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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 .............................................................................................. 10
Electrical control of liquid crystal displays .................................................................................................. 10
Glare detection using computer vision ....................................................................................................... 11
GRIP computer vision engine ...................................................................................................................... 12
Research Plan .............................................................................................................................................. 12
Engineering Problem Statement ............................................................................................................. 12
Engineering Goal ..................................................................................................................................... 13
Procedure................................................................................................................................................ 13
Analysis ................................................................................................................................................... 13
References .................................................................................................................................................. 14
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
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
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.
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
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)
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
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%.
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
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
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
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
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.
References
Charoudi, Day (1976). “Electrically activated thermochromic optical shutters.” Retrieved from
http://patft.uspto.gov/netacgi/nphParser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearchbool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F4307942 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, 2016.
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, 2016.
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 http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=6c7d2744b6af-47fd-9a51-9b5c6a2f48ad on November 30, 2016.
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 http://lcd.creol.ucf.edu/Publications/2004/APL%20Lin.pdf.
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: 10.1109/LEOS.2004.1363099
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 http://www.dailymail.co.uk/news/article2461972/Glare-sun-contributes-3-000-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.
PJWH (2007) “Polymer Dispersed Liquid Crystals (PDLCs)” Retrieved from http://wwwg.eng.cam.ac.uk/CMMPE/res_mat_pdlc.html on December 1, 2016.
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, 123-127. Retrieved from
http://ieeexplore.ieee.org.ezproxy.wpi.edu/document/587464/ on November 27 2016.