Manufacturers appear to be gearing up to produce 4K displays, with a resolution of 3840×2160 — each dimension having twice the number of pixels of an HDTV display, providing an astounding 8 million pixels. Is bigger always better?

Sony recently announced that it will begin a worldwide phased launch of its latest Bravia LCD television by the end of 2012. The new Bravia sports an 84-inch 4K LCD panel — that’s 6 feet wide — that incorporates Sony’s 4K X-Reality Pro technology and is 3D as well. The new XBR-84X900 will be available for $25,000. LG similarly announced the U.S. availability of an 84-inch 4K set, also including 3D. Even before 4K content is available, a proprietary “Resolution Upscaler Plus” could deliver better performance from current HD/SD external sources. The LG UD 84LM9600 is expected to be available in October at an MSRP of $19,999.99. Sony, SIM2 and JVC have announced 4K projectors, and professional rigs from Sony, Barco and Christie have already been available for some time.

So what is the practical viewing distance for a 4K resolution a display? People with “20/20” vision have a visual acuity that can resolve 60 features per degree or 30 cycles per degree. From this, we can calculate that the “optimum” distance from which to observe a 1080-line display is about 3.2 times the picture height, where the vertical viewing angle is 18 degrees. Further than that, a person with 20/20 corrected vision can’t resolve the smallest displayed details; closer than that, and you’ll start to see individual pixels. Stated in screen diagonals, this works out to 1.55 times the diagonal measure of a 1920×1080 display.

So, let’s say you’ve got an 1080-line monitor with a 42” diagonal. The optimum viewing distance is about five-and-a-half feet. But at 4K resolution, the optimum distance becomes about 1.5 picture heights, or about 0.75 screen diagonals. For an 84” set, that means sitting at about 5.3 feet from the screen — a truly immersive experience, as the horizontal angle subtended by the display would be about 60 degrees, or about half of the normal binocular range of human vision. Funny enough (or is it?), the 84-inch Sony Bravia is said to have a viewing angle of just that — 60 degrees.

These calculations, however, assume that there are no other limiting conditions; in reality, factors based on Kell factor, interlace, the inter-pixel grid, contrast and the sharp edges of the image details must all be taken into account. Also, because most people view their TV from a larger distance of about 9 feet (the so-called “Lechner distance,” named after TV researcher Bernie Lechner), the required optimum screen size grows proportionally.

NHK researchers wrote in a 2008 paper that test subjects could distinguish between images with effective resolutions of 78 and 156 cycles per degree.   This suggests that some people can tell the difference between a display with 1080 lines and one with 2160 lines, when viewed within the practical confines of a living room. Time to get a bigger house?