Mobile display resolution: how far should you go?
And if so, how much higher can resolution go before we do reach the limits of human vision? Sharp Devices Europe conducted a controlled study to find out.
Throwing down the gauntlet on ultra-resolution
The first mobile phone that was claimed to out-resolve the human eye appeared as early as 2010. Just how much resolution is required to achieve this feat? According to some, the magic number is exactly 300 pixels per inch.
As a ground-breaking new feature, this claim was undoubtedly valuable from a marketing perspective. But was it true? Is it possible to see pixels on a screen with more than 300 PPI? And what about more recent mobile displays with even higher pixel densities?
Try to see it my way
At the time, the jury was split. Some (most notably Wired Magazine) reported they could see pixels even at resolutions slightly greater than 300 PPI. Display expert Raymond Soneira of DisplayMate Technologies weighed in, stating “it will be a lot better … if everyone sticks with the true objective values instead of values exaggerated by marketing departments.”
Yet others such as astronomer Phil Plait came to the defence of the new ultra-high-resolution display technology, claiming 300 PPI “is safely higher than can be resolved by the normal eye, but lower than what can be resolved by someone with perfect vision.”
A matter of perspective
All this talk of pixel density and the limits of human vision might lead one to believe these are absolute limits. Quite the contrary – the impact of resolution is of course relative to viewing distance. The claimed 300 PPI limit, for instance, applies only at a distance of 30 cm.
With the eye’s ability to focus from very near to extremely far, one thing does remain constant, however. Research conducted for television screens has revealed that the maximum resolving power of human vision expressed as angular resolution is the same for televisions as it is for mobile displays. The pixel density needed to resolve detail close to this upper limit, however, varies significantly between applications.
Measuring resolution as the interior angle created by two lines extended from the eye to the outer edges of an object yields a theoretical maximum for a perfect eye of 0.6 arcminutes. At 30cm, this translates to nearly 500 PPI. But most people’s vision is far from perfect, making 300 PPI a realistic number for 20/20 vision at 30cm.
A perfect display for a perfect eye?
To better understand why it is difficult to define the “perfect” display, we must first consider the human eye. Certainly the fovea or central area of the retina, where the most densely packed cones in the eye relay information to the brain, represents a hard limit on the amount of detail humans can perceive. But only considering the retina leaves out a number of other important considerations. Other structures on the eye’s visual axis including the cornea, lens and the vitreous and aqueous humours also affect how much detail we perceive.
Visual acuity also varies greatly from one person to the next. Luckily, there is consensus as to what constitutes healthy vision, known as 20/20 eyesight. 20/20 represents the ability to read the eighth line of a Snellen chart from a distance of 20 feet (6 meters). This corresponds to the ability to resolve details at a level of approximately one arcminute. 300 PPI at 30 cm also comes out to roughly one arcminute of angular resolution, making it a valid yardstick. But many people possess better than 20/20 eyesight.
And there are more ways to measure visual acuity than simple resolving power. A person’s ability to detect differences in contrast (contrast sensitivity) or the alignment of two lines (vernier acuity) can also be measured, for instance.
The common method of measuring human vision, however, defines acuity as the ability to resolve discrete entities as separate objects. This is a significant point because it turns out that the human eye is able to detect differences represented by even finer structures. Tests of vernier acuity (distinguishing the relative alignment of parallel lines) for instance show that humans can distinguish details five to ten times smaller than standard resolution measurements predict.
So even if some individuals are able to discern individual pixels in the latest generation of ultra-high-resolution mobile displays, these screens are indeed already quite good. The question is, how much better can they get?
How to know the unknown
To test the upper limits of our ability to notice resolution differences, we would ideally use actual displays of varying resolutions. But there was a problem. Smartphone displays with pixel densities as high as the 1000 PPI we examined in our study are not available.
So we simulated displays of varying resolutions. We used a photographic process to create backlit transparencies of six different images. These images were downsampled to simulate displays with resolutions of 960×540 (254 PPI), 1280×720 (339 PPI), 1920×1080 (508 PPI), and 3840×2160 (1016 PPI). Each set of six images was produced both as a simulation of aliased and anti-aliased digital images by using Gaussian blur.
Test Images used in experiment, clockwise from top left: Vernier Test Pattern, Text, Web^a , Family Photo^b , Video Game (CG)^c , Kitchen Photo^d . a © 2012 BBC, b © 2012 Applied Photography, c GT5© 2010 Sony Computer Entertainment, d ©2012 Kitchen Design Perth
For all but the highest resolution images, print quality very closely mirrored the expected quality of the corresponding digital display. For the purposes of our study, loss of resolution due to optical imperfections and film grain was not, however, statistically significant.
Step right up and see for yourself
We selected 50 subjects at random to participate in our study. Their average vision score was 20/16 based on a near-view Snellen eye test.
In our double blind testing, we asked participants to evaluate the resolution of each of the six image types (vernier pattern, text, website, family photo, video game, and kitchen photo) in a controlled environment. The distance from the simulated displays was 300 mm and was maintained by using a head mount. The subjects were given a time limit of three minutes to rate the images.
The results clearly showed that higher resolution images resulted in significantly better ratings by our subjects for all images and types, both aliased and anti-aliased. Since improvements in ratings were observed for the 1016-PPI images over the corresponding 508-PPI images, we conclude that the perceptible advantage of higher resolution smartphone displays has not been exhausted, even at 1000 PPI.
Interview results: Ranked data (8 – worst, 1, best) showing how subjects perceived the benefit of increased resolution across different image types for a) aliased images, b) anti-aliased images.
The new frontier: 1080p and beyond
Certainly the difference in rankings between the two highest resolutions was not as great as for the lower pixel densities. But the ramifications for future screen development are clear: many consumers can and do perceive differences in resolution far beyond even the latest 440 PPI, 1080p smartphone displays.
To give device manufacturers a visible edge, Sharp has focused considerable resources on pioneering screen technology. Our new IGZO displays make higher resolutions possible while providing previously unheard-of efficiency and brilliance. These advances put longer battery life and more immersive experiences within reach for manufacturers.
So no matter how poor your eyesight, one thing is clear: we have yet to see the end of display technology advances in the mobile sector. The enhanced resolution of the next generation of display technology will not be lost on consumers, no matter which way you look at it.
About the author
Lee Spencer is Senior Project Manager bei at Sharp Devices Europe and a Lead Engineer for LCD solutions – www.sharpsde.com
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