a second label on the outside of the device. If we assume by the labeling that there is only a red laser in the device, then how can the reticle change to green? This was very puzzling. In a test to determine whether an image of the reticle could be seen from outside the front window, we went into a dark room and placed the front of the sight against a wall. The reticle could not be seen at all, but a faint red light in the shape of a rectangle (since the glass window is in a rectangular shape) larger than the front window could be seen on the wall. As the brightness of the reticle was increased, the brightness of the rectangular box of red light increased as well. A magnifying glass was then held in front of the sight at various distances from the wall, but the rectangular box of red light didn't change. When the reticle was toggled to green, the same effects were observed, but the green light couldn't be seen as easily on the wall. When the sight was turned around with the rear window facing the wall, the same effects were observed.

Friday, 10 June 2011

I spent most of today reading more about holography. I'm particularly interested in holographic sights or holosights. A holographic sight is an electro-optical device placed on firearms to assist in aiming. Very simply put, a holographic sight is composed of a hologram of a reticle image built into a tiny window within the device and when turned on, a laser diode illuminates the hologram of the reticle. When looking through the sight, the reticle appears to be superimposed at a distance away on the field of view. In order to adjust the accuracy of the sight, screws located on the side of the device can be turned which change the windage (left and right) and elevation (up and down) of the reticle by moving the window which the hologram is built. The sight is on the same axis as the weapon that it is mounted so after the sight is properly adjusted, wherever the reticle points is where the weapon will shoot. This provides users with a tremendous advantage over traditional iron sights since the user does not have to line up his or her eyes with the front and rear sights and the target. Instead, users can simply place the reticle on the target and fire, eliminating the need to have their eyes perfectly in line with the iron sights and target. I personally own a replica holographic sight which I will bring in for discussion on Monday.

Also today we had a delicious Thai lunch, which was the first time I had ever eaten Thai food. I had the Pad Thai, which were rice noodles stir-fried with egg, bean curd, sweet turnip, bean sprout, scallion, and ground peanut. It was very similar to the variants of Chinese noodles that I have had.

Thursday, 9 June 2011

Today we each were guided in the setup of our LTC web pages. I haven't had any experience in programming in html nor have I ever used a Linux-based operating system, so I'll have to overcome the learning curve associated with both features.

Later that afternoon, we spent some time in the library exploring possible project ideas. I did some reading on holography, a topic that I know very little about. I learned that holography is a technique by which the interfering patterns of light from a three-dimensional object are recorded and then later reconstructed in the absence of the original light field. Holography has applications ranging from data storage and encryption to art and entertainment. I've only begun to scratch the surface of the field of holography and there is much yet to be learned.

Wednesday, 8 June 2011

Today we began to discuss the small angle approximation. The small angle approximation states that sinθ ≈ θ, cosθ ≈ 1-(θ²)/2, and tanθ ≈ θ. The small angle approximation can be justified graphically, geometrically, and mathematically. When the line y = x is drawn on the same axes as the sine or tangent curves, from 0 degrees to approximately 30 degrees their values are very similar and the same is true for the cosine curve when comparing it to the curve y = 1-x^2/2. From the graphs, you can see that as you approach 0 degrees, the approximation becomes better. In the unit circle, there is a similarity between the lengths of the hypotenuse and the adjacent sides of a right triangle, as well as between the arc length and opposite side of a right triangle. Since those sides are approximately equal in length, it can be concluded that sinθ ≈ tanθ ≈ θ. Using the Taylor Series, one can also justify the small angle approximation for sine, cosine, and tangent functions since all the terms beyond the first term (or the first two terms for cosine) diminish very rapidly due to their large exponents. The small angle approximation may be used in circumstances where you are dealing with an angle less than approximately 30 degrees or 0.5 radians, depending on the desired accuracy.

Next, we began a discussion of Young's double-slit experiment. The variables involved are the wavelength λ, separation of slits d, and the distance to the screen L. When rays are drawn from the two slits, they are assumed to be parallel to one another as they hit the screen, although in actuality they arrive at the same location. The angle θ between a slit and a ray is the ratio of the distance y between the rays at the screen and the distance L to the screen and also θ is proportional to the wavelength λ divided by the separation of the slits. Using the small angle approximation, tanθ is approximately equal to θ. The angle created by the screen and a ray creates another right triangle, with the opposite side measuring dsinθ. Finally we arrive at mλ = dsinθ, where m is an integer for the number of maxima produced by constructive interference.

