Switchable Glass: A possible medium for
Department of Computer Science,
Faculty of Mathematics and Computer Science,
Babes-Bolyai University, Kogalniceanu 1
Cluj-Napoca, 400084, Romania.
NASA conference on
Adaptive Hardware Systems,
IEEE CS Press, pp 81-87,
The possibility of using switchable glass (also called smart windows)
technology for Evolvable Hardware tasks is suggested in this paper.
Switchable glass technology basically means controlling the transmission of
light through windows by using electrical power. By applying a variable
voltage to the window we can continuously vary the amount of transmitted
light. Three existing technologies are reviewed in this paper:
Electrochromic Devices, Suspended Particle Devices and Liquid Crystal
Devices. An Evolvable Hardware application for a light-based device is
described. The proposed device can be used for solving an entire class of
problems, instead of one problem only as in the case of other dedicated
Intrinsic Evolvable Hardware (EHW) employs the Darwinian principle of evolution
directly into hardware. Several devices have been used so far in this purpose:
Field Programmable Gate Array , Field
Programmable Transistor Array , Field
Programmable Analog Array  and Liquid
In this paper we suggest the use of switchable glass (commercially known as
smart windows) for Evolvable Hardware purposes. Because switchable glass affects
only the transmitted light, we can used this technology for light-based
In the area of light control, there are three main technologies: Electrochromic
Devices, Suspended Particle Devices and Liquid Crystal Devices. Electrochromic
Devices technology is based on chemical reactions whereas Suspended Particle
Devices technology uses field effects. These technologies are widely used to
manufacture smart windows . These types
of glass can be automatically controlled to adjust the amount of light passing
Suspended Particle Device refers to rod-like particles suspended in a fluid.
With no applied voltage, the particles are randomly oriented and block light
(dark state). When a voltage is applied, the particles align with the electric
field and let light though (light state). By varying the applied voltage, we can
continuously vary the amount of transmitted light.
Electrochromic glass becomes translucent when voltage is added and are
transparent when voltage is taken away. Like suspended particle devices,
electrochromic windows can be adjusted to allow varying levels of visibility.
Liquid crystal glasses become transparent when a voltage is added and have an
opaque behavior when there is no electrical power applied. However, liquid
crystal glass has only 2 states: opaque and transparent with no other degrees of
The paper is structured as follows: The properties of intrisic EHW systems are
briefly described in section 2. Section
3 reviews the current directions in the field of switchable glasses. A
possible EHW application of switchable glass is presented in section
5. Conclusions and further work directions are given in section
2 Properties of EHW devices
According to  there are several
characteristics that a material should have in order to become a possible
candidate for EHW tasks:
In this paper we suggest the possibility of using switchable glass as a platform
for EHW tasks. This kind of material has all the properties required by the EHW
- the material should be configurable
by applying some electrical power or any other source of energy (such as
- the material should affect an incident signal (optical or
- the material should be able to be reset to its original
Because switchable glass affects only the transmitted light, we can used this
technology for light-based computations only.
- the switchable glass can change its
degree of opaqueness by applying some electrical power,
- the switchable glass affects the quantity of light that
pass through it,
- the switchable glass is able to be reset to its original
state by removing the source of power.
3 Overview of the switchable glass
Three existing technologies in the field of switchable glasses are briefly
described in this section. These technologies are: Suspended-particle devices,
Electrochromic devices and Liquid crystal devices.
3.1 Suspended-particle devices
Suspended Particle Devices (SPDs) [7,
19], also called light valves use either a liquid suspension
or a film within which droplets of liquid suspension are distributed.
Light-absorbing microscopic particles are dispersed within the liquid
suspension. The liquid suspension or film is then enclosed between two glass or
plastic plates coated with a transparent conductive material. The mechanism
behind SPD is similar to that of the dielectric in a parallel-plate capacitor
which means that the atoms of the dielectric are polarized by the electric
field. When an electrical voltage is applied to the suspension via the coatings,
the particles are forced to align.
