Computer Displays
The PPI of a computer display is related to the size of the display in inches and the total number of pixels in the horizontal and vertical directions. This measurement is often referred to as dots per inch, though that measurement more accurately refers to the resolution of a computer printer.
For example, a 15 inch (38 cm) display whose dimensions work out to 12 inches (30.48 cm) wide by 9 inches (22.86 cm) high, capable of a maximum 1024×768 (or XGA) pixel resolution, can display around 85 PPI in both the horizontal and vertical directions. This figure is determined by dividing the width (or height) of the display area in pixels by the width (or height) of the display area in inches. It is possible for a display’s horizontal and vertical PPI measurements to be different (e.g., a typical 4:3 ratio CRT monitor showing a 1280×1024 mode computer display at maximum size, which is a 5:4 ratio, not quite the same as 4:3). The apparent PPI of a monitor depends upon the screen resolution (that is, the number of pixels) and the size of the screen in use; a monitor in 800×600 mode has a lower PPI than does the same monitor in a 1024×768 or 1280×960 mode.
The dot pitch of a computer display determines the absolute limit of possible pixel density. Typical circa-2000 cathode ray tube or LCD computer displays range from 67 to 130 PPI.
The IBM T220/T221 LCD monitors marketed from 2001 to 2005 reached 204 PPI.
The Toshiba Portégé G900 Windows Mobile 6 Professional phone, launched in mid 2007, came with a 3" WVGA LCD having “print-quality” pixel density of 313 PPI.
In January 2008, Kopin Corp. announced a 0.44 inch (1.12 cm) SVGA LCD with an astonishing pixel density of 2272 PPI (each pixel only 11¼ μm). According to the manufacturer, the LCD was designed to be optically magnified to yield a vivid image and therefore expected to find use in high-resolution eye-wear devices.
Holography applications demand even greater pixel density, as higher pixel density results in a larger image size and viewing angle. Spatial light modulators can be used to reduce pixel pitch to 2.5 μm, giving a pixel density of 10,160 PPI.
It has been observed that the unaided human eye can generally not differentiate detail beyond 300 PPI; however, this figure depends both on the distance between viewer and image, and the viewer’s visual acuity. Modern displays having upwards of 300 PPI pixel densities, combined with their non-reflective, bright, evenly lit and interactive display areas, may have vastly more appeal to users than the best prints available on paper. Such high pixel density display technologies would make supersampled antialiasing obsolete, enable true WYSIWYG graphics and, further, pave the way towards the elusive “paperless office” era. For perspective, such a device at 15 inch (38 cm) screen size would have to display more than four Full HD screens (or WQUXGA resolution).
Development of a display with ~900ppi allows for three pixels with 16bit color act as sub-pixels to form a "pixel cluster". These "pixel clusters" act as regular pixels at ~300ppi to produce true 48bit color display.
The PPI pixel density specification of a display is also useful for calibrating a monitor with a printer. Software can use the PPI measurement to display a document at “actual size” on the screen.
Read more about this topic: Pixel Density
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