CRT's & LCD's: The tale of the tape.
A friend asked me today what could be expected between a CRT's life and a LCD's life. His question came from two perspectives. His experience using CRTs and Flat Panel displays in the consumer sector and in medical displays. These are distinctly different applications and the life of each is affected by the application. The two technologies age differently, but there are some things that can be compared.
CRT life is generally defined as the point at which luminance declines to one half of its original luminance. There are two principle controlling causes.
1. cathode emission current reduction
2. phosphor aging (general loss in overall electron to photon conversion efficiency)
A subset of uniform phosphor screen aging known as differential aging or image burn creates noticeable localized areas of lower screen luminance which hastens the end of the CRT’s useful life.
A CRT's brightness will decline rapidly in the first few thousand hours and level off. So there could be a very noticeable difference in a CRTs performance in those first few thousand hours. In entertainment TV application a CRT could last well over 50,000 hours. Due to the relatively low cathode emission current densities which is inversely proportional to the relatively limited number of TV lines of resolution output.
Medical applications can limit life to as little as 5,000 to 10,000 hours. High resolution medical applications require higher overall cathode current and higher current densities, causing more rapid emission current decline. CRT life in a desktop monitor application generally falls in between with an expected 20,000 hours of useful life. In industrial applications static image burn in is a very real possibility as an industrial application will typically show the same images all the time. Image burn will cause the CRTs life to dwindle to 10,000 to 15,000 hours.
LCD's luminance is controlled by the luminosity of the backlight / edgelight. The backlights in LCD monitors are almost always CCFL (cold cathode florescent lamps). The life of the backlight is determined by how long it takes until the lamp reaches half of its original luminance. Similar to CRTs phosphors in CCFL’s age and their efficiency declines. LCD's don't suffer from flicker, or image burn just a few years ago that time was about 30,000 hours. Newer lamp technology has increased that time to 60,000 hours to reach half brightness.
As LED's (light emitting diode) become a more cost effective alternative to CCFL's, LCD life will approach 100,000 hours. In medical applications the LCD backlight assemblies will be driven harder as well, but their life is not affected as greatly as a CRT's. In industrial applications the backlight life should approach 50,000 hours, because there is no image burn.
Maybe this is more of an answer than my friend had in mind but engineers aren't exactly known for short answers.
Sources:
Al Gard, Planar Systems
Adi Abileah, Planar Systems
posted by Jay at 11:16 AM
3 Comments:
I have two questions.
Is the life of an CCFL significantly affected by turning it on and off?
Also, how significantly does the brightness setting on the monitor affect the expected life of the CCFL?
Nic,
Good questions.
In our best engineering speak…
(1) Yes - turning the backlight ON / OFF many times will shorten the life of the CCFL. Each time that you turn it ON, there are ions accumulating near the electrodes and are causing sputtering. This is the black sputter that you see near the edges of fluorescent lamps that are old. This will be even more harmful if the starts are at cold temperatures, where there is not enough mercury vapor in the gas mix.
(2) It is significant - the lower the CCFL current, the less ion bombardment on the electrodes, and less aging of the phosphor. Both mechanisms that make the lamps age (electrodes coating, and phosphor aging) are reduced with less current. I don't have the exact relation - it is not linear...
Again, many thanks to Adi,
OVL (http://www.eefl.cn) LCD cold cathode lamp (CCFL) with long life, it is more than 50’000 hours, with Japanese new cathode, phosphor, glass, and other materials, special made by OVL, they are perfect for all LCD backlight application and other purpose, such as instrumentation, medical devices, and all type devices.
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