LITERATURE REVIEW. Tao Lin

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1 LITERATURE REVIEW Tao Lin Contents Dangers of Glare... 2 Technologies to Reduce the Impact of Glare... 2 Sun Visors... 2 Sunglasses... 3 Subtractive Displays... 3 Micro-Blinds... 4 Thermochromic Coatings... 5 Electrochromic Coatings... 5 Liquid Crystal Technology... 5 Specifics of Liquid Crystal Displays... 7 Polymer Dispersed Liquid Crystal... 9 Twisted Polymer Dispersed Liquid Crystal Electrical control of liquid crystal displays Glare detection using computer vision GRIP computer vision engine Research Plan Engineering Problem Statement Engineering Goal Procedure Analysis References... 14

2 Dangers of Glare Everyone has experienced the temporary blindness and pain caused by a car s bright headlights or the sun reflected off a windshield when driving. Bright objects that obscure your vision, known as glare, have a negative effect that varies with how close they are to the driver s center of vision and how bright they are compared to its surroundings (Mitra 2011, p. 1). The effect is momentary: by the time one arrives at his or her destination the blindness is forgotten, and the dangers of glare are often overlooked. However, solar glare s impact on drivers is measurable and, unfortunately, it causes approximately 3000 car crashes every year (Massey, Ray 2013). These crashes are most likely to occur during the early morning, when the sun is low enough on the horizon to obscure other cars, signs, or pedestrians (Mitra 2011, p. 4). According to a study on the effect of glare on road safety (Choi 2014, p. 2), an increase in car crashes as well as police reports of solar glare affecting those crashes coincide with the sun s position in the sky. Technologies to Reduce the Impact of Glare Many technologies exist for reducing the effects of glare, the most common of which are sun visors and sunglasses. Sun Visors The flip down visor, which covers the top portion of the windshield and side windows, is the most basic glare blocking device. It blocks out everything above a certain adjustable angular height above the horizon, which includes direct sunlight whenever the sun is above this angular height. Visors are very effective at blocking most glare, but they don t protect form the rarest or most dangerous forms of glare, namely headlights and the early morning sun. Sun visors can also cause car crashes in intersections with steep streets and high traffic lights. When a car is

3 facing downhill into an intersection with a high traffic light, the sun visor can block the driver s view of said traffic light, causing them to run a red by accident which can cause a car crash. Although visors are useful, they still leave much to be desired. Sunglasses Sunglasses are another popular form of glare protection. They simply lower the light levels of the entire field of vision, preventing higher intensity light from reaching the eye. This functionality is helpful in bright conditions, but is completely unsuitable for blocking headlights, and only minimally effective at blocking the sun when the sun s angle is particularly low. Some sunglasses have additional features such as polarization or tint that block more of the sun s light, but these features generally have limited success. Polarized sunglasses block all light of a particular orientation and are advertised as blocking reflections of the sun. However, this effect is actually much smaller than it is claimed to be, and has not proven increased safety over ordinary sunglasses. Blue lenses have also been used because they block out more yellow light than other colors (Gray et all., 2012, p. 3). The eye is not used to blue light, however, and the brain perceives higher contrast in yellow tinted light, mitigating the effects of blue tinted sunglasses. Even though existing technologies attempt to protect against glare, none completely eliminate its effect. Subtractive Displays A novel approach to blocking glare is a wearable subtractive display. A subtractive display is a transparent plane made up of pixels that can be independently darkened by digital control. A subtractive display, combined with a camera or light sensor, could detect areas of high glare and darken them compared to the rest of the environment. This change could be accomplished

4 by a multitude of different methods including micro-blinds, nanoparticle coatings, and LCD screens, as well as transition coatings. Transition coatings are made of materials that change color when exposed to light, heat, electricity, or some other factor, and are controlled using resistors, projectors, or other devices. Micro-Blinds These micro-blinds are electrostatically actuated window shutters, as depicted in Fig. 1. They are made of nanoscale strips of alloy metal partially bonded to one side of a thin sheet of glass and a transparent conductive coating bonded to the other. When a voltage of the same polarity is applied to the front and back coatings they repel each other, pushing the micro-blinds away from the glass surface and allowing some light to pass through. Fig 1. Micro-blinds in their transparent state. From Boris Lamontagne, In-Hyouk Song - National Research Council Canada When voltage is not applied, the flaps on the metal strips fall flat against the glass, covering it completely (Lamontogne 1-2). This technology uses cheap raw materials, but due to its delicate texture, it may not be durable and it can only be manufactured through very precise and expensive processes.

