Key Features for Choosing a Sunlight Readable Display

A sunlight readable display is a screen that stays legible in direct sunlight, typically by combining a high-brightness backlight of 1,000 nits (cd/m²) or more with ways of cutting reflected light, such as optical bonding and anti-reflective glass. Standard industrial screens at 250 to 400 nits wash out outdoors, because the sunlight reflected off the glass is brighter than the image behind it.

When you are outdoors on a bright day it can be difficult, if not impossible, to read the display or HMI you are trying to operate. There are two reasons: the screen is not bright enough, and the outer glass reflects the sun back at you. Sunlight readable displays, also called high bright or high brightness displays, deal with both. This guide explains how brightness is measured, how much you need for each environment, why contrast matters more than nits, the technologies that make a display readable, and how heat, power, dimming and touch affect the choice.

Display brightness is the luminance of the screen, the amount of light each square meter of screen sends towards the viewer. It is measured in candela per square meter (cd/m²), the SI measure of luminance, and usually quoted in nits, a common name for the same quantity:

1 nit = 1 cd/m²

The candela itself is one of the seven SI base measures, covering luminous intensity. It descends from the old “candlepower” measured against a standard candle, but it is now defined precisely in physical terms, so cd/m² and nits are exact, interchangeable measures. You will see both on our datasheets for high bright sunlight readable displays. Our explainer on what nits are covers how the figures are measured.

Datasheet brightness is normally the panel’s typical luminance at the centre of the screen, at full white, with the backlight at maximum and without a touch screen. A touch sensor or protective glass in front of the panel reduces what reaches the viewer, often by 10 to 20% for a resistive touch screen, so compare like with like.

How an LCD makes an image

LCD technology is widely used in commercial and industrial displays because it offers a good balance of brightness, resolution, power and price. Traditionally lit by fluorescent tubes, most LCDs now use edge-lit or direct LED backlights. The layers behind an LCD, before protective glass or a touch screen is added, are shown below.

• High brightness options up to about 6,000 nits for signage
• Resolutions up to 4K and beyond
• A good balance of brightness and contrast
• Low power consumption at standard brightness

For the difference between LCD and LED in display specifications, see LCD vs LED.

The brightness you need depends on how much light falls on the screen and whether the sun can reach it directly. The figures below are typical guidance for industrial HMIs and monitors, not guarantees; screen angle, shading and reflections change the answer for every site.

Higher is not always better. A brighter backlight uses more power, makes more heat and can dazzle at night, so the right specification combines enough brightness with the lowest practical reflection. Choosing the screen is one part of choosing an industrial HMI; for outdoor positions the sealing matters as much, and our IP rating chart explains front IP65 and IP66 ratings. For a quick explanation of the threshold, see what makes a display sunlight readable.

Not quite. High brightness describes the backlight: a high brightness display or monitor is one rated well above the 250 to 350 nits of an office screen, typically 700 to 1,000 nits or more. Sunlight readable describes the result: the image stays legible in direct sun. Brightness is the biggest part of that, but a very bright panel behind a reflective cover glass can still wash out, while a moderately bright panel with optical bonding and an anti-reflective front may read well.

Datasheets and suppliers use the terms loosely, and “daylight readable” usually means the same as sunlight readable. When you compare high brightness and sunlight readable models, look past the label to the nits figure, the surface reflection and how the front glass is bonded.

For an industrial screen in direct sun, treat high brightness as a starting point rather than a guarantee: specify the nits you need for the site, then check that the front is bonded and treated to keep reflections down.

In bright light the eye does not judge a screen by its brightness, but by how much the image stands out from the light reflected off its surface. That is the ambient contrast ratio:

Ambient contrast ratio = (L_white + L_reflected) ÷ (L_black + L_reflected)

L_white and L_black are the luminance of the screen showing white and black, and L_reflected is the luminance of the light the screen reflects. Indoors, L_reflected is small and the native contrast ratio of perhaps 1,000:1 is what you see. In sunlight, L_reflected can be larger than L_white, and contrast collapses towards 1:1.

A worked example

Take a screen in bright daylight with 50,000 lux falling on it, and treat the reflection as diffuse, so reflected luminance is roughly reflectance × illuminance ÷ π. The figures are simplified but show the pattern.

Cutting reflections from about 8% to about 1% does more for readability than raising the backlight from 1,000 to 1,500 nits. A ratio of roughly 3:1 to 5:1 is often quoted as the minimum for legible text, which is why sunlight readable specifications pair brightness with optical bonding and an anti-reflective cover glass.

There are several ways to make a screen more viewable and reliable in sunlight. The primary one is a backlight bright enough to overcome the surroundings. Others reduce reflections or use the ambient light itself: optical bonding, transflective panels, direct backlighting with local dimming, and anti-reflective or anti-glare treatments. Most sunlight readable displays combine two or three.

