If
one were to simply compare the visual appearance
of an optically bonded LCD display to that of
a traditional “air-gapped”, non-bonded
display one will quickly observe that the optically
bonded LCD looks noticeable better. This significantly
improved visual aspect will be true in any type
of ambient light condition, not just in direct
‘sunlight’ environments. So, why does
an optically bonded display always produce a much
better viewable image, what other major resulting
benefits occur, and how does optical bonding work
in general? The answers are abundant and many
relate to optical physics as well as human vision.
This paper intends to explain all of these effects
and benefits.
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Human
Vision and Liquid Crystal Displays |
There exists two primary distinguishing characteristics
of human sight, which are directly related to
the workings of human binocular eyes. The first
revealing quality is full color. Humans can see
across the range of the visible spectrum or the
portion of the electromagnetic spectrum that can
be detected by the human eye. Electromagnetic
radiation in this range of wavelengths is called
visible light. A typical human eye will respond
to wavelengths from about 390 to 750 nm. In terms
of frequency, this corresponds to a band in the
vicinity of 400–790 THz. A light-adapted eye generally
has its maximum sensitivity at around 555 nm (540
THz), in the green region of the optical spectrum.
The spectrum does not, however, contain all the
colors that the human eyes and brain can distinguish.
Unsaturated colors such as pink, or purple variations
such as magenta, are absent, for example, because
they can only be made by a mix of multiple wavelengths.
The
second distinct characteristic of human vision
is contrast or the difference in visual properties
that makes an object (or its representation in
an image) distinguishable from other objects and
the background. In visual perception of the real
world, contrast is determined by the difference
in the color and brightness of the object and
other objects within the same field of view. Because
the human visual system is more sensitive to contrast
than absolute luminance, we can perceive the world
similarly regardless of the huge changes in illumination
over the day or from place to place. Subsequently,
it is for this very reason that contrast
plays the biggest role in viewing images in direct
sunlight or high ambient light conditions. Luminance
is only a small factor. Try this exercise….as
the contrast
level decreases
images become
less visible
even though the background brightness remains
the same. The human eye significantly notices
contrast changes, so simply making an LCD display
brighter does not necessarily make it visually
appear better nor easier to view in direct sunlight.
One can increase the luminance of a display to
the point of greatly reduced contrast and thus
progressively diminishing image visibility.
With
regards to Liquid Crystal Displays (LCD), the
critical optical unit of measure is defined as
“contrast ratio”, or a measure of a display system,
defined as the ratio of the luminance of the brightest
color (white) to that of the darkest color (black)
that the system is capable of producing. For example,
a particular LCD panel might have a hypothetical
white luminance of 400 cd/m2 or nits (candelas
per meter squared, a photometric unit of light
measurement) and a black luminance of 2 cd/m2
(or nits). The contrast ratio would then be measured
as 200:1. It should be noted here that the average
human eye can not see adequate differences in
contrast below 5:1 on the low-end or beyond a
ratio of 100:1 on the high-end. Current LCD panels
typically operate at contrast ratios of 300:1
or higher, which means that the LCD operates beyond
the limits of human vision.
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LCD's
and Optical Physics |
LCD modules currently manufactured by NEC, Samsung,
LG, Optrex, AUO and many others are engineered
to be highly optimized optical devices specifically
designed for human vision. Active matrix liquid
crystal displays are standard on most laptop computers
as well as commercial and industrial grade display
systems. Two properties of liquid crystal are
used as tiny switches to turn picture elements
(pixels) off and on. First the crystals are transparent
but can alter the orientation of polarized light
passing through them. Second, the alignment of
their molecules (and their polarization properties)
is changed by applying an electric field.
In
a color display the liquid crystals are held between
two glass plates or transparent plastics. These
plates are usually manufactured with transparent
electrodes, typically made of indium tin oxide,
that makes it possible to apply an electric field
across small areas of the film of liquid crystal.
The
outsides are coated with polarizing filters. Only
light with a perpendicular polarization can pass
through these filters. (a). See figure 1.1 below.
Inside
the plates are transparent electrodes and color
filters, which form very small picture element
regions called subpixels. A grouping of a red,
a green and a blue subpixels defines the color
that the pixel transmits. Fluorescent (or LED)
backlighting illuminates a display from the rear.
