The ultimate measure of any display technology is viewability.
The minute you engage a display outdoors, that display competes
with the sun, and loses every time.
Emissive displays, for instance, can become washed out in diffuse
sunlight and are completely overpowered in direct sunlight. This is
illustrated by the fact a user must typically shield their display
when they are outdoors in bright sunlight. While emissive displays
work well at low ambient light levels, when the ambient light
exceeds the light levels being emitted from the display, the
display’s contrast is reduced.
One technology aiming to address this issue is a reflective display
technology from Qualcomm, called mirasol displays.
The technology behind the displays is called interferometric
modulation (IMOD) and uses micro electromechanical systems (MEMS)
technology. MEMS-based display technologies have been under
development for over a decade, but have recently started to gain
significant traction. Using IMOD technology, the displays work by
reflecting light so that specific wavelengths interfere with each
other to create pure, vivid colours.
Figure 1. An overview of the mirasol IMOD pixel structure and
operation
The display physically manipulates light using micron and
sub-micron-sized mechanical elements. As shown in the left-hand
side of figure 1, each pixel within a mirasol display is composed
of MEMS elements. The display is built on a glass substrate, and
each MEMS element functions as a resonant optical cavity that
strongly reflects a specific portion of the visible spectrum. The
related visual colour that is created is directly proportional to
the cavity’s depth.
Thin films deposited on the substrate comprise one wall of this
cavity, and the other wall is a highly reflective flexible
membrane. When electrostatic force is applied across the cavity,
the membrane collapses against the substrate films, the cavity
becomes very thin, and the resonant wavelength moves into the
ultraviolet spectrum. Consequently, the viewer perceives a
collapsed MEMS element as being black or “off.”
As shown in the right-hand side of figure 1, colour displays are
made by composing a single pixel from MEMS elements of different
thickness. Varying the cavity depth results in variations of
resonant wavelengths, which yield variations of colour. Based on
the phase difference, some wavelengths will constructively
interfere, while others will destructively interfere. As a result,
the human eye will perceive different colours as certain
wavelengths will be amplified in respect to others.
This pixel construction utilises no colour filters, polarisers, or
organic compounds. Colour generation via interference is much more
efficient in its use of light compared to traditional colour
filters and polarisers, which work on the principle of absorption
and waste much of the light entering the display.
The nature of reflective displays allows consistent contrast
ratios, and hence consistent viewing quality, across the full range
of illumination environments. “Just Noticeable Difference” (JND) is
a recognised method of expressing the number of separate
discernable image levels available to the viewer. One can associate
the ability to discern small levels of difference in an image with
perceiving the image as having high quality. Conversely, a decrease
in JND count represents a decrease in the quality level of the
viewed image.
Another advantage to reflective display technologies, outside of
superior viewability, is the elimination of the need for
supplemental illumination. The mirasol display is energy
efficient, provides greatly extended battery life in handset usage
models, and dramatically increases the features available to users,
the design space available to developers, and the revenue streams
available to operators.
James Cathey is v-p of business development for Qualcomm
MEMS Technologies