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Original transparency a few minutes
Introduction:
A glass that darkens when exposed to light but regains its original
transparency a few minutes after light is removed; the rate of clearing
increases with temperature.
Photosensitive glass that is chemical treated to become dark upon exposure
to ultraviolet light and regains full transparency when the UV rays are
removed. When exposed to sunlight, dark spots form in photo chromic
glasses which impede the sun's rays. Formation of these dark spot results
from a rearrangement of the oxides within the glass. See this video:
What is a glass?
Glass is an amorphous solid that has been around in various forms for
thousands of years and has been manufactured for human use since 12,000
BCE. The status of glass as a liquid versus a solid has been hotly debated.
The short story is that glass is a super cooled liquid, meaning that it is rigid
and static but does not change molecularly between melting and solidification
into a desired shape. Glass is one the most versatile substances on Earth,
used in many applications and in a wide variety of forms, from plain clears
glass to tempered and tinted varieties, and so forth.
What is photo chromic glass?
Traditional photo chromic eyeglasses are generally alkali boroalumino
silicates with 0.01 to 0.1 percent silver halide and a small amount of copper.
Upon absorption of light, the silver ion reduces to metallic silver, which
nucleates to form colloids about 120 angstroms in size.
This is small enough to keep the glass transparent, but the colloids are dense
enough to make the glass look gray or brown. In photo chromic eyeglasses,
darkening is reversed either by the removal of light (optical bleaching) or by
raising the temperature (thermal bleaching)
How can my glasses change from transparent, when I'm inside, to dark when
I go outdoors?
Sunglasses or prescription eyeglasses that darken when exposed to the sun
were first developed by Corning in the late 1960s and popularized by
Transitions in the 1990s. In fact, because of the extreme popularity of the
Transitions brand, these lenses are usually referred to as transition lenses.
The correct term for these glasses is photo chromic or photo chromatic, which
refers to a specific chemical reaction the lenses have to ultraviolet (UV)
radiation.
Photo chromic lenses have millions of molecules of substances such as silver
chloride or silver halide embedded in them. The molecules are transparent to
visible light in the absence of UV light, which is normal for artificial lighting.
But when exposed to UV rays, as in direct sunlight, the molecules undergo a
chemical process that causes them to change shape. The new molecular
structure absorbs portions of the visible light, causing the lenses to darken.
The number of molecules that change shape varies with the intensity of the
UV rays.
When you go indoors and out of the UV light, a different chemical reaction
takes place. The absence of the UV radiation causes the molecules to "snap
back" to their original shape, resulting in the loss of their light absorbing
properties. In both directions, the entire process happens very rapidly.
In the original Photo Brown and Photo Rey products made by Corning, the
lenses are made of glass, and the molecules are distributed evenly
throughout the entire lens. The problem with this method was apparent in
prescription glasses where different parts of the lens were of varying
thickness. The thicker parts would appear darker than the thinner areas. But
with the increasing popularity of plastic lenses, a new method has been
developed. By immersing the lenses in a chemical bath, the photo chromatic
molecules are actually absorbed to a depth of about 150 microns into the
plastic. This is much better than a simple coating, which would only be about
5 microns thick and would not provide enough molecules to make the lenses
sufficiently dark. This plastic lens absorption process has been popularized
by Transitions, the leading manufacturer of photo chromic lenses.
An important note about photo chromic lenses: because they react to UV light
and not to visible light, there are circumstances under which the darkening
will not occur. A perfect example of this is in your car. Because the windshield
blocks out most UV light, photo chromic lenses will not darken. For this
reason, most sunglasses with photo chromic lenses also have a certain
amount of tint already applied to them.
Chemistry behind it:
Many people who wear eye glasses prefer those made with photo chromic
lenses or glass lenses which darken when exposed to bright light. These
eyeglasses eliminate the need for sunglasses as they can reduce up to 80%
of transmitted light. The basis of this change in color in response to light can
be explained in terms of oxidation-reduction reactions. Glass consists of a
complex matrix of silicates which is ordinarily transparent to visible light. In
photo chromic lenses, silver chloride (AgCl) and copper (I) chloride (CuCl)
crystals are added during the manufacturing of the glass while it is in the
molten state and these crystals become uniformly embedded in the glass as it
solidifies. One characteristic of silver chloride is its susceptibility to oxidation
and reduction by light as described below.
Cl- -----------> Cl + eOxidation
Ag+ + e- -----------> Ag
Reduction
The chloride ions are oxidized to produce chlorine atoms and an electron.