5 Thermochromic Coatings Thermochromic coatings are a type of transition coating that changes color when exposed to change in temperature. These coatings are usually thin, clear films of oxide molecules that become dark gray when heated resistive films bonded to the glass beneath them (Charoudi 1976). These technologies are relatively simple to manufacture, but they require time to cool down when transitioning into their clear state and require constant current in their opaque states. Electrochromic Coatings In some transition coatings, electricity initiates chemical reactions that change the optical properties of molecules in the coating. These reactions are reversible, can sustain over one million cycles (Silver 1997), and involve oxidation reactions of multiple different metals. The mixture of metals in thermochromic coatings can tailor the transmission of different frequencies of light. Control of specific frequencies of light could be useful in passive sun protection, as well as custom settings to block colored light sources. Nano-structures made up of transitioning compounds can be used to further control the transmission of different frequencies by matching the frequency of a nanostructure to the frequency of light that the structure is designed to block. Electrochromic coatings, however, have not seen extensive use in industry or commercial marketplaces, and has a low contrast ratio. Liquid Crystal Technology The oldest and most widely used type of subtractive display is the Liquid Crystal Display (LCD). LCDs use the unique properties of liquid crystals to selectively block light (Kumar 2001, p. 3). Liquid crystal is a state of matter that some substances experience where the molecules are

6 free to flow around each other, but prefer to orient themselves in patterns similar to a crystal lattice. The patterns they orient themselves in are called liquid crystal phases, and each compound with a liquid crystal state can exhibit any number of these phases at the same temperature and pressure based on electric fields. The two liquid crystal phases that are most commonly used in subtractive displays are nematic and chiral (also known as cholesteric) (Kumar 2001). In the nematic phase, molecules are distributed randomly, but all oriented in the same direction. In the chiral phase, the liquid crystal layers itself into sheets of uniform orientation, but with each sheet rotated slightly compared to the sheets above and below it, as shown in Fig. 2. Fig. 2. Diagram of arrangement of molecules in nematic and cholesteric liquid crystal. (Principles of General Chemistry)

7 Specifics of Liquid Crystal Displays Liquid crystals are useful in subtractive displays because of the way the chiral liquid crystal phase interacts with polarized light and how the phase of liquid crystal can be manipulated using electric fields. When light passes through a chiral liquid crystal perpendicular to the orientation of its layers, each layer rotates the polarization of the light slightly as shown in Fig. 3. Light passing through liquid crystal in the nematic phase experiences a different refractive index depending on the alignment of the molecules: if the molecules are aligned with the direction of travel of the light, light will behave differently than if they are perpendicular. In the presence of an electric field, a liquid crystal will tend toward the nematic phase instead of the chiral phase, and the uniform orientation becomes oriented with the direction of the electric field. The most basic LCD display is a very thin liquid crystal layer sandwiched between two panes of glass, with one pane covered in a continuous conductive coating and the other covered with an array of transparent conductive patches that can be controlled using transistors. Polarizers angled perpendicular to each other are adhered to each pane. A constant voltage is applied to the continuous conductive coating. When the opposite voltage is applied to a particular conductive patch, the electric field created by the voltage difference between the two sides causes the liquid crystal to switch from its chiral to its nematic phase. When the liquid crystal is in chiral phase, light is rotated 90 in the liquid crystal layer, allowing it to pass through both polarizers. When voltage is applied and the liquid crystal transitions to nematic phase, however, the light doesn t change polarization in the liquid crystal layer and, because it oriented 90 to the orientation of the second polarizer, the light is blocked. This technology is

8 used on all digital watch faces, clocks, most computer monitors and many phones (Kumar 2011). Fig. 3. Diagram of basic LCD display. (Encyclopedia Britannica) Because they are so widely used, LCD screens come in many varieties, are affordable, and have well established control mechanisms. LCD screens also have limitations that make them suboptimal for use directly in front of the eye. Because all light that passes through them in their transparent state must pass through a polarizer, the maximum transparency for an LCD display is 50%.

