How much higher can mobile display resolution go?

A revolutionary advance in display technology has been achieved when the first smartphone with a pixel density of over 300 PPI to hit the market. Now with 1080p displays as small as 4.7 inches readily available, the question is: can we really see the difference?

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 (6m). 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.