The electron is then transferred to silver ions to produce silver atoms. These
atoms cluster together and block the transmittance of light, causing the
lenses to darken. This process occurs almost instantaneously. As the degree
of "darkening" is dependent on the intensity of the light, these photo chromic
lenses are quite convenient and all but eliminate the need for an extra pair of
sunglasses.
The photo chromic process would not be useful unless it were reversible. The
presence of copper (I) chloride reverses the darkening process in the
following way. When the lenses are removed from light, the following
reactions occur:
Cl + Cu+ ------> Cu+2 + ClOxidizing agent reducing agent oxidized species reduced species
The chlorine atoms formed by the exposure to light are reduced by the
copper ions, preventing their escape as gaseous atoms from the matrix. The
copper (+1) ion is oxidized to produce copper (+2) ions, which then reacts
with the silver atoms as shown.
Cu+2 + Ag ------> Cu+1 + Ag+
Oxidizing agent reducing agent reduced species oxidized species
The net effect of these reactions is that the lenses become transparent again
as the silver and chloride atoms are converted to their original oxidized and
reduced states.
Photo chromic glass lenses
A specific mode of action: Silver atoms The photo chromic mechanism that
takes place in a glass photo chromic lens isvery different from what happens
with plastic.
The light-reactive properties of silver atoms have long been known.
Photography is a key example, but in this field, the process is not reversible.
Photo chromic glass lenses, on the other hand, where the reaction is
mediated by atoms of silver in the matrix of a specially constituted vitreous
environment, are capable of continuous and reversible change between the
clear and darkened states. The number of atoms present, and their
distribution within the lens, will directly influencetransmissionlevels and lens
color. Additionally, the basic composition of the lens determines the rate of
change through the darkening-clearing cycle.
The basic principles governing this change of state in the atoms of silver are
schematically represented in our animation. Influenced by UV-A or short-wave
visible spectrum radiation, an atom is able to modify its external electronic
structure by deployment of the electrons present in the vitreous structure.
When such radiation is no longer present, the system returns to its original
state.
Stable and durable
Laboratory tests involving several tens of thousands of darkening-clearing
cycles, corresponding to long years - or even decades - of normal use, have
revealed no significant fatigue in this phenomenon. Indeed, it shows
remarkable reversibility under natural lighting conditions.
This, added to the natural transparency of glass and the in-mass presence of
the photo chromic agent, explains how stable glass photo chromic lenses can
be in the long run.
A complex process
Optimum precision in product manufacturing is required to regulate photo
chromic performance, especially during the critical stage ofannealing. After
melting and pressing, the carrying of a lens through an annealing Lahr (at
temperatures up to 700°C depending on the nature and softening point* of
the glass) is a commonly used internal stress-reduction process. But when it
comes to photo chromic glass, annealing is also the way to activate thesilverhalidecrystals, and its temperature has to be regulated with a precision of
1°C, taking into account the size and number of crystals, the glass
composition and volume, and the photo chromic characteristics - avery
complex process which Corning pioneered in the mid 1960s and never
stopped improving since then.
The Need for Photo chromic glass:
A discussion of photo chromic begins with the discussion of the human eye
and how it adapts to light. When eye encounters light, it does three things
with it Changes its speed Changes its direction. Changes the amount
entering the eye Potential
Problems Light Causes:
The Light outdoors is 25 times more power than indoor light Bright light &
glare can be a problem causing fatigue, headaches and eyestrain Night vision
may be affected if filters are not worn during the daytime .UV poses a threat
to the long‐ term health of the eye. The solution the way to alleviate these
concerns is to regulate light levels Control glare protecting the eye from
harmful UVR.
How?
Use photo chromic lenses to recreate natural vision.
It makes vision sharper, clearer, bolder and more
comfortable photo chromic lenses do all of this without any
kind of medication for the patient
Q: How Photo chromic Lenses Change UV is a high
energy light invisible to the eye?
Answer: When a photo chromic molecule is exposed to UV light, a chemical
bond in the photo chromic molecule is broken Molecule rearranges & reduces
visible light Thermal energy (heat) drives the photo chromic molecules back
to their clear state.
Factors That Affect Photo chromic Performance
Two main factors ‐ UVR and temperature
Photo chromic lenses do not perform in an “on” or “off”
manner
Geographical location
The time of year,
The time of day and the orientation of the lens to sunlight
History:
TWO-FOR-ONE EYEGLASS LENSES that darken when exposed to
ultraviolet rays from the sun and return to their clear state indoors have come
a long way since they were first invented in the 1960s. Early versions of these
so-called photo chromic lenses were plagued by slow darkening or lightening,
and sometimes the lenses would get stuck midway. Newer models of the
changeling spectacles are much quicker to activate and fade, and they come
in both glass and lightweight plastic.