This allows a range of transparency where light transmission can be rapidly
varied to any degree desired depending upon the voltage applied.
An example of how SPD works is given in Figure 1.
The way in which SPD glass works is very simple, if one thinks of SPDs as light
valves. In a SPD window, millions of SPDs are placed between two panels of glass
or plastic. When electricity comes into contact with the SPDs via the conductive
coating, they line up in a straight line and allow light to flow through. Once
the electricity is taken away, they move back into a random pattern and block
light. When the amount of voltage is decreased, the window darkens until it's
completely dark after all electricity is taken away.
Figure 1: The materials within a SPD-based glass (from left
to right): glass or plastic panel, conductive material - used to coat the
panes of glass, Suspended Particle Devices - millions of these black
particles are placed between the two panes of glass, a second glass or
plastic panel. We have kept the glass panels in order to show how this
technology works. When the SPDs are switched on, via the conductive coating,
they line up in a straight line and allow light to flow through. When
switched off the SPDs move back into a random pattern and block light.
3.2 Electrochromic devices
Electrochromic windows [4,
19] are made of special materials that have electrochromic
properties. Electrochromic basically describes materials that can change
color when energized by an electrical current. Electricity generates a chemical
reaction in this material. This reaction (like any chemical reaction) changes
the properties of the material. In this particular case, the reaction changes
the way the material reflects and absorbs light. In some other electrochromic
materials, the change is between different colors. In electrochromic windows,
the material changes between colored (reflecting light of some color) and
transparent (not reflecting any light).
At its most basic level, an electrochromic window needs this sort of
electrochromic material and an electrode system to change its chemical state
from colored to transparent and back again.
Electrochromic glass is made by sandwiching certain materials between two panes
of glass. Figure 2 shows the materials inside one
basic electrochromic window system and the way in which this system works.
In the design shown in Figure 2, the chemical
reaction at work is an oxidation reaction – a reaction in which molecules in a
compound lose an electron. Ions in the sandwiched electrochromic layer are what
allow it to change from translucent to transparent. It's these ions that allow
it to absorb light. A power source is wired to the two conducting oxide layers,
and a voltage drives the ions from the ion storage layer, through the ion
conducting layer and into the electrochromic layer. This makes the glass opaque.
By shutting off the voltage, the ions are driven out of the electrochromic
layers and into the ion storage layer. When the ions leave the electrochromic
layer, the window regains its transparency.
Figure 2: The materials within a ECD-based glass (from left
to right): glass or plastic panel, conducting oxide, electrochromic layer,
such as tungsten oxide, ion conductor/electrolyte, ion storage, a second
layer of conducting oxide, a second glass or plastic panel. We have kept the
glass panels in order to show how this technology works. When switched off,
an electrochromic window remains transparent (left side). When switched on,
a low volt of electricity makes the electrochromic window translucent (right
With an electrochromic smart-window, it only requires electricity to make the
initial change in opacity. Maintaining a particular shade does not require
constant voltage. One only needs to apply enough voltage to make the change, and
then enough to reverse the change.
3.3 Liquid crystal devices
Polymer Dispersed Liquid Crystals (PDLCs), [8,
19] or Liquid Crystals Devices (LCDs) are another major application
in the field of switchable windows.
An example of how LCD works is given in Figure 3.
In the opaque state, the glass diffuses direct sunlight and eliminates 99
percent of the ultraviolet rays.
