August 2014
12 OPINION: ON THE AGENDA
Bob Simpson How to avoid visible pixels
A simple formula dictates minimum viewing distance for a given pixel pitch if individual pixels aren’t to be seen S
alesmen always like a metric: for example in the case of projectors the lumen has been the favourite competition unit. Recently, however, a new front has opened for displays in general – that
of pixel pitch. Apple was the first company to promote this with its Retina displays for its handheld devices, touted as having ‘329 pixels per inch (ppi)’.
What is the significance of this figure? Apart
from the obvious point that a display with more pixels can convey better image detail, its significance is actually related to visual acuity. This is the ability of the ‘average’ human eye to resolve detail, found to be the ability to resolve features subtending an angle of one minute of arc (one sixtieth of a degree). Ideally a viewer should not be able to see the individual pixels in a display, so, as a starting rule of thumb, individual pixels should not subtend more than a minute of arc at the intended viewing distance. The diagram below shows the principle. If it is applied to 329ppi, a viewing distance of 26cm is the minimum. In the handheld market, vendors
LG promoted big UHD LCD displays at ISE 2014 – as did other companies, including arch rival Samsung
are now offering products with nearly 500ppi. Is there any point in doing this? There are people with higher than ‘normal’ acuity and everyone has a ‘vernier acuity’ which in practice means that higher resolution displays appear subjectively better. Recently a team at Sharp Laboratories
in Oxford concluded that, for a viewing distance of 30cm, a majority of viewers could tell the difference between displays of 339 and 508ppi, and many saw an improvement at 1016ppi. The formula in the diagram also applies to
large displays, whether flat panel or projected image. A 50in HDTV set has pixels of 0.57mm width, implying a minimum viewing distance of 1.96m. A ‘2K’ projected image of 10m width in the cinema has pixels as large as 5mm, which implies a minimum viewing distance of 17m; however for many applications the theoretical minima can be halved, especially when moving images are involved. The advent of big LCDs is encouraging higher
This figure relates pixel width to viewing distance. If angle is one minute of arc (=0.000291 radians), then D is the distance at which a pixel of width w can be resolved
resolutions, for example a 95in LCD with an image size of 2096mm × 1179mm showing an ‘HD’ image 1920 pixels wide has a theoretical minimum viewing distance of 3.75m. However
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