9 Polymer Dispersed Liquid Crystal Another much less common type of liquid crystal display is called Polymer Dispersed Liquid Crystal (PDLC). In Polymer Dispersed Liquid Crystal, a type of liquid crystal molecule that only exhibits the nematic phase is used. PDLC makes use of the fact that when light passes through nematic liquid crystal, it experiences a refractive index dependent on the angle at which the light strikes the liquid crystal molecules. In PDLC, bubbles of nematic liquid crystal are suspended in a polymer plastic which is placed between two transparent conductors. Without voltage, the LC molecules in the bubbles are arranged around arbitrary poles of the bubbles (PJWH 2007). Fig. 4. Polymer Dispersed Liquid Crystal scattering and transmission states. (World of Liquid Crystal) In the voltage off state, light will be refracted in a random direction multiple times within the panel, causing much of it to be directed backward or absorbed within the panel. When voltage is applied, the LC molecules in the bubbles all orient in the same direction, allowing light to

10 travel through the device (PJWH 2007). In order for light to be redirected a sufficient number of times within the panel, the separation between the conductor plates must be larger than in a standard LCD display. The increased separation means that voltages of between 5 and 15 volts are required between the two plates to completely orient the liquid crystal bubbles. Because lithium batteries produce around 3.7 volts, a dc to dc converter is required for higher voltage PDLC display. Solutions to the problem of high voltage requirement are to use a small cell gap or add conductors to the polymer matrix (PJWH 2007). Twisted Polymer Dispersed Liquid Crystal Twisted Polymer Dispersed Liquid Crystal (T-PDLC) is a refinement on PDLC technology made by researchers at the University of Central Florida (Lin, Y. Hsin, Hongwen Ren, Shin-Tson Wu 2004). In T-PDLC, the glass substrates are brushed orthogonally, which increases the bonding energy needed for the liquid crystal to adhere to the surface, which in turn limits the size of liquid crystal bubbles on the surface and prevents them from joining together. This technology increases the contrast ratio of the panel, while also retaining a low activation voltage. These qualities make it useful in practical transparent display applications (Yi-Hsin Lin, Hongwen Ren, Yung-Hsun Wu, Xiao Liang and Shin-Tson Wu 2004). Electrical control of liquid crystal displays Each LCD display has a slightly different control system. LCD screens are usually connected to control chips by ribbon cables, which have the highest number of channels of any cable type. These cables still don t have as many channels as the LCD has pixels, so the LCD screen has its own small PCB board to control every pixel in the display based on input from a number of channels which is usually less than a hundredth the number of pixels in the display. Each LCD display receives data from the ribbon cable in

11 its own encoding specific to that display which requires special driver software installed on the chip that controls it. Desktop monitors have chips that convert the input image from a standard type such as HDMI to the specific encoding for its display. Compact and integrated devices such as smartphones use customized screens that can only be controlled by the circuitry on the phone itself. Therefore, older and less optimized devices with generic control are more suited for modification and reuse in new devices. Glare detection using computer vision Detecting objects that are much brighter than their surroundings is one of the most basic operations of computer vision. The simplest way to detect bright pixels is a threshold test, which produces a binary black and white image, also known as a binary bitmap, where all pixels that have RGB values above the threshold are white, and all pixels with RGB values below the threshold are black (B. Miller, personal communication, November 13, 2016). Simple threshold tests often lets some noise pixels through, necessitating noise reduction (Cyganek 34). Transformations used to reduce noise include dilate, blur, and size threshold, which all produce a resultant image composed of a black background and any number of large, roughly convex white blobs. The computer then creates a contour map of the image, which is the set of edges dividing black from white in the binary bitmap. If this is done correctly, then the contours should be closed loops around the white areas. The computer vision software then calculates the area and location of the contours and sends them to the next stage of the program where they are used to control a device such as a PDLC screen. The most comprehensive and widely used software library for computer vision is OpenCV (Open Computer Vision), an open source library of computer vision functions. OpenCV is very powerful and used for a vast range of