Figure 3: The materials within a LCD-based glass (from left
to right): glass or plastic panel, interlayer film, liquid crystal film, a
conductive coating, liquid crystal layer, a second conductive layer, a
second liquid crystal film, a second interlayer film, a second glass or
plastic panel. We have kept the glass panels in order to show how this
technology works. When switched off the liquid crystal droplets are randomly
oriented, creating an opaque state. When switched on the liquid crystals
align parallel to the electric field and light passes through, creating a
LCDs operate on the principle of electrically controlled light scattering. They
consist of liquid crystal droplets surrounded by a polymer mixture sandwiched
between two pieces of conducting glass. When no electricity is applied the
liquid crystal droplets are randomly oriented, creating an opaque state. When
electricity is applied the liquid crystals align parallel to the electric field
and light passes through, creating a transparent state.
LCD windows can only be in one of the two states: transparent or opaque. There
is nothing between these states, regardless how much electrical power is
4 Advantages and weaknesses
A comparison of the features of the switchable glasses technologies is given in
An important advantage of Suspended-Particles Devices and Electrochromic Devices
is their ability to continuously vary the amount of transmitted light based on
continuous variations of the applied electrical power. This feature makes SPDs
and ECDs very suitable for a large number of problems whose parameters are
real-valued. By contrast, Liquid Crystal Devices are of ON-OFF type: they can
have only 2 states.
A possible drawback is the speed of performing the changes. As a general remark,
the speed is directly connected to the glass surface. A bigger surface requires
more time to change its state than a smaller surface. However, for EHW tasks the
required surface can be microscopic, which requires less than 1 microsecond to
perform a complete cycle (in the case of SPDs).
Because ECDs depend on ion injection and chemical reactions, the process is
inherently slow. However, SPDs depend on a field effect, thus responding in very
short time. As the technology advances we can expect to have smart glass which
will respond faster to the applied electrical power.
In this section we describe an application for solving the 0/1 knapsack problem
10]. For this purpose we have designed a special system which
consists in two main parts:
A schematic view of the proposed system is depicted in Figure
- A computer which runs a standard
Evolutionary Algorithm (EA) with binary encoding and
- an evolvable hardware light-based device for computing
the fitness of a chromosome.
Figure 4: A schematic view of a light based-computer which
performs a EHW task for solving a knapsack problem with 5 items. The weight
of each item is encoded by the SPD array of glass pieces. The LCD array
encodes the structure of the chromosome (the light is blocked if the
corresponding gene xi has value 0 and the light is
allowed to pass if the corresponding gene xi has
value 1). When a source of light is applied, the first two arrays perform
xi*wi operation. The biconvex
lens will focus all rays into a single spot which is equivalent with
computing the sum x1*w1+...+xn*wn.
Finally a solar cell will convert the light into electrical power which will
be sent back to the computer
5.1 The knapsack problem
In this problem we have a set M of n items. Each item has its own
weight wi. We also have a knapsack of capacity C.
We have to fill the knapsack using some objects in the set M so that:
Without losing the generality we may assume that each weight is a real number
between 0 and 1. Otherwise we can scale them to that interval.
- the total weight of the objects in
the knapsack does not exceed the given capacity C,
- the difference between C and the sum of the
weights of the objects in the knapsack is minimal.
The knapsack problem belongs to the class of NP-Complete problems [2,
6]. No polynomial-time algorithm is known for solving it.
Evolutionary algorithms have been extensively used for solving this problem and
its countless variants [1,
5.2 Evolutionary Algorithms for the
We use a standard representation of a chromosome: a binary string x of
length n. Each position xi is filled with either
1 (meaning that the object is in the knapsack) or 0 (meaning that the object is
not in the knapsack). The fitness is equal to the difference, in absolute value,
between C and the sum of the objects in the knapsack. If the capacity C
is exceeded, the corresponding chromosome will have the fitness equal to ∞. The
fitness of a chromosome is computed by the EHW device (see section
5.3 The EHW device
The EHW device (used for computing the fitness of a chromosome) consists in 5
- several continuous sources of light.
Their number is equal to n (the number of objects in the set M).
- an array of n LCD cells. Each cell is an
reconfigurable LCD glass which lets or does not let the light to pass through.
