EW guide: Sunlight-readable LCDs
To be readable under the sun’s glare, a display needs to be bright and have low reflectivity.
Even how it reflects light is important.
“Anti-reflective is better than anti-glare,” Martin Whitehead, director of technology at Display Solutions, told Electronics Weekly. “A matt screen is anti-glare because it spreads light out – so the whole surface is bright. With anti-reflection you get a mirror-like reflection, but it is dim – you cannot see the display through the sun’s reflected circle, but the circle remains small and you can see the display all around it.”
And sometimes a little reflectivity is an advantage, but only if the parts of the display that are supposed to be bright are more reflective than the parts that are supposed to be dark – leading to ‘transflective enhancement’.
“The transflective enhancement will increase the apparent backlight brightness by reflecting 0.5 to 1% of the incident backlight back through the LCD crystals,” said Whitehead. “So with sunlight at 50,000nits [nits=cd/m2], a 500nit backlight would perform like a 500nit backlight plus a 250-500nit backlight.”
‘Performs like’, he notes, not ‘will be as bright as’: “The eye is over 1,000 times more sensitive to contrast than brightness.”
Reflections can be cut by reducing the number of air-glass and air-plastic interfaces between the actual pixels and the viewer.
One technique for this is called ‘optical bonding’ – filling the air spaces between the display and its front glass or touch screen with a transparent liquid that sets to be a transparent solid.
“An extrinsic contrast ratio of at least 7:1 is required for sunlight readability of colour displays,” said Whitehead. “Optical bonding is combined with anti-reflective coatings to reduce a typical LCDs reflections from 14% with a cover glass, to under 1%. This increases the extrinsic contrast ratio from 1.2:1 to 14.5:1.”
Outdoor cash machines are frequently unreadable on sunny days, but this does not have to be so, according to Whitehead. “If you use a high-brightness LCD and optical bonding, if your body shades half of the display, you cannot actually see where your shadow ends.”
How bright a display has to be to get its message across in direct sunlight is somewhere above 1,000cd/m2.
This has to come from the backlight, where LEDs are replacing traditional compact fluorescent tubes.
“The biggest problem is heat with sunlight-readable displays. LEDs reduce by 50% the backlight heat,” said Whitehead. “Also sunlight viewable displays are used in sunlight, and infra-red and ultra-violet are absorbed through front which also adds to heat, so we incorporate UV and IR filters.”
The problem with too much heat, apart from reduced reliability, is that when many LCDs get hot, and around 55°C according to Whitehead, they go black. The image returns as they cool, although permanent damage is caused if this process is repeated enough times. “If you want wider temperature, you have to change the structure and use a more expensive substrate, which really does push the price up,” he said.
Optical bonding can play a part in heat reduction.
“Optical bonding improves contrast so you can use less brightness,” said Whitehead. “And it also conducts heat always from LCD to the front as the air is normally less than 55°C, except in places like Saudi.”
Display Solution’s sunlight-readable range comes from Litemax, which offers amorphous and polysilicon LCDs, in both TN and PVA technology, all with LED backlighting.
There are standard products – for example a 32in 1,000cd/m2 version which dissipates 50W, and a 52in 2,000cd/m2 type dissipating 110W – both around half the power of equivalents lit by older backlight technology, claimed Whitehead.
As a backlighting note, the 2,000cd/m2 product uses an array of LEDs behind the panel, but 1,000cd/m2 can be, and is in this case, produced by edge-lighting through a light guide which is generally a cheaper and thinner way of doing things.
Litemax will also turn around custom sizes, said Whitehead: “A 46in. full HD LCD has 1920×1080 resolution. Litemax can cut this down to a thin letterbox. A popular size is 1920×512 which is used for digital signage where there are low ceilings like busses, trains and malls, or in portrait mode for menus and advertising.”
For ultimate sunlight readability, “we would take a standard display, replace polarisers and reflector, add high-brightness backlight, and optically bond to the front surface or touch screen”, said Whitehead. “The proposals we offer can include any or all three methods. All three together produces the very best performance, at the highest cost.”