12 applications such as robot control and facial recognition. For basic applications such as glare detection, other software packages such as GRIP are just as effective and easier to use. GRIP computer vision engine GRIP is an open source project created by students at WPI to be a user-friendly computer vision system for First Robotics Competition (FRC) robots. FRC robots often have to shoot objects very accurately at targets lined with retroreflective tape. Retroreflective tape is tape which reflects light at high intensity directly back to the source of the light. FRC robots use cameras surrounded by high intensity LEDs to detect the apparently very bright retroreflective tape and turn towards it. Techniques used for locating retroreflective tape are very similar to those used in detecting glare because they are both vastly brighter than their surroundings. GRIP is a graphical programming language with an interface similar to NXT-G that is designed to produce a computer vision pipeline, in which an image is processed by a sequence of transformations until the desired output is produced (B. Miller, personal communication, November 13, 2016). GRIP s main limitation is that it is a desktop application that can only be run on a relatively powerful computer, bigger than or equal to a Raspberry Pi, which measures 3 by 2. For further miniaturization, simpler and more efficient software is necessary. Research Plan Engineering Problem Statement Glare, from either the sun or headlights, causes 3000 traffic accidents every year. These accidents occur most often in the early morning on intersections with cardinal facing streets, when the sun is low on the horizon directly in front of drivers (Choi, Eun-Ha, Santakh Sing

13 2014). The close angular proximity between the glare and other cars requires precise and targeted glare reduction technology, rather than global or fixed-area glare blocking devices such as sunglasses or sun visors. Engineering Goal The goal of the project is to engineer a digitally controlled pair of sunglasses that can block out light sources with brightness over a certain threshold. Procedure Polymer Dispersed Liquid Crystal (PDLC) display will be created using parts from LCD displays and raw materials. A mixture of polymer and liquid crystal will be sandwiched between one Indium Tin Oxide (ITO) coated glass panel and one thin film transistor (TFT) array panel from an LCD screen. Analysis In order to determine the impact of the glasses on vision and awareness, the fraction of light that passes through the lenses from a bright light source will be measured when the lenses are activated and deactivated. A light sensor will be used to determine the amount of ambient light that is inadvertently blocked by the device. This measurement will be made while the device is displaying a set pattern of light blocking pixels created when the device was exposed to a small, bright light source of known angular size. Statistical methods will be used to calculate consistency and rank effectiveness of the prototypes based on these data.

14 References Charoudi, Day (1976). Electrically activated thermochromic optical shutters. Retrieved from Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearchbool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F on November 25, 2016 Choi, Eun-Ha, Santakh Sing (2014). Statistical Assessment of the Glare Issue-Human and Natural Elements. National Highway Safety Traffic Administration. Retrieved from fcsm.sites.usa.gov on November 27, Cyganek, Boguslaw. (2013) Object detection and recognition in digital images : theory and practice. John Wiley & Sons Inc. Gray, Rob, Warren Hill, Brooke Neuman, Dianne Houtman,Richard Potvin (2012). Effects of blue light-filtering intraocular Lens on Driving Safetly in Glare Conditions. Retrieved on September 9, Kumar, Satyendra. (2001) Liquid crystals : experimental study of physical properties and phase transitions. Cambridge, Cambridge University Press. Lamontogne, Boris, Pedro Barrios, Cristophe Py, Suwas Nikomb (2009). The next generation of switchable glass : the micro-blinds, NRC Institute for Microstructural Sciences. Retrieved from b6af-47fd-9a51-9b5c6a2f48ad on November 30, Lin, Y. Hsin, Hongwen Ren, Shin-Tson Wu (2004). High Contrast Polymer Dispersed Liquid Crystal in a 90 Twisted Cell. Applied Physics Letters, 84,20. Retrieved on October 4, 2016 from Lin, Y. Hsin, Hongwen Ren, Yung-Hsun Wu, Xiao Liang and Shin-Tson Wu (2004). High contrast reflective display using a polymer-dispersed liquid crystal. Lasers and Electro-Optics Society,11. DOI: /LEOS Massey, Ray (2011). The dazzling sunsets that kill 36 drivers in 12 months: Glare contributes to 3,000 accidents and is particularly dangerous at this time of year. Dailymail.com. Retrieved on September 29, 2016 from /Glare-sun-contributes road-accidents-particularly-dangerous-timeyear.html Mitra, Sudeshra. Sun glare and road safety: an empirical investigation of intersection crashes. Elseveir Safety Science. Retrieved from on September 27, 2016.

15 PJWH (2007) Polymer Dispersed Liquid Crystals (PDLCs) Retrieved from on December 1, Silver, J, M.T. Ahmet, J. Billingham, J.G.F. Littler(1997). Development of wavelength selective shutters for device application for filters and smart windows. IEE Proceedings - Circuits, Devices and Systems, 144, 2, Retrieved from on November

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