Retina Display is a brand name used by Apple for liquid crystal displays which they claim have a high enough pixel density
that the human eye is unable to notice pixelation at a typical viewing
distance. The term is used for several Apple products, including the iPhone, iPod Touch, iPad, and MacBook Pro. As the typical viewing distance would be different depending on each device's usage, the pixels per inch claimed as retina
quality can be different for the smallest devices (326, iPhone and iPod
Touch): greater than the mid-sized devices (264, iPad) and greater than
the larger devices (220, MacBook Pro).
The displays are manufactured by different suppliers. Currently, the iPad's display comes from the same suppliers Samsung use while the Macbook Pro displays are mostly made by LG's suppliers, along with the iPhone and iPod Touch displays.[citation needed] Apple has applied to register the term "Retina" as a trademark in regard to computers and mobile devices with the United States Patent and Trademark Office, Canadian Intellectual Property Office, and in Jamaica. On November 27, 2012 the US Patent and Trademark office approved Apple's application and Retina® is now a registered trademark.
source - http://en.wikipedia.org/wiki/Retina_Display
The displays are manufactured by different suppliers. Currently, the iPad's display comes from the same suppliers Samsung use while the Macbook Pro displays are mostly made by LG's suppliers, along with the iPhone and iPod Touch displays.[citation needed] Apple has applied to register the term "Retina" as a trademark in regard to computers and mobile devices with the United States Patent and Trademark Office, Canadian Intellectual Property Office, and in Jamaica. On November 27, 2012 the US Patent and Trademark office approved Apple's application and Retina® is now a registered trademark.
Retina Display trigonometrical pixel density
Retina Display's real unit of measure is PPD: Pixels Per Degree. When introducing the iPhone 4, Steve Jobs said the magic number for a retina display is about 300 PPI for a device held 10 to 12 inches from the eye, which correlates to a PPD (at 10 inches) of 53. If a display's pixel density is more than 53 PPD (57 PPD for the iPhone 4 display) viewed from its optimal distance, that is a Retina Display. 53 PPD pixel density means that there are 53 pixels visible in one degree view angle. For example, 53 PPD means that when viewed from the expected distance, if an imaginary triangle is drawn between two chosen pixels and the eye of the viewer, with a one-degree angle formed at the vertex of the triangle that is at the viewer's eye, then there will be 53 pixels along the opposite side of the triangle (between the other two vertices of the triangle, which are at the chosen pixels). Any display's viewing quality (from phone displays to huge projectors) can be described with this size-independent universal parameter. Note that the PPD parameter is not an intrinsic parameter of the display itself, unlike absolute pixel resolution (e.g. 1024 x 800 pixels) or relative pixel density (e.g. 72 PPI), but is dependent on the distance between the display and the eye of the person (or lens of the device) viewing the display; moving the eye closer to the display reduces the PPD, and moving away from it increases the PPD in proportion to the distance. It can be calculated by multiplying the distance to the screen times the PPI resolution of the screen times π divided by 180 degrees.source - http://en.wikipedia.org/wiki/Retina_Display
