This article is here for historical reference. Modern cameras no longer use TVL to measure resolution like they used to and any cameras still listing TVL over Effective Pixels (eg. 1920 x 1080) should be avoided.
TVL is often misunderstood as the deciding factor on the quality of a camera. Unfortunately, it’s not.
TVL is the number of horizontal lines of an analogue camera’s resolution. While TVL is one of the most important resolution measures in a video system, a much more important factor in CCTV terms is ensuring you have the correct lens type for your application.
Your camera’s lens is the defining factor in quality and cannot capture detail beyond its own capabilities. If you had both a 420TVL camera and a 700TVL camera which both had the same lens then the 700TVL camera wouldn’t be able to see much further than the 420TVL. You may have a slightly better picture due to the increase in resolution but if you can’t read a number plate at 20m using the 420TVL then the problem is the lens, not the resolution.