Sunlight readability

<Figure 1> Readability under different illuminance level*
<Figure 2> Illuminance measured from roda office rooftop, from 11:00am ~ 15:00pm.

Sunlight readability, as the name suggests, is the readability of screen output under sunlight.
Most matters in the natural world reflect light, which is also the basis for
human eyes to “see” things.    <Figure 1> Readability under different illuminance level*
However,  as  the  intensity  of  reflected  light  increases,  the  brightness  contrast decreases. The screen clarity might decrease to a point where information onscreen is no longer readable. Figure 1 illustrates the effects of reflected light on readability of screen
output.
*:
1.    Illuminance is the total luminous flux incident on a surface, per unit area (unit: lux).
2.    Illuminance in an office building is approximately 300~500 lux (dependent on distance from the light source).
3.    Outdoor illuminance differs significantly depending on weather and air quality.

As the needs to deploy military-industrial grade display equipment outdoors continue to grow, the industry is also showing greater interest in researches relating to sunlight readability.
The sunlight readability of display equipment can be evaluated by the following formula:

Effective Contrast Ratio

<Figure 3> Illustration of Optical Bonding solution
<Figure 4> Polarizer + 1/4 Lambda film illustration

ECR = 1 + (Emitted Light / Reflected Light)

For display equipment, Effective Contrast Ratio is the ratio of equipment’s emitted light to light reflected from external light source(s).

A higher ECR denotes better readability under sunlight. An ECR of 5 and above indicates that the monitor is readable under sunlight.

To improve sunlight readability, a practical approach is to increase luminance and reduce reflection.

Increase Luminance
As LED technology continues to advance, switching from CCFL to LED lap can increase luminance from 200 nits (CCFL) to 500 nits to 1,200 nits (LED). The maximum sustainable luminance with the current technology is about 1,200 nits to 1,500 nits; when ambient reflected light is beyond 500 nits, readability will be unsatisfactory.

Reflection Reduction: Coating
Reflection can be reduced via anti-reflection coating on the surface of display equipment. Common coating methods, such as dipping, evaporation deposition, Physical Vapor Deposition, etcetera, are used to reduce light reflections. Of these, dipping is employed in Anti-Reflection glass manufactured by roda computer.

Reflection Reduction: Reduce Reflecting Surface
When light passes through a transmission medium, reflection and refraction are created. A beam of light can create both reflection and refraction as it passes a layer of glass. For two-layer glass, reflection and refraction are created twice, once on each glass.This reflective disturbance can be reduced by reducing the amount of reflecting surfaces, using a method shown in Figure 3.

This method, known as optical bonding, can reduce the number of reflecting surfaces by “sealing” the gap between two layers of glass, thereby eliminating some reflection and refraction disturbances. roda computer has been mass producing optical bonding solutions in the 5” lineup, with  other   sizes   entering
Pre-Production.

Reflection Reduction: Light Polarization
The natural light is a mixture of different light sources, such as direct sunlight, scattering light through the clouds, refracting lights from other surfaces, etc., and as such is unpolarized, without a specific direction. The touch panel developed by roda computer has polarizer and ¼ Lambda film implemented to reduce reflected light. The polarizer has the capability to filter out light, allowing only specific directional light to pass through. The filtered light is then circularly polarized via the 1/4 Lambda film. The resulting  circular polarized light will be filtered again by the polarizer, thus eliminating reflection. Please refer to figure 4 for illustration.

CONCLUSION:

True sunlight readability is achieved by incorporating several methods, dependent on the practicability of such implementation. Sophisticated display equipment, such as touch screen, would require almost all methods discussed above, in order to achieve true sunlight readability.