The first photo chromic lenses were made of glass and developed by
scientists in the U.S. at Corning. The company has made improvements to its
original process over the years, but in general, the science remains the same,
says Lionel M. Tanguy, a glass application engineer at Corning Ophthalmic, in
France.
Glass photo chromic lenses contain silver halide crystals embedded in a
glass substrate. In the presence of UV-A light (wavelengths of 320-400 nm),
electrons from the glass combine with the colorless silver cautions to form
elemental silver. Because elemental silver is visible, the lenses appear
darker. Back in the shade, this reaction is reversed. The silver returns to its
original ionic state, and the lenses become clear. Although these lenses are
still manufactured, the market for glass has dwindled since the
commercialization of plastic photo chromic lenses in the 1990s.
Applications:
WINDOWS OF THE FUTURE:
Photo chromic Windows
Still in the development stage, photo chromic windows respond to changes in
light, much like sunglasses that darken when you move from a dim light to a
bright one
While this type of technology may seem like a good idea, it has its drawbacks
for saving energy. Photo chromic windows work well to reduce glare from the
sun, but they don't control heat gain. That's because the amount of light that
strikes a window doesn't necessarily correspond to the amount of solar heat it
absorbs. Because the sun is lower in the sky during the winter months, for
example, its rays may strike a window more intensely in the cold season than
in the summer, when the sun is higher in the sky. In this case, a photo
chromic window would darken more in the winter than in the summer,
although winter is the time when solar heat would be beneficial.
Another problem is that, while this technology works fine on small, eyeglasssized pieces of glass, it has yet do be done successfully on a large-scale,
commercial level for window-sized pieces.
Despite some problems, "smart windows" hold the promise of reducing
energy demand and cutting air conditioning and heating loads in the future.
They offer the next major step in windows that are increasingly sophisticated
and energy efficient.
Electro chromic Windows
Flip a switch and an electro chromic window can change from clear to fully
darkened or any level of tint in-between.
Electro chromic window in Boeing 787:
The technology has been suggested for cars, where with a touch of a switch
the driver can tint the mirror or sunroof. In buildings, the changeable windows
allow for privacy, to cut down glare, and to ward off increases in solar heat.
The action of an electric field signals the change in the window's optical and
thermal properties. Once the field is reversed, the process is also reversed.
The windows operate on a very low voltage -- one to three volts -- and only
use energy to change their condition, not to maintain any particular state.
To make an electro chromic window, a thin, multi-layer assembly is
sandwiched between traditional pieces of glass. The two outside layers of the
assembly are transparent electronic conductors. Next is a counter-electrode
layer and an electro chromic layer, with an ion conductor layer in-between.
When a low voltage is applied across the conductors, moving ions from the
counter-electrode to the electro chromic layer cause the assembly to change
color. Reversing the voltage moves ions from the electro chromic layer back
to the counter-electrode layer, restoring the device to its previous clear state.
The glass may be programmed to absorb only part of the light spectrum, such
as solar infrared.
Early research indicates that the technology can save substantial amounts of
energy in buildings, and electro chromic glazings may eventually replace
traditional solar control technology such as tints, reflective films and shading
devices.
Photo chromic
lenses change in color and light transmission when exposed to different light
intensities. The rate of darkening depends on the ambient temperature.
These lenses do not become as dark when driving because the car's
windshield absorbs some of the ultraviolet light that causes the lenses to
darken. Photo chromic lenses are now available in plastic as well as glass,
and they may be either brown or gray in color.
Advantages:
Absorb the sun not reflect it!
Will greatly reduce global warming!
Protect your interior furnishings against the sun with 100%
UV blockage!
Greatly reduce fossil fuel energy consumption from Air
Conditioning!
No electric needed to power Photo chromic Windows, all
you need is sunlight!
Watch your big screen TV's and monitors with ease day
and night!
Consumers will actually see the windows working nonstop
to greatly reduce there
Expensive energy bills!
Watch your Photo chromic Windows darken automatically
with the sun and lighten at dusk!
Photo chromic Windows will lighten and darken forever
without fatigue!
Photo chromic glass reacts only to sunlight. In the winter
the Earth may be closest to
the Sun but because of the tilt of the Earth, the Northern
Hemisphere has less direct
Sunlight than the Southern Hemisphere. Thus, in the winter
when the Sun is out,
Photo chromic Windows in the Northern Hemisphere will
only darken slightly compared to
Photo chromic Windows in the direct sunlight of the
Southern Hemisphere.
Spring,
summer, fall, or winter watch them adjust perfectly
right before your eyes!