This array has the same structure as the structure of the chromosome whose
fitness is computed: 1 - the light is not blocked and 0 - the light is blocked.
This array will be reconfigured (by applying a predefined electric power) by the
computer everytime when the fitness of a chromosome needs to be computed.
- an array of n SPD cells. Each cell is an SPD glass
which allows a variable degree of light to pass through. This array encodes the
weights of the given objects. When we have defined the problem (see section
5.1) we have imposed a restriction that each weight is between 0 and 1. Thus
each SPD cell is configured to let wi*100% of the
incoming light to pass through. If wi = 1 it means that
the corresponding glass is fully transparent allowing to pass 100% of the
incoming light. When wi is near to 0 means that the
corresponding glass blocks almost all of the incoming light. This array of SPD
cells actually encodes the problem to be solved. This is why it should be kept
fixed during a run. However, when the problem is changed (i.e. the weights of
the objects are changed) we can reconfigure this SPD array too. This feature
gives us a great generalization ability.
- The effect of these arrays (LCD and SPD), when a light is
applied, is equivalent to performing, for each object, the multiplication xi*wi.
For computing the sum of the previously generated values we can use a biconvex
lens. This lens will focus all rays in a given point also called focal point.
The lens is a very important piece of this device, because it can compute a sum
of numbers in O(1) time. The conventional devices (electrically powered) can do
this operation in O(n) time.
- The light is then captured by a solar cell (photo cell)
which transforms it into electric power, which is later sent to computer as a
analog signal. The computer will calculate the actual fitness based on the
differences between C and the value generated by the EHW device.
5.4 How the system works?
The system works as follows (see also Figure 4):
- The SPDs cells are reconfigured at
the beginning of the search in order to encode the knapsack problem (i.e.
the weights of the objects). These cells are not reconfigured anymore during
the current run. They are kept fixed until a new knapsack problem will be
- A computer runs an Evolutionary Algorithm for solving the
knapsack problem as described in section 5.2.
- When the fitness of a chromosome needs to be computed its
structure will be downloaded into the LCD array. This means that the LCD array
will be reconfigured (by applying/removing some electrical power to each of its
cells) in order to reflect the structure of the chromosome. The light rays will
pass through a cell only if the corresponding object is in the knapsack
(according to the current chromosome).
- When the light rays will pass the second layer (SPDs
array), they will have a power which reflects the degree of opaqueness of each
cell (which actually encodes the weight of each object).
- Finally, the light rays are focused by a biconvex lens
and the resulting ray will be captured by a solar cell. The signal, which is
sent back to the computer, by the solar cell encodes the sum of the weights of
the objects in the knapsack.
- The computer calculates the fitness by comparing value
C with the sum of the objects in the knapsack.
Because of its two levels of reconfigurability, this EHW device has an important
advantage: it can be used for solving an entire class of knapsack problems,
without changing the device. The second array (the array of SPD cells) allows us
to reconfigure the weights, thus permitting to encode and solve all knapsack
problems which have no more than n objects. However, the maximum number
of objects, for a given device, cannot be modified.
Another important advantage is given by the use of the biconvex lens. This type
of lens allows us to compute the sum x1*w1+x2*w2+...+xn*wn
in O(1) time by focusing the entire light to a single spot. Note that a standard
computer needs O(n) steps to do this operation.
Table 1: The differences between the compared smart glass
technologies. Second column shows when the glass is transparent. Switched
ON means that an electrical current is applied to obtain the transparent
state. Third column shows whether the glass can be kept in other states
between opaque and transparent. Fourth column shows whether glass requires
power in order to maintain its state after an initial electrical power has
been applied for changing the state
||When is transparent?
||Continuous states between opaque and transparent?
||Requires power to maintain the state?
|Suspended Particle Devices
|Liquid Crystal Devices
6 Conclusion and future work
The possibility of using switchable glass for Evolvable Hardware tasks has been
suggested in this paper. Three commercially available technologies in the field
of smart windows have been reviewed. Strengths and weaknesses of these
technologies have been deeply presented.
