| Two
terms used to talk about the impact of ambient
lighting on displays are reflection and glare.
The reduction of these is done using surface treatments
for the display which are termed antireflection
and antiglare.
Glare
Defined
Glare
is a phenomenon related to the difficulty of seeing
in the presence of bright light such as direct
or reflected sunlight or artificial light such
as car headlamps at night.
Glare
is caused by a significant ratio of luminance
between that which is being looked at and the
source of glare. Factors with significant impacts
on the experience of the display are the angle
at which the viewer is looking and the glare source
itself as well as individual human eye adaptation.
Glare can be generally divided into two types,
discomfort glare and disability glare. Discomfort
glare results in an instinctive desire to look
away from a bright light source or difficulty
in seeing a task. Disability glare renders the
task impossible to view, such as when driving
westward at sunset. Disability glare is often
caused by the inter-reflection of light within
the eyeball, reducing the contrast between task
and glare source to the point where the task cannot
be distinguished.
Relating
to LCD monitors users, however, glare refers to
the reflection at the display surface of intense
or at least very noticeable light which is deemed
by the viewer as being highly distracting. Often,
a viewer will consider a noticeable reflection
of let’s say a white colored shirt in a display
as a ‘glare’ which reduces his/her ability to
perform view the screen image adequately. This
is more correctly defined as a specular reflection
(see image below).
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.
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Antiglare
Surface Treatments
Antiglare
properties are produced by ‘roughening’ the surface
of the display. This roughening can be done by
any one of a number of innovative processes; be
it mechanical design, chemical treatments or spray
deposition. Chemical processes or spray treatment
are the two most commonly used enhancement treatments
of glare for LCD monitors. In the chemical process,
the glass or polycarbonate overlay to be applied
to the display is ‘etched’ with an appropriate
solvent; then buffered with hydrofluoric acid
(for glass) or an organic solvent (polycarbonate).
This removes material in such a manner as to leave
a microscopically roughened surface. Spray treatment
involves spray or dip coating of the protective
overlay with a unique solution which, on drying,
will leave a roughened layer behind. A common
method involves using a nano-particle suspension
of SiO2 which leaves behind a random distribution
of particles when dried. Subsequently, this surface
treatment changes the ratio of specular to diffuse
(Lambertian) reflections.
The
degree to which this occurs is measured by a “glossmeter”.
This instrument measures light intensity over
a small range of the reflection angle. The intensity
is dependent on the material and the angle of
illumination.
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Reflection
Defined
The
appearance of objects, their color and surface
finish, which can be noticeably viewed as reflected
image from the surface of the LCD monitor, and
in many cases cause significant inadequate viewability.
It can further be characterized by the way in
which the LCD reflects and transmits light (visible
part of the spectrum of electro-magnetic radiation).
LCD
screen reflections have three negative effects:
(1) they reduce the contrast of the displayed
visual information by adding (reflected) luminance
to the emitted luminance, (2) reflected white
light reduces the saturation of displayed colors
(bleaching), and (3) the image of light sources
reflected in the screen cause the human visual
system to focus on that image which is usually
at a much farther distance than the information
shown on the screen.
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Antireflection
Surface Treatments
In
contrast to antiglare processes, antireflection
treatments are usually in the form of very thin,
multilayer films which are typically laminated
to the surface of the protective overlay substrate,
be it glass or polycarbonate material. Careful
design of the film involves specification of the
refractive index of the glass or plastic and of
the surrounding medium (typically air). With this
information, the designer of the film can make
a determination of which materials to use and
the thickness to be deposited. These films are
able to reduce the specular reflectance of a surface
from the Fresnel value (about 4% for glass) to
less than 0.5% over the visible range.
Other
relative articles:
Optical
Bonding Technical Description
LCD
OPTICAL STACK FILMS FOR IMPROVED READABILITY IN
DIRECT SUNLIGHT
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