In pixels that are off, light passes through the
rear polarizing filter, the crystals (b) and the
color filters, only to be blocked (absorbed) by
the front polarizing filter. To the eye , these
pixels appear dark. When a pixel is turned on,
the liquid crystals reorient their position, and
they in turn repolarize the light so that it can
pass through the front polarizing filter (c).
The
active matrix provides a superior method of electronically
addressing (turning on ) an array of pixels. For
an image to appear on screen, one row of pixels
receives the appropriate voltage. At the same
time, software in the computer dictates that voltage
be applied to those columns holding active subpixels.
Where an activated row and column intersect, a
transistor turns on a subpixel electrode, generating
an electrical field that controls the orientation
of the liquid crystal. This process repeats sequentially
for each of the rows which can take 16 to 33 milliseconds.
One
of the fundamental problems and inherent limitations
with all LCDs in real-world environments is the
delicate nature of the polarizer material. The
frontal polarizer is easily scratched and physically
damaged which will permanently destroy the quality
of the display image. Another problem is that
this polarizer material is as well very hydrophilic
(absorbs water) and can be damaged with prolonged
exposure to moisture, such as rain, melting snow,
dew, etc. It is because of these fragile characteristics
of the front polarizer that system manufacturers,
such as VarTech, deem it a necessity to protect
the delicate frontal LCD surface with some type
of protective window, be it cover glass or polycarbonate
or touch screen. And here’s where everything
starts to fall apart. Once a cover window or touch
screen is placed in front of the LCD, an “air
gap” is formed between the front polarizer
of the LCD and the overlaying protective cover
window. This ‘air gap’, regardless
of thickness, causes undesirable optical and performance
conditions. From an ‘optical’ standpoint,
this ‘gapped’ cover window causes
reduction in display contrast, decreases in visible
luminance from the LCD, and increases both specular
and diffuse reflection levels.
Reflections
can be divided into two types: Specular reflection
and Diffuse reflection. Specular reflection describes
glossy surfaces such as mirrors or LCD cover glass,
which reflect light in a simple, predictable way.
This allows for production of reflected images
that can be associated with an actual (real) or
extrapolated (virtual) location in space. Diffuse
reflection describes matte surfaces, such as paper
or rock.
So,
in the end, the protective cover window or touch
screen is the direct cause of reduced display
viewability….and environmental performance, which
will be discussed later on in this article.
The
‘air gap’ has such an adverse effect on the quality
of the LCD image because of the optics of Index
of Refraction (Refractive Index) of transparent
surfaces. Transparent materials, such as touch
screens, Lexan overlays, glass protective windows,
heater and/or EMI windows, etc. transmit light
at slightly different rates. This variation is
measured on the Refractive Index (RI) scale. The
polarizer material typically used by the Original
Equipment Manufacturer (OEM) has Refractive Index
of 1.45; air has a value of 1.00; and the various
types of glass substrates such as borosilicate
and soda-glass have an average Refractive Index
of closer to 1.50. The existence of an Refractive
Index mismatch of more than 0.10 units between
contacting surfaces is enough to cause significant
light reflection to occur at the interface between
those substances.
The
greater the Refractive Index discord, the greater
the interface light reflection levels. Consider
this, the three reflective layers of Refractive
Index mismatches typical of most LCD “air gap”
monitors have several optical effects. First,
is external ambient light shining on the display
surface at an angle of incidence greater than
zero degrees. A subsequent result is specular
reflections at each of the three traditional interfaces;
(1) Polarizer to Air, (2) Air to Glass and (3)
Glass to Air. The second effect is light generated
by the LCD’s backlight. This generated light causes
internal specular reflections at the interfaces
of (A) Air to Glass and (B) Glass to Air. The
third effect is external ambient light shining
onto the display surface at zero degrees of incidence
resulting in ‘diffuse’ reflections (generally
referred to as “glare”) at the interfaces of (X)
Air to Glass and (Y) Glass to Air.
The
cumulative effect of the internal specular reflections
of A & B alone result in an average loss of 9.0%
light transmission (luminance) from the display’s
backlight(s). Depending on the angle of incidence
and intensity of external light, both specular
and diffuse reflections can cause image “washout”
(see "Reflection Washout" image below).