Optical bonding fills the gap between the cover glass or touch screen and the LCD panel with a clear adhesive, making a solid laminate with no air pockets. In an air gap display, light reflects at each surface where it crosses between air and glass, about 4% each time, and those internal reflections come back to the viewer as glare. Bonding removes two of them, so more of the light that reaches the viewer is the image. It helps a screen look brighter and more contrasted without changing the backlight, and it also stops internal condensation.

The adhesive is either a liquid resin or a film of optically clear adhesive. For the front surface, a coating does the rest: the difference between anti-glare and anti-reflective explains which suits direct sun.

Transflective enhanced LCDs

A transflective, meaning transmissive-reflective, panel adds a partly reflective layer behind the liquid crystal. Ambient light passes through the image, reflects off that layer and comes back out, so the brighter the surroundings, the brighter the picture. Indoors the backlight does the work as normal. It lets a display stay readable in strong light with a lower-power backlight and less heat, which suits small screens, battery-powered equipment and open frame displays built into kiosks, though colors are usually less vivid than on a high-brightness transmissive panel.

Local dimming to reduce heat and save energy

Local dimming uses a direct LED backlight split into zones that can be dimmed independently. Zones behind dark areas of the image turn down while bright areas stay at full output, which raises dynamic contrast, cuts average power and reduces the heat a very bright display generates. It is most common on large-format screens and digital signage, and on large format industrial monitors.

LCDs are sensitive to heat and ultraviolet light. Over time, UV radiation from the sun can degrade the panel and its polarisers until image quality becomes unacceptable; UV and infrared filters and films reduce the damage. Some outdoor monitors offer a UV film as an option for solar radiation protection.

Heat has a faster effect. Liquid crystal only works below its clearing point, the nematic-isotropic transition temperature (TNI). Above it, the crystals lose their alignment and the affected area turns black until the panel cools. Standard panels typically have a TNI of about 70 to 80 °C, which a dark screen in full sun or a display in a sealed, unventilated enclosure can reach. High-TNI panels raise the clearing point to around 100 to 110 °C, so the image survives the surface temperatures of window displays, outdoor kiosks and vehicle dashboards.

Thermal design matters as much as the panel. A brighter backlight adds heat inside the enclosure, so fanless sunlight readable displays rely on conduction to the housing, and a heater may be needed at the other end of the range: some outdoor monitors extend their operating range down to −25 °C with an optional heater. Our guide to outdoor enclosures covers sun loading and ventilation.

A screen bright enough for midday sun is uncomfortable, and on a ship’s bridge or in a vehicle cab can be dangerous, after dark. Most industrial displays and panel PCs with sunlight readable LCDs offer manual or automatic dimming. An ambient light sensor on the front bezel measures the incoming light and adjusts the backlight to match, so the display is bright in the day and comfortable at night without the operator touching it.

Look at the dimming range as well as the peak brightness.

Bridge, cockpit and cab screens all need low-level dimming that stays stable and flicker-free. Marine bridge equipment must dim to very low levels for night watch, and military vehicle and cockpit displays may need a separate NVIS mode compatible with night vision goggles, as covered under MIL-STD-3009. Our marine monitors and military monitors list the dimming and NVIS options for each model.

Touch screens in direct sunlight

Touch adds layers in front of the panel, and each layer can add reflection. Projected capacitive (PCAP) touch sits behind a single cover glass and bonds well, which keeps reflections low; many outdoor PCAP screens add glove and water-rejection modes so rain and gloved hands do not cause false touches. Resistive touch works with any glove or stylus but places a flexible film over the image, which reflects more light and reduces brightness.

For HMIs in sunlight, PCAP touch monitors with bonded glass are the usual starting point. Our guide to touch technologies and the resistive vs capacitive comparison cover the trade-offs.

Polarised sunglasses

LCDs emit polarised light, so a screen can appear dark when viewed through polarised sunglasses at certain angles. For drivers, pilots and outdoor operators, ask whether the panel’s polariser orientation or an added retarder film keeps the screen visible in both portrait and landscape.

Power draw

Backlight power rises roughly with brightness, so a 1,000 nit panel can draw two to three times the backlight power of a 300 nit version of the same size. Wide viewing angles matter too for screens read from the side; an IPS panel keeps colors and contrast steadier off-axis. On vehicles and solar-powered sites, check the display’s maximum power draw against the supply, and use dimming to cut average consumption.

High MTBF and 16/7 or 24/7 operation

Most of our industrial and commercial sunlight readable monitors are designed for at least 16/7 operation, 16 hours a day, seven days a week, which covers long shifts. Some models are rated for 24/7 continuous use, which suits signage and retail screens that run around the clock. Backlight lifetime, often quoted as hours to half brightness, and the panel’s MTBF are worth comparing for screens that never switch off.