UHD is a term coined by the consumer electronics industry for this new high-resolution imagery. The image size for UHD is 3840 pixels by 2160 lines – exactly 4 times the size of an HD picture (which is 1080×1920). So, twice as many pixels horizontally and twice as many lines – therefore 4x the size.
4K is the term used by the professional video market to denote this higher resolution as used in film and TV production work. The image size in this case is 4096 pixels wide by 2160 lines, so it is slightly MORE than 4x the image size for HD.
In many cases, this slight difference in horizontal resolution doesn’t matter, but be aware that a UHD TV set cannot display a 4K image without either throwing away 256 pixels on each line, or rescaling the image to fit.
For more information about 4K, 8K, HDR and UHD, download our guide here.
Sincerely,
Mark Wronski