Why should you consider optical bonding
for your portable design? Do you need
anti-glare or anti-reflective enhancements
in the product? Should you plan
on a more powerful backlight for your thin-film-transistor
(TFT) LCD display? Start by evaluating the ambient light
levels your product is most likely to encounter.
REFLECTION MANAGEMENT
Everyone has experienced unwanted glare obscuring information
they need to see on a cell phone, TV, GPS navigation
system, or kiosk display. Such reflections can be mild inconveniences
in consumer items or severe safety hazards in missioncritical
equipment, such as cockpit avionics displays.
Glare on vehicle navigation systems or vehicle PC screens
can contribute to accidents if it impedes time to perception and
causes drivers to spend critical time away from
the primary task of piloting the vehicle. On a
marine vessel, glare can obscure vital navigation
information about oncoming hazards.
Fortunately, reliable techniques can combat
glare. Reflection management is a key concept
that designers of products used in brightly lit
ambient environments should understand.
Many devices that are intended for outdoor
use employ transmissive TFT LCDs.
These devices generally incorporate a protective
overlay or touchscreen atop the TFT
that serves to protect the soft polarizer surface
from damage and seal the device from dust or
liquids. While necessary for durability, these
overlays impair optical performance by creating
external and internal reflective surfaces
that decrease the display’s legibility in bright
ambient environments.
However, additional steps like optical bonding and antiglare
(AG) or anti-reflective (AR) surfaces can be added to
the overlays to maintain the product’s performance. Optical
bonding and AG and AR surface treatments also improve display
legibility without impacting power consumption, making
reflection management key to beating power budget and battery
weight design constraints in mobile devices.
Typically, most protective windows and touchscreens adhere
to TFT displays with double-sided tape, leaving an air gap
between the polarizer surface and the overlay. Yet light reflects
when it travels between various mediums with differing indexes
of refraction. If you place a transparent protective window on
top of a display, light will reflect back at the viewer where the
interface of air and glass meet. Light will also
reflect back when it exits the medium.
A simplified model demonstrates that
4251 nits (cd/m2) of specular illumination
reflect from a protective window and the surface
of a typical transmissive TFT when an
air gap is present (Fig. 1). Note that in this
example there is no AR surface treatment. Yet
optical bonding and an AR coating on the
front surface of the protective window can
reduce specular reflection by eliminating the
air gap (Fig. 2).
Reducing reflections from 4251 nits down
to 239 nits is fundamental to increasing readability.
A designer can now begin to contemplate
a backlight to overpower the luminous
intensity of the ambient reflection.
Reflections also occur as light emitted
from a backlight source transits various interfaces as it exits the
display toward the viewer. These additional reflections further
lessen the total emission of usable light if an air gap exists
between the display and protective overlays.
Furthermore, reflected ambient luminescence is brightness
that competes against the luminescence of the information
you desire to see. By reducing unwanted internal and external
reflections, you minimize the intensity of backlighting required
to overpower the ambient light.
CONTRAST RATIOS HELP DETERMINE LEGIBILITY
Display-usability experts generally say a 7:1 minimum contrast
ratio is required for data legibility on full-color graphic displays, while 3:1 suffices for black alphanumeric data on a white
background. Consider the formula for contrast ratio:
(White brightness area + total specular and diffuse reflections)/
(black brightness area + total specular and diffuse reflections) =
contrast ratio
As an example, for a display in a dark room with an air gap
between it and a protective window with a contrast ratio (CR) of
333:1 and a luminance of 1500 nits, we get for a full ambient daylight
(diffused) environment of 10,000 fC (107,600 Lux):
CR = (1500 + 4351)/(3 + 4351) = 1.3
This CR is too low for display information to be legible. When
the same display is bonded to the protective window and an AR
coating is applied on the front, the CR will improve:
CR = (1500 + 240)/(3 + 240) = 7.1
This improvement will make even color images
legible. The display in Figure 1 requires an
impractical 26,000 nits of brightness for color
images to be legible, while the display in Figure 2
requires only 1500 nits. Legible black-on-white
alphanumeric data requires 8502 nits for the display
in Figure 1, as opposed to 478 nits for the
display in Figure 2 (see the table).
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