This is the point where reflected light intensity
is greater than the emitted light intensity from
the display image. As the level of reflected light
increases, the contrast ratio of the display image
decreases below the level of 5:1, and no longer
is visible to the human eye. It is for these reasons
that ‘air gapped’ LCD products are not considered
as sunlight viewable. The use of anti-reflective
(AR) coatings on the front and back surfaces of
the cover glass substrate, and even on the surface
of the front polarizer, serves to help minimize
these reflection levels by index matching the
glass and polarizer surfaces closer to the 1.00
Refractive Index of air. Although the usage of
multiple anti-reflective (AR) coatings (commonly
referred to as ‘passive enhancements’) improves
the viewability of ‘air gapped’ display products,
the limited efficiency of these AR coatings (see
Image 2 below) still permits reflections to occur
at all interface surfaces. So, in the end, passively
enhanced only displays are marginal at best for
achieving LCD image readability in direct sunlight
or very high ambient lighting conditions.
 
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VarTech's
Optical Bonding Solution |
The
ultimate solution to the problem of internal and
external light reflections and its limitations
on human vision is to eliminate all of the Refractive
Index (RI) interfaces between the display’s
polarizer and the LCD product’s cover window.
This technique is typically referred to as “active
enhancement” and is achievable by a unique
use and application of special silicon based optical
material which has a Refractive Index close to
1.44. This RI value is very close to the polarizer
index of 1.45 and within the tolerance range of
the nominal glass index of 1.50. VarTech takes
this advanced silicon material and utilizes a
unique bond design (described below) to join the
glass and polarizer surfaces which results in
an elimination of the ‘air gap’. The
ending result is an optical Refractive Index match
of materials which allows for uniform light transmission
and very low reflection. By utilizing display
cover glass with a front surface anti-reflective
coating treatment, the following effects are achieved:
External
light strikes the display cover glass at various
angles of incidence. About 5% of this light (2)
is reflected off of the AR coated surface. More
than 93.5% of this external light (1) passes through
the now index matched bonded solution. As light
passes through the front polarizer, it becomes
polarized light. When this polarized light then
reflects off of the LCD cell, its polarization
axis is rotated to where it is then absorbed and
blocked by the front polarizer material.
In addition, due to the operating switching state
of the liquid crystal cell sub-pixels, much of
this external light will pass directly through
the rear polarizer and reflect off of the internal
backlight films. At this point this reflected
external light (Z) is fundamentally the same as
the internal light generated by the display backlight
(B).
The
end result is that the external light is optically
directed, polarized, and utilized to enhance the
color brightness of the display image. This enhanced
color brightness maintains high contrast ratio
levels, independent of the luminance intensity
of the external light source. With the contrast
ratio maintained and reflection “washout” eliminated
the display image is easily seen and thus becomes
truly readable in direct sunlight conditions.
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LCD
Ruggedization Effects From Bonding |
Impact
Testing
VarTech’s optical bonding technology with 3mm
AR-treated cover-glass dramatically improves impact
resistance over non-bonded displays in steel ball
drop tests performed on LCDs. In fact, the bonding
provided up to a 300% increase in impact resistance
on even simple notebook PCs.
-
Metal ball drop tests used 1.5” diameter steel
balls (225g)
- Ball drop height increased 2 inches with each
drop until failure occurred
- LCD only (no bond) failed at approximately
22 inches
- LCD with non-bonded 1.1mm protective cover
glass failed at approximately 24 inches
- LCD with bonded 0.7mm protective cover glass
failed at approximately 37 inches
- LCD with bonded 1.1mm protective cover glass
failed at approximately 40 inches
- LCD with bonded 3.0mm protective cover glass (the type VarTech uses) finally failed at approximately 63 inches
Scratch
Resistance
VarTech’s bonding process increased scratch resistance
by 300% over non-bonded display surfaces in scratch
tests.
Pencil
grades from 2H to 9H were selected and applied
to VarTech bonded and non-bonded LCD surfaces
using sufficient pressure to allow the pencil
lead to just crush.
Non-bonded LCD surfaces experienced scratches
at 3H while the VarTech bonded surface showed
no marring or scratches up to 9H.
Vibration
Testing
Large displacements of the display glass can occur
when LCD monitors are subjected to vibrations.
