Goshark: bleeding

[compuhan]

Goshark,

Of all the monitors I've seen, they seem to have bleeding of white on black background on the right edge of the screen. This is only very noticeable when using a long light scroll bar against a dark background. Why is this? There seems to be a pattern. Is it because of the way a monitor scans? Is this a legitimate reason to return an other wise good monitor. I obviously want to get rid of this, but wonder if this flaw cannot be eliminated.

Thanks.

Quality is better than name brand, even regarding beloved AMD.

 

[GoSharks]

This is commonly known as phosphor bleed. Black backgrounds are the worst possible thing for a monitor any CRT monitor.

Glow in the dark toys all use a long persistence phosphor similar to the phosphor used in T.V.’s and computer monitors. It doesn’t take much of a beam or light wave to make them glow. If you have ever turned your TV or monitor off when the room is completely dark, you will notice the phosphors glow for a period of time after the unit is off.

Basically the light from the active white phosphors glow or have a halo affect into the black region. As the monitors size gets larger this phenomena becomes more apparent. There is no fix to this short of using a lighter color background.

Jim Witkowski
Chief Hardware Engineer
MonitorsDirect.com

<A HREF="http://www.monitorsdirect.com" target="_new">MonitorsDirect.com</A>

 

[compuhan]

Thanks.

I dont' think it's just phosphor bleed because it's limited the right side of every monitor I've seen. As I move the window to the left, the effect dissappears.

Does it have to do direction in which the beam is scanned? Could you explain how a the beam scans? Does it zig-zag from left to right down the screen while continually on or does it work like a typewriter (on in direction and then off as it returns to repeat). This may explain how the phosphors most recently lit (right), persists while the left (longer time left unlit) has no glow? Just a theory.

Quality is better than name brand, even regarding beloved AMD.

 

[GoSharks]

On a normal monitor, the beam starts at the left top corner scans across one line to the right side then turns off (this period is call the horizontal blanking time). The deflection yoke the resets itself to the second line on the left and scans across. This process continues until the beam scans to the lowest line turns off (this period is the vertical blanking time. Deflection circuit then resets to the upper left corner and the process is repeated.

I have seen phosphor bleed almost anywhere on the screen, I doubt it has anything to do with how the phosphors are scanned. Older monitors are less susceptible to phosphor bleed due to the fact that the phosphor is close to or has already reached half brightness.

Jim Witkowski
Chief Hardware Engineer
MonitorsDirect.com

<A HREF="http://www.monitorsdirect.com" target="_new">MonitorsDirect.com</A>

 

[compuhan]

Thanks for explaining the direction and mechanism of monitor scanning.

I understand that phosphor bleeding can occur at any area of the screen, but I have noticed that all crt monitors (home, work, stores, friends houses, etc.) show a disproportionate bleeding on the right side of the screen in dark backgrounds. Is this an eerie coincidence or have I noticed a pattern that speaks of monitor design limitations?

I think this asymmetry can be explained by the asymmetrical scanning of monitors that you have explained.

Consider an illustration. Imagine a mouse pointer moving from left to right. It leaves a decaying trail that is stronger at the most recently passed part of the screen. Now, imagine it blanks out and returns the start point (for simplicity lets not consider vertical movement). Obviously, no trace in right to left direction is observed as their is no beam. Now, repeat the movement to the right again. NOw imagine cranking up the speed and repeating. It's not hard to imagine the right side of the screen showing a trace gradient of phosphor bleed at the right speed as the duration of the last decay is greater than than scan rate. Now if the blanking time is reduced, that also helps exxagerate the effect. Is this not a plausible explanation? I have a feeling it's way too oversimplified, but I'm convinced that the right side phosphor bleed is a real trend of some sort.

What else do you suggest causes the disproportionate phosphor bleed on the right? Or do you hold that my observations of right side phosphor bleed coincedental?

Quality is better than name brand, even regarding beloved AMD.

 

[GoSharks]

Honestly I have not studied this problem in any great detail I usually tell people to stop using black backgounds and the problem is solved.

Just like the guy who goes into the doctors office and says doctor it hurts when I do this and the doctor says “well don’t to that”

Jim Witkowski
Chief Hardware Engfineer
MonitorsDirect.com

<A HREF="http://www.monitorsdirect.com" target="_new">MonitorsDirect.com</A>