Give a display geek an Integrated Hemisphere and he’ll think that all the world’s a sunny day. And that’s just what happened when Raymond Soneira of DisplayMate locked himself in his lab with a stack of smartphones and tablets. He wanted to find out just how well their screens performed under a wide range of lighting conditions, from pitch black darkness to full sunshine.
If you’re not familiar with an Integrating Hemisphere, you’re not alone. It is a scientific apparatus that can be used to provide even illumination from all directions. In this case, a 6500 Kelvin light source was used to produce light in eight levels of brightness, ranging from absolute darkness (a black cat in a coal cellar at midnight) to 40,000 lux (equivalent to sunshine from a window falling on a nearby desk). Direct sunlight outdoors typically ranges from 100,000 to 120,000 lux.
Why would anyone do this? Soneira is a display-expert’s expert, and he has a long history of rigorous independent testing of displays. His company provides software tools and other products designed to help display manufacturers get the most out of their devices. And he also spends a lot of time delving into the deepest details of how these devices actually perform in the real world.
So it should come as no surprise that he would look into how well mobile devices hold up under bright lighting conditions. He recently published his test results for four smartphones and four tablets. The smartphones tested were the Apple iPhone 4, HTC Desire, Motorola Droid X, and Samsung Galaxy S. For the tablets, he tested the Apple iPad 2, Amazon Kindle Fire, Motorola Xoom, and Samsung Galaxy Tab 10.1. And he found out significant differences in their performance.
What a Washout!
Soneira published photos of all the screens at each of the eight different lighting levels, but here are examples from the smartphone shoot-out that illustrate some of the results. This first image is of all four screens at 0 lux; in other words, they are in pitch black.
[Image courtesy of DisplayMate]
It should be no surprise that all the screens look great in the dark. The only light is that emitted by the display itself. The graduated shades of color and gray in the test image show that none of the displays differentiate the last three or four shades, but show the rest well. In addition, the black level is truly black; the border of the image is pure black to provide a comparison.
As you crank up the ambient lighting, however, the image quality starts to deteriorate. By the time you reach 20,000 lux – the level of sunshine through a window landing on a desk – the images look a lot different.
[Image courtesy of DisplayMate]
The pure black border is still present in these illustrations to provide a comparison, and the first thing you notice is that the darkest shade show on any of the four screens would be characterized as “light gray” at best. It no longer resembles black.
You can also see that there are differences in the images. The HTC and Motorola models clearly show fewer shades of color than the Apple and Samsung models. This is a visual demonstration of what Soneira calls the “DisplayMate Contrast Rating for High Ambient Light.” How this differs from the industry standard “contrast ratio” measurement requires some explanation.
Compare and Contrast
The concept of contrast ratio is simple: Just take the brightest image a display can create and divide it by the darkest image it can create. The industry standard method is to conduct these measurements on full-screen images – all white or all black – in absolute darkness. This sounds useful until you start looking into it more closely.
First and foremost, you rarely use your display in a pitch black setting, so a number generated under those conditions may not be a good predictor of what you actually experience. Next, you probably don’t look at an all-white or all-black screen when you’re trying to do anything useful. Those images give the manufacturer the highest contrast ratio measurement, but again it is probably not a good predictor of what you’ll see.
When you have light and dark areas mixed on a screen, such as the familiar black text on a white background, those pesky photons don’t always go where they are supposed to. As a result, some of the white light from the background “leaks” into the black parts of the image, and they won’t be as “black” as it would be if the whole screen were black.
And finally, displays are getting better and better at producing little light when set to “black.” If they emit no light at all – which is something OLED displays should be able to do – then the contrast ratio will be infinite, no matter how much or how little light it produces when set to “white.” That’s because the contrast ratio divides by the amount of light output for “black” and that will be zero if it does not emit any light. So what good is a specification that says a display has an infinite contrast ratio if that does not hold true under the harsh light of day (so to speak)?
Soneira’s approach is to take what many might describe as “real world conditions.” He defines the DisplayMate Contrast Rating for High Ambient Light as “the ratio of Peak Brightness (Luminance in cd/m2) to the Average Screen Reflectance (in percent).” In other words, he measures how much ambient light is reflected by the screen, separate from the image being displayed. This is compared with the brightest image that the screen can display. At some point, the reflected light overpowers the displayed image. The more light that is reflected by the screen, and the less light that it can produce in the first place, the sooner the image washes out and become unreadable.
In the case of these four smartphones, the HTC Desire has a measured peak brightness of 234 cd/m2 and a screen reflectance of 15.5%. This results in a DisplayMate Contrast Rating for High Ambient Light of 15. Compare that with the results for the iPhone: peak brightness of 541 cd/m2 and 7.0% reflectivity for a rating of 77. What is behind these differences?
On the peak brightness side of the equation, some panels can put out more light than others. For LCD panels, this is largely determined by how bright the backlight is. Manufacturers can design brighter backlights, but this can negatively impact other features. For example, brighter LEDs can create heat dissipation problems. Brighter backlights also require more power, which can mean a larger and heavier battery, or shorter run times.
The reflectivity part of the equation is also complex. Smartphones and tablets don’t just have a simple sheet of glass on the top of their display. In most cases, this is a complex sandwich of display panel, touch panel, and protective layers. Each layer reflects some of the ambient light, and adding an aftermarket screen protector can reflect even more. Just as with the backlight brightness, engineers can take steps to reduce reflectivity, but these can have a negative impact on other display characteristics, such as its brightness.
Can You See Me Now?
So what’s the take-away from all this? First and foremost, don’t rely on specifications to predict how a mobile display will perform when you take it out into the daylight.
Also, give a tip of your virtual cap to the engineers who design these devices in the first place. They have to perform a difficult balancing act where any decision to improve one aspect of performance is likely to have a negative impact on one or more areas in return. Sure, we’d all like our phones and tablets to be even better, but consider that they are pretty incredible already.