Maximum displacements – where the rear of the
LCD glass comes in contact with the backlight
unit – can result in film damage and unwanted
mura defects. VarTech’s optical bonding process
improved an LCD modules’ resistance to mechanical
shock and vibration over non-bonded LCD modules
by 300% in testing.
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Other
Benefits – The Greenhouse Effect |
Aside
from the optical and ruggedization qualities discussed
above with the elimination of the air-gap, optical
bonding prevents heat build-up (see “Going Isotropic”
below) from the "greenhouse" effect and prevents
fogging from moisture or contamination from dirt
or particles.
Going
Isotropic
Liquid crystals (LCs) are a state of matter that
has properties between those of a conventional
liquid and those of a solid crystal. Most modern
electronic displays are liquid crystal based (LCD).
LC devices usually work at different thermal regimes,
within various temperature intervals and in different
climatic conditions. LC displays (LCD) have a
well-defined isotropic or operating temperature
limit, above which the actual liquid crystal molecules
will lose their orientation and will assume a
random orientation instead of ‘twisting’ through
the light valve. If the temperature rise is too
high, thermal motion will destroy the delicate
cooperative ordering of the LC phase, thus forcing
the material into a conventional isotropic liquid
phase. In other words, the rod-like molecules
will no longer lie in well-ordered planes stacked
upon each other and will not be able to pass through
the light valve. Isotropic conditions will cause
positive image displays to become dark (see image
below), while negative image LCD's become transparent.
This is the Nematic-to-Isotropic Transition Temperature
or NI Transition.
So,
optical bonding eliminates the ‘air gap’,
an area where heat can get trapped and begin to
‘cook’ the delicate liquid crystals.
Isotropic state can still occur with bonded displays,
but at a much slower rate.
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The
Bond Design |
VarTech’s
VBOND technology combines an innovative bonding
process with an industry-leading proprietary adhesive
to optically bond an anti-reflective glass, plastic
or touch sensor directly to the front of an LCD
display. VarTech’s bonding technology enhances
display performance by improving sunlight readability
up to 400% and impact and scratch resistance up
to 300%. It is ideal for use in consumer, military,
marine, and other industrial applications requiring
outdoor viewability and the durability to withstand
impact, vibration, extreme temperatures, altitudes
and dust.
Optical
Bonding is the affixing of two optical elements
to one another, using a liquid adhesive. In this
way, we differentiate bonding from lamination;
a lamination process is currently performed by
alternate “bonding’ companies as its solution
for the ‘air gap’ problem. By lamination, we are
referring to the affixing of two optical elements
to one another using a pressure sensitive adhesive.
Bonding is suitable for use with elements which
are rigid and may be substantial in size, while
lamination is suitable for affixing a thin membrane,
such as an antireflective-coated plastic film,
to a more or less rigid substrate, such as an
LCD, OLED, Plasma display, touch sensor or anti-vandal
shield.
Using
the qualifier "optical", implies that
the bonded adhesive material is transparent, has
a suitable refractive index and is made under
adequate control that there are no significant
variations in optical properties within a single
bond. On a practical side, the adhesive must also
provide adequate bond strength, have a reasonable
pot life after preparing, not present any health
or safety issues, be available at reasonable cost
from reliable sources and cure to the finished
bond condition using temperatures and time which
are friendly to flat panel display manufacture.
Additionally,
when considering the optical bond as a useable
material, it is important to analyze the impact
of each substance and component to be used jointly
as well as the associated properties of each material.
Introduction of new materials into the process
must be accompanied by a study of such things
as environmental stability of the adhesive may
be affected by the new materials and cure time(s).
VarTech has devised an innovative bonding formula to address these necessities.
An
alternate adhesive agent that some bonding companies
use is an epoxy based formula. This makes a much
more rigid bond than silicone. However, it is
not re-workable in the event of any issues during
production or use (including in-warranty or post-warranty
period repair situations). The biggest drawback,
however, is the ‘yellowing’ effect. This type
of material exhibits a severe yellowing over time
when exposed to high ambient (solar) lighting
conditions. Because of this tendency to yellow,
VarTech does not use this type of adhesive.
