20/20 vision. Perfect vision. The best that the human eye can resolve. Therefore there’s no benefit in creating a display system that has a higher resolution than that of the 20/20 line of an eye chart. Right?
20/20 vision means that at 20′ (6m) a viewer can identify characters constructed of lines 1.75mm wide and 1.75mm apart. It’s the ability to clearly see this gap and discern the letters of the eye chart that tests a person’s visual acuity. The line thickness of the characters on an eye chart are 1 arc minute (one 60th of a degree) wide when viewed from 20′ away.
Therefore on that premise, a high resolution display in an Immersive theater needs pixels no larger than 1 arc minute wide. That equates to around 10, 800 pixels for a dome (180 degrees x 60 pixels per degree), when viewed from the centre of the theater (also assuming the viewer is at the spring-line of the dome), and 4, 320 pixels across in a giant flat screen theatre (DIGSS1.1)1 when viewed from the central seat row.
Well, actually that’s wrong. We can “see’ objects far smaller than lines that are 1 arc minute wide. Look around you now. How much of what you see is made up of regular shaped black and white lines equally spaced? Not much. The World is not an eye chart. We can see details far smaller than 1 arc minute across. The leaves on a tree, the fur of an animal. The carpet. A hair on the page of a book. Also consider that we do not need to accurately resolve an object and recognise it to see that it is there, that it exists. In fact we typically enjoy the intrigue of getting closer to an object to discover what it is. Fully and clearly resolving an object is quite different to seeing that it’s there. If 1 arc minute was all that we could see then we would never see the hair on our arms!
It’s a misnomer that 20/20 vision equates to what is referred to as “eye limiting resolution’. It’s easy to understand why these terms get mixed up and interchanged. How we see is a complicated subject and one that this short article cannot fully describe. Part of the complication is that the resolution of the human eye is not an absolute. How much we can see depends upon the viewing conditions. The amount of light and the contrast of the image play an important part. Leaving these variables to one side, it is generally agreed that under optimum viewing conditions the human eye can resolve detail as small as 0.59 arc minutes per line pair2 (pair of pixels) which equates to a pixel size of 0.3 arc minutes. This therefore is the generally accepted figure for “eye limiting resolution’, more than three times the resolution of 20/20 vision.
Resolution is an important metric for the flight simulation industry for pilot training applications. A level-D simulator (suitable for commercial aircraft) stipulates a display resolution of only 1.5 arc minute sized pixels (3 arc minutes per line pair). Significantly this is because pilots need to be able to resolve the runway lines which are 4 feet apart at 6, 876 feet away as they come into land. Fast jet simulators on the other hand are targeting a much more demanding specification of 0.5 arc minutes per pixel. These fast jet simulators are designed as close as possible to eye limiting resolution so that the pilots can identify fast moving objects in the sky.
If we are ever to experience eye limiting resolution in immersive Theaters we would need to capture and display 0.3 arc minutes of resolution. This equates to 36, 000 pixels across the centreline of a dome and 14, 400 pixels across a 70′ giant flat screen when viewed from the central row of seats.
Furthermore, consider that in a dome every audience member bar one is closer to some part of the screen than the person in the center, and that the front half of a giant flat screen audience are significantly closer to the screen than the center row. These audience members would need even more pixels on the screen to enjoy eye limiting resolution (in fact up to 22, 600 pixels wide from the front row of a 70′ wide flat screen).
As of 2014 the current state of the art for domes offers around 6, 600 pixel resolution (optimistically referred to as “8k’) and for flat screens 4, 096 pixels. These specifications are driven by the “4k’ projection systems now in the market (domes use an array of five or six 4k projectors to cover the dome surface). Once “8k’ projectors come on-line at comparable prices and at the brightness levels required (probably 3 to 5 years away) the capability of flat screens will quickly jump to 8, 000 pixels and domes to around 12, 000 pixels (assuming that the capture resolution and production workflow can keep up). This may appear unrealistic and out of reach for now, but for those of us who can remember marvelling at 1280 x 1024 resolution at the turn of the century will know that we soon recalibrate to expect higher resolution once we’ve experienced it.
Thus far this only considers spatial resolution (how many pixels). Of increasing importance is temporal resolution (how often), otherwise known as frame rate. Frame rates or fps (frames per second) have been quite stable for many years. 24fps and 30fps have been with us for many years. HFR (high frame rate) 48fps and 60fps are with us now and could quickly be overtaken by frame rates as high as 120fps or even more. High frame rates smooth out the strobing or judder that one can experience, particularly on large field of view screens. With 3D systems HFR can significantly help with edge definition, the means by which we determine the depth of an image using current 3D technologies. HFR can help us see the resolution that is there in the image that is otherwise blurred-out during panning and fast action shots.
The combination of higher frame rates and resolutions approaching eye limiting resolution will help deliver increasingly realistic immersive experiences.
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