Using a laser, we were then able to find the approximate thickness of a human hair. First we need to find the beam divergence of the laser. The beam divergence, θ is proportional to the wavelength λ divided by the spacing d between bands. We used a 500 nanometer green laser and when it was shined on to a single strand of hair, there was a diffraction pattern with a 1 centimeter spacing between bands, resulting in a beam divergence of 5 milliradians. A high quality laser would have a beam divergence of only about 1 milliradian. The spacing between bands d is equal to the wavelength λ divided by the beam divergence θ. A 500 nanometer green laser with a beam divergence of 5 milliradians would yield a spacing between bands of 100 micrometers, which would be the thickness of a single strand of hair.

Later in the day, we finally figured out the reason for the difference in the sunspot image produced by the large and small pinholes in the pie plates. The larger pinhole produces a brighter sunspot because the larger aperture allows more light to pass through, but the larger pinhole produces a less clear sunspot because light can enter from more angles, which gives the sunspot a very fuzzy edge. The size of the sunspot remained constant at close distances to the screen because the large size of the pie plate created a shadow, which kept the sunspot at a small and consistent size until the pie plate is moved far enough away from the screen.

Tuesday, 7 June 2011

Today we observed an interesting toy that puzzled us for quite some time. The toy, called Mirage, was set up with two parabolic mirrors facing each other, the top mirror had a hole in its center. Two small plastic pigs were placed in the center of the bottom mirror. When one looks at the hole, the two small plastic pigs appear floating in the opposite orientation as they were placed in the bottom mirror. To explain this, we drew a ray diagram showing the light shining in through the hole and the path it would take as it reflects off of the mirrors. In our ray diagram, we drew rays that were incident from an angle as reflecting outwards parallel and rays that were incident parallel as reflecting outwards at an angle. Each ray took a total of three reflections before it left through the hole in the top mirror. All the rays converged at a single point, the hole in the top mirror, which formed a real image of the two plastic pigs and real images are inverted, explaining the opposite orientation of the pigs.

Another interesting phenomenon that we uncovered was the reason why a full body mirror only needs to be half as tall as a person. The angle at which light strikes the mirror is equal to the angle at which light is reflected, so the light will bounce off the mirror from your head to your toes and vice versa. This symmetry is responsible for allowing your eyes to see all the way down to your toes, since the angle needed to see your toes is at the half-way point of your body. Also, since this symmetry exists, it does not matter how far or close you are to the mirror because the incident and reflected light will always be at the same angle. This symmetry is better known as the Law of Reflection, where the angle of incidence is equal to the angle of reflection. The image formed by a full body mirror, which is a plane mirror, is a virtual image. The outgoing rays from a point on the object diverge in a virtual image, so the image will appear to form behind the plane mirror.

The latter portion of the day consisted of going outside to perform a series of experiments. First, we used magnifying glasses to focus the sun's light on to pieces of black paper. As the sunlight passed through the magnifying glass, a convex lens, all the rays of light converged to a small point on the black paper, which generated enough heat to light the black paper on fire. Next, a lens from a pair of reading glasses was shined on to a piece of white paper until the image (a small circle) could be seen clearly. The distance at which the image could be seen clearly was 0.8 meters, which was the lens's focal length. The image had a diameter of 0.9 centimeters with an uncertainty of 0.1 centimeters. Next, a much larger lens with only a very slight curvature was held up to the sunlight and an image was shined on to a piece of white paper. This image had a diameter of 2.5 centimeters and a focal length of 2.84 meters. The larger size and smaller curvature of the large lens were responsible for its larger image and longer focal length. The size of the image is proportional to the focal length of the lens. Following those experiments, we used two pie plates, each with a pinhole in the center and held them up to the sun. A piece of white paper was held below each pie plate, acting as a screen. One pie plate had a larger pinhole, approximately 5 millimeters in diameter, whereas the other pie plate had a pinhole approximately 1 millimeter in diameter. At small distances, the size of the sunspot remained constant for both pie plates, but at larger distances the size of the sunspots increased. The trend could be seen graphically (size of sunspot vs. distance to screen) starting from the origin and moving toward infinity, appearing almost as a hyperbola. We observed that a larger pinhole produces a brighter and less clear sunspot, while a smaller pinhole produces a dimmer and sharper sunspot. The explanation for this phenomenon was not immediately apparent to us and we were given some time to think about it. The last outdoor experiment that we conducted involved holding a mirror normal to the ground as the sun shined on it and reflected sunlight on to a wall. The distance from the mirror to the wall (34 meters) was measured very crudely by simply taking steps from the mirror to the wall. The width of the reflected light on the wall was measured to be 30 inches or 76.2 centimeters. The reflected light resembled more of an oblong or elliptical shape rather than the shape of the circular mirror. Since the wall was not perpendicular to the mirror, that could be a factor in why the reflected light looked so elliptical. From this experiment, we were able to conclude that the angular diameter of the sun is about 1/100 of a radian. The angular size of an object in radians is the ratio of the arc length, which is the size of the object, to the radius, which is the distance to the object.