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Uses
and Applications |
VarTech's
bonding technology provides valuable benefits
for displays used in consumer and industrial applications
requiring outdoor viewability and the durability
to withstand impact, vibration, extreme temperatures,
altitudes and dust. Here are some examples of
how VarTech bonding technology is currently used:
Marine
Electronics
LCDs used in marine electronics are regularly
exposed to harsh environmental elements such as
high humidity and heat, extreme lighting conditions,
rain, salt water, shock and vibration. VarTech
bonded displays have been widely deployed in marine
electronics and we have led the industry in providing
the now standard display enhancement solution
that:
-
Prevents condensation and fogging
- Improves outdoor readability up to 400%
- Increases the durability to withstand extreme
temperatures, shock and vibration
Medical
Medical applications require mobility and reliability
features in LCDs and also present demanding challenges
such as ambient lighting conditions, shock, vibration,
sterile environmental requirements and constant
temperature changes. VarTech has been a leading
provider of bonding technology to the medical
and emergency health care fields and has worked
closely with medical display manufacturers to
adapt to fast-changing medical technologies where
VarTech is used to:
-
Improve outdoor readability 400%
- Resist stains, dirt, dust, scratches and moisture
- Increase durability to withstand shock, vibration
and temperature changes
- Enable thinner and lighter display designs
Military
and Avionics
LCDs installed in military and commercial avionics
applications operate in notoriously rugged conditions.
Challenges can include rough terrain, extreme
temperature changes, high altitudes, harsh ambient
lighting, electromagnetic interference (EMI),
shock and vibration. VarTech has been successfully
deployed to break down the highly demanding environmental
barriers faced by these displays. In military
and avionics applications, VarTech bonding technology:
-
Enables stable performance in extreme temperatures
and altitudes
- Resists stains, dirt, moisture and scratches
- Increases sunlight readability 400%
- Increases impact resistance 300%
Portable
LCD Devices, KIOSK or Public Information Displays
The portability and/or versatility features of
portable LCD displays and/or KIOSK or outdoor
information displays present challenges such as
frequent exposure to harsh outdoor lighting and
temperatures, ongoing surface stress, shock and
vibration. To protect against these challenges,
VarTech has been successfully driving implementation
of its optical bonding as a standard feature on
many of its products. As consumers become more
mobile and/or demand more from their outdoor visual
devices, VarTech bonding can:
-
Improve outdoor readability up to 400%
- Increase impact and scratch resistance by 300%
- Enable thinner and lighter display designs
- Improve overall durability against vibration,
extreme temperatures and dust
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Conclusion |
By
using an optical bond, it is possible to provide
a low reflectance outermost surface by coating
only one side of one piece of glass. This outer
coating, as stated, will be a very low reflectance
and the bond will eliminate the optical transitions
within the rest of the viewable portion of the
display. This is true, even if the refractive
index of the adhesive, the cover glass and the
display are not exactly matched. Optical bonding
of an LCD with an outer polarizer that is antiglare
treated will also eliminate the diffuse reflections
seen from this surface. As seen in the image below:
The
antiglare treatment amounts to a controlled roughening
of the surface. This causes a bright glow on the
face of the display which cannot be avoided, regardless
of the relative angles between the light source,
the display and the viewer. It is a Lambertian
reflectance (see image below) and will significantly
reduce the viewability of the display in high
ambient lighting. By bonding an anti-reflective
(AR) glass to the face, this Lambertian reflectance
is converted to a specular (image forming) reflection.
It is now possible to adjust the angles of the
display and user to minimize the distraction caused
by the reflections.
A
side benefit of bonding is an increase in usable
light output. Not only have we reduced the reflections
of ambient light (an optical noise), but the light
from the display which was being reflected out
of the path toward the viewer’s eye is now directed
to the user; this is an increase in the signal,
or light output.
Another
point to be considered arises when the display
is to incorporate a conductive surface, such as
an EMI filter or heater. The most commonly used
conductor is indium tin oxide (ITO), which has
a refractive index of about 1.95. This will yield
a Fresnel reflectance (see below images) in air
of about 10% per surface. If normally deposited
ITO is bonded without regard to index matching,
the reflectance will be reduced to less than 2%.
By using a graded coating which is index matched
to a nominal 1.5 for the bond, the reflectance
can be reduced to less than 0.5%.
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