A potential application of these technologies for evolution in materio
tasks has been described.
Further efforts will be focused on two directions:
- implementing the proposed hardware,
- finding new applications which are suitable for this
- applying the switchable glass to reversible computing .
This task could be possible because the glass is able to restore its original
state after removing the source of energy.
C. Cotta, J.M. Troya, A hybrid genetic algorithm for the 0-1 multiple knapsack
problem, Artificial Neural Nets and Genetic Algorithms, Springer-Verlag, (3) pp.
V. Chvatal, Hard knapsack problems, Operations Research 28, pp. 1402-1411, 1980
S. J. Flockton and K. Sheehan, Intrinsic circuit evolution using programmable
analogue arrays, Proceedings of The Second International Conference on Evolvable
Systems: From Biology to Hardware, LNCS, vol. 1478, Springer-Verlag, pp.
C. Granqvist, Handbook of inorganic electrochromic materials, Elsevier Press,
Amsterdam; New York, 1995
C. Grosan, Improving the performance of evolutionary algorithms for the
multiobjective 0/1 knapsack problem using epsilon -dominance, IEEE Congress on
Evolutionary Computation, pp. 1958-1963, IEEE Press, 2004
M.R. Garey, D.S. Johnson, Computers and Intractability: A Guide to
NP-Completeness, Freeman & Co, San Francisco, CA, 1979.
J. M. Harary, Production and Performance of Suspended Particle Device (SPD)
Products, in Proceedings of the 5th International Meeting on
Electrochromism (IME-5) Golden, Colorado, 2002
I. C. Khoo, Liquid Crystals: physical properties and non-linear optical
phenomena, Wiley, 1995
M. R. LaPointe, G. M. Sottile, 2000 survey of window manufacturers on the
subject of switchable glass, Society of Photo-Optical Instrumentation Engineers
and Switching Materials, Vol. 4458, pp. 112-119, 2001
S. Martello, P. Toth, Knapsack problems: Algorithms and Computer
Implementations. John Wiley & Sons, 1990
J. Miller and K. Downing, Evolution in materio: Looking Beyond the Silicon Box,
Proceedings of the 4th
NASA/DoD Conference On Evolvable Hardware, pp. 167-176, IEEE Computer Society
S. Harding, J. Miller, Evolution in materio: Initial Experiments with Liquid
Crystal, Proceedings of the 6th
NASA/DoD Conference On Evolvable Hardware, pp. 167-176, IEEE Computer Society
R. Mortimer, Electrochromic materials, Chemical Society Reviews, Vol. 26(3), pp.
147 - 156, 1997
M. Oltean, Evolving reversible circuits for the even-parity problem, EvoHOT
workshop, Applications of Evolutionary Computing, F. Rothlauf, (et al.) (Eds.),
LNCS 3449, pp. 225-234, Springer-Verlag, Berlin, 2005
A. Stoica, (et al.), Evolution of analogue circuits on Field Programmable
Transistor Arrays, Proceedings of the 2nd NASA/DoD Workshop on
Evolvable Hardware, pp. 99 - 108, IEEE Computer Society Press, 2000
A. Thompson, An evolved circuit, intrinsic in silicon, Proceedings of the 1st
International Conference on Evolvable Systems: From Biology to Hardware, LNCS,
vol. 1259, Springer-Verlag, pp. 390 - 405, 1997
A. Thompson, On the Automatic design of Robust Electronics through Artificial
Evolution, Proceedings of the 2nd International Conference on
Evolvable Systems: From Biology to Hardware, LNCS, vol. 1478, Springer-Verlag,
pp. 13 - 24, 1998
E. Zitzler, Evolutionary algorithms for multiobjective optimization: Methods and
Applications. Ph. D. thesis, Swiss Federal Institute of Technology (ETH) Zurich,
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