The term "retina display" is tossed around with increased frequency and decreased meaning. Is there really a strict definition anymore? Screen mega-expert Ray Soneira of DisplayMate points out that the biggest monitor in your house has been retina all along.
There is a widespread misunderstanding of exactly what constitutes an Apple "Retina Display" - or any display that has an equivalent visual sharpness. While the first Retina Display on the iPhone 4 has 326 Pixels Per Inch PPI, all Retina Displays don't need 326 PPI because the eye's resolution is not based on linear Pixels Per Inch, but rather on angular resolution, so visual acuity depends on the viewing distance. That is why you take a standard vision test at 20 feet and aren't allowed to walk right up to the chart and read the smallest letters on the bottom line - visual acuity depends on the viewing distance.
Apple's Retina Display definition is equivalent to standard 20/20 Vision - your eyes won't be able to resolve the individual pixels on the display provided you don't look at the screen from too close a viewing distance. If you have 20/20 Vision and view the iPhone 4 from 10.5 inches or more its display will appear "perfectly" sharp to your eyes - meaning the display appears at the visual acuity limit of your eyes. If the display were any sharper with a higher PPI or higher pixel resolution your eyes wouldn't be able to see the difference. That is what "Retina Display" means... Let's see what PPIs are needed for other displays in order to qualify as a Retina Display:
iPhone 4, new iPad 3, and MacBook Pro Retina Displays
The new iPad 3 and MacBook Pro have much lower PPIs than the iPhone 4 but Apple correctly markets them as Retina Displays because they are typically held further away from the eyes and therefore still appear "perfectly" sharp at their proper viewing distance. Below we have calculated the viewing distances needed to qualify as a 20/20 Vision Retina Display (defined as 1 arc-minute visual acuity). For a discussion on the difference between the Acuity of the Retina and 20/20 Vision Acuity see this article.
The iPhone 4 with 326 PPI is a Retina Display when viewed from 10.5 inches or more
The new iPad 3 with 264 PPI is a Retina Display when viewed from 13.0 inches or more
The MacBook Pro with 220 PPI is a Retina Display when viewed from 15.6 inches or more
On the other hand, the average viewing distance for living room HDTVs in America is around 7 to 10 feet, depending on the screen size. So to appear "perfectly" sharp with 20/20 Vision like the iPhone 4 Retina Display, HDTVs only need a proportionally much lower PPI in order to achieve "Retina Display" status and have the HDTV appear "perfectly" sharp and at the visual acuity limit of your eyes.
Existing 40 inch 1920x1080 HDTV is a "Retina Display" when viewed from 5.2 feet or more
Existing 50 inch 1920x1080 HDTV is a "Retina Display" when viewed from 6.5 feet or more
Existing 60 inch 1920x1080 HDTV is a "Retina Display" when viewed from 7.8 feet or more
Since the typical HDTV viewing distances are larger than the minimum distances listed above, the HDTVs appear "perfectly" sharp and at the visual acuity limit of your eyes. At the viewing distances listed above the pixels on a 1920x1080 HDTV will not be visible by a person with 20/20 Vision in exactly the same way as the Retina Displays on the iPhone 4, new iPad 3, and MacBook Pro at their viewing distances. So existing 1920x1080 HDTVs are "Retina Displays" in exactly the same way as the existing Apple Retina Display products. If the HDTVs had a higher PPI or a higher pixel resolution your eyes wouldn't be able to see the difference at their proper viewing distances. So existing 1920x1080 HDTVs are already equivalent to what Apple calls a "Retina Display." When Apple launches its own Apple Television it will almost certainly have a resolution of 1920x1080 and it will be a True Retina Display [for humans with 20/20 Vision at standard HDTV viewing distances].
4K HDTVs and Projectors
Some manufacturers are introducing HDTVs with resolutions that are at least double the existing standard 1920x1080 resolution - 3840x2160 or more. They are often called 4K displays. Some reviewers have already claimed dramatically improved picture quality and sharpness - but that is impossible unless they have significantly better than 20/20 Vision or are watching from an absurdly close viewing distance. However, the higher resolutions are important for Digital Cinematography and cinema projectors that have large 10 foot or more screens. But note that there isn't any consumer content available yet for resolutions higher than 1920x1080, so save your money and wait for the Apple Television with a true 1920x1080 Retina Display...