Sunlight-readability
Sunlight-readability, as the name suggests, indicates that displays can be read in sunlight. Most material surfaces reflect light, which in turn is necessary for the human eye to perceive them. However, when the intensity of the reflected light increases, the brightness contrast decreases. The visibility of screen displays decreases, to a point at which the displayed information is no longer legible. Figure 1 illustrates the effect of light reflection on the readability and presentation of a display.
- Illuminance (unit: lux) is the photometric equivalent of radiometric irradiance E (unit lumen/square metre)
- The normal illuminance within an office building amounts to approx. 300–500 lux (depending on the distance of the light source)
- The outdoor illuminance varies considerably, depending on the quality of water and light
The sunlight-readability of displays can be calculated using the following formula:
Effective contrast ratio
Effective contrast ratio = 1 + (radiated light / reflected light)
With displays, the effective contrast ratio specifies the ratio between the radiated light and the reflected light from external light sources. A higher effective contrast ratio means better readability in sunlight. Contrast ratios of 5 or more indicate that displays can be read in sunlight. An effective method to improve readability in sunlight is to lower brightness and reduce reflection.
Decrease brightness
With the continuous further development of LED technology in terms of brightness and service life, brightness can be increased from 200 nits (CCFL) to 500 or up to 1,200 nits (LED) when switching to LED backlighting.
The maximum brightness with current LED technology is 1,200 to 1,500 nits. (If the reflective ambient light intensity is above 500 nits, readability will be unsatisfactory at a display brightness of 1200–1500 nits.)
This is why surface reflection must be reduced to increase legibility. To this end, various technologies are used, ranging from coating, reduction of the reflecting surface, as well as the polarisation of light.
Reduce reflection: Coating
Reflection can be reduced by applying an anti-reflection coating on the surface of the display. Common coating methods to reduce light reflection include immersion, chemical vapour deposition, physical vapour deposition, and the like.
Reducing reflection: Reduction of the reflecting surface
When light passes through a transmission medium, reflected and refracted light are generated. A beam of light can produce both when it passes through a layer of glass.
With double-coated glass, reflection and refraction are produced twice, once as the light passes each layer.
This reflective interference can be reduced by reducing the number of reflective surfaces. This method, which is called "optical bonding",
is illustrated in Figure 3. With optical bonding, the number of reflective surfaces is reduced by "sealing" the gap between the two glass layers. This eliminates a certain amount of reflection and refraction interference. (roda computer GmbH uses optical bonding solutions with its mass-produced 5" displays. Other sizes are currently being introduced, and are in pre-series production.)
Reduce reflection: Polarisation of light
Natural light is composed of various different light sources, including direct sunlight, dimmed light, refracted light from other surfaces, and the like. This means that it is naturally unpolarised, without a certain direction. The touch panel developed by roda has an integrated polariser and a ¼-lambda film, which helps reduce reflective light. The polariser can filter the incoming light and only allow light heading in a certain direction to pass through. The filtered light is then polarised in a circular manner by the ¼-lambda film. The resulting orbital polarised light is again filtered through the polariser, to prevent reflection.