Sunlight Readable Displays

Brightness alone does not decide it. What decides it is ambient contrast ratio: the light your display emits divided by the light it reflects back at the viewer. A 1,000-nit panel with a glossy untreated surface can be less readable outdoors than a 600-nit panel that reflects less.

As a working guide for a transmissive TFT:

• Indoor office, roughly 300 to 500 lux ambient: 250 to 350 nits is normal.
• Shaded outdoor or a bright window: 600 to 800 nits.
• Direct sunlight, which runs 10,000 to over 100,000 lux: 1,000 nits and up, and only with reflection control as well.

Past about 1,000 nits you are usually paying more in power, heat and backlight lifetime than you gain in readability. Reducing reflection is the cheaper lever above that point.

• Transmissive is the standard LCD. All the light comes from the backlight behind it. It looks best indoors and washes out as ambient light rises, so outdoor versions need a brighter backlight and more power.
• Reflective has no backlight at all. It uses ambient light as its light source, so it gets better as the sun gets brighter and is unreadable in the dark without a front light. ePaper is the most familiar reflective technology.
• Transflective does both. A partially reflective layer behind the LCD passes backlight through indoors and reflects ambient light outdoors. It is the usual answer for a device that has to work in a dark cab at night and full sun at noon.

The trade-off with transflective is that some of the backlight is lost to the reflector, so indoor brightness is lower than a transmissive panel of the same backlight power.

Read more about the differences in our Blog.

Yes, and it is usually the most effective change after brightness.

An unbonded display has an air gap between the cover glass or touch panel and the LCD. Every air-to-glass boundary reflects light back at the viewer, and in sunlight that reflected light is what washes the image out. Optical bonding fills the gap with an optically matched adhesive, removing those internal reflection surfaces.

It also stiffens the stack, which improves impact resistance, and it stops condensation forming in the gap in equipment that goes through temperature cycles. The cost is that it is not reworkable: a bonded assembly that fails is scrapped, not repaired.

Many of our displays are optically bonded and are noted on their product pages. If in doubt, please reach out to our U.S. Based technical support for a quick answer. Or to see about customizations that are available.

Decide on power budget and on what the display has to show.

Choose high-brightness transmissive when the content is full colour, changes often, or is video, and when you have the power and thermal headroom for a 1,000-nit-plus backlight. This is the usual answer for signage, kiosks and vehicle dashboards on vehicle power.

Choose transflective when the device is battery powered, when the content is mostly static, or when it has to be readable from darkness to direct sun without a large power budget. This is the usual answer for handheld instruments, marine and field equipment.

Two practical notes. A high-brightness backlight in an enclosed outdoor housing is a thermal problem before it is an optical one. And a transflective panel viewed in a dark room looks dimmer than a customer expects unless you set that expectation.

Can OLED or AMOLED be used in direct sunlight?

OLED has excellent contrast in a dark or indoor environment because its black pixels are genuinely off. In direct sunlight that advantage mostly disappears, because what you are fighting is reflection off the front surface rather than the black level behind it.

You can drive an OLED hard enough to compete, but OLED brightness and lifetime trade against each other directly: running a panel at high luminance to overcome sunlight shortens its usable life and accelerates differential ageing on static content. For a device that sits in the sun all day for years, a transflective LCD or a high-brightness transmissive TFT will normally outlast it.

Where OLED does win outdoors is wide viewing angle and fast response at low temperature.

Crystalfontz stocks passive-matrix OLED, character and graphic, and transparent OLED and AMOLED.

They do not have to fight the sun, so their colour behaviour is more stable than a backlit panel’s. A reflective display’s colour comes from filtering the ambient light that falls on it, so the perceived colour tracks the colour temperature of the environment rather than diverging from it. In practice it looks like printed paper: it shifts with the light the way everything else in the scene shifts.

The limits are saturation and gamut. A reflective colour filter discards a large share of the incoming light, so colours are muted compared to an emissive display, and colour ePaper in particular has a narrow gamut and a slow update.