Backlight color problem? zr30w and others

Mr.Pixel

Limp Gawd
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Oct 30, 2010
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138
Has anyone ever seen color after-images on a LCD with cfl backlight? It looks like what you would see from a DLP projector, only not quite as obvious.

A simple test it to view an entirely black screen with a small (5-10 pixel wide) vertical white line near the center. I can see what appear to be blue, yellow, and magenta after-images when looking from one side of the line to the other. In the worst case at minimum brightness there appear to be 2 cycles of these colors. The image itself is static, so I assume the effect must somehow be due to the backlight. I don't know how this could be happening, since I thought there can't be cycling colors with cfl backlights.

I have seen this effect on some TN monitors when used at less than full brightness, but have never seen it on my older dell 2007wfp (PVA version).

I just purchased a zr30w and am sending it back because this is extremely annoying to me (and it buzzes). At full brightness the image appears to not have this effect, but when using at the minimum brightness (the only usable setting) there are multicolored trailing images visible in high contrast areas when visually scanning across the display.

Most people I have asked can just barely see what I am talking about, so apparently I'm more sensitive to this than average, but someone else must have encountered this at some point.

Is there a name for this effect? I can only assume it is caused by the backlight since it disappears at full brightness. I need to find a monitor that doesn't have this problem.


Monitors that don't have the problem:
Dell 2007wfp (PVA version)

Monitors that do have the problem:
Samsung 245BW
HP zr30w
 

Walker

Limp Gawd
Joined
Oct 24, 2004
Messages
355
I don't know what causes it but this exact same effect is extremely noticeable on my plasma. Only with white objects on a black background, not any other colors, but very noticeable even from distance. I have never seen it on an LCD though.
 

Mr.Pixel

Limp Gawd
Joined
Oct 30, 2010
Messages
138
Thanks for the reply Walker.

I don't have a plasma to compare with, but mainly I just want to find a way to avoid buying something with this problem.
 

Sycraft

Supreme [H]ardness
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Nov 9, 2006
Messages
4,985
Probably you are seeing an optical illusion. I would guess it is a mechanism of action similar to a Benham Disk. With that you have nothing but black and white, yet with rapid movement you appear to get colors.

So my guess would be that at high brightness, when your eyes are getting over saturated, the rapid movement of your eyes stimulates this phenomena.
 

tk-don

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Jun 15, 2010
Messages
324
I don't know what causes it but this exact same effect is extremely noticeable on my plasma. Only with white objects on a black background, not any other colors, but very noticeable even from distance. I have never seen it on an LCD though.
Here's a good explanation:
http://www.hdtvtest.co.uk/news/plasma-phosphor-trail-2007040133.htm

It doesn't happen with LCDs as they are inherently monocrome (i.e. blocks or passes white light). The color filter is completely passive. For plamas, to produce white, three phosphor types are "excited" similarly to a CRT.
 

Snowdog

[H]F Junkie
Joined
Apr 22, 2006
Messages
11,267
Probably you are seeing an optical illusion. I would guess it is a mechanism of action similar to a Benham Disk. With that you have nothing but black and white, yet with rapid movement you appear to get colors.

So my guess would be that at high brightness, when your eyes are getting over saturated, the rapid movement of your eyes stimulates this phenomena.

My guess as well.
 

Mr.Pixel

Limp Gawd
Joined
Oct 30, 2010
Messages
138
Sycraft & Snowdog,
That's what I thought was happening at first, but there are 2 arguments against it:
1. The effect never happens at max brightness on any monitor I've tried, regardless of the actual brightness at the max level (across several monitors I'd estimate vary by 4:1 or more in max brightness). This may be in part due to the effect you mentioned.
2. It does not occur on some monitors at all regardless of the brightness setting.

I should mention that the effect becomes more pronounced as the brightness is decreased on all monitors where it is visible.

Just tested 2 more screens (both TN) from others, 1 shows the effect when at less than max brightness, and the other never shows it. Both models are several years old.


tk-don,
All I'm looking at are LCDs. I'm confused because as you said I thought it would be impossible to see on screens using cfl backlights.


Thanks for the help everyone, I'm just trying to figure out what's going on.
 

Mr.Pixel

Limp Gawd
Joined
Oct 30, 2010
Messages
138
I may have a possible cause of the problem, but no solution to it yet.

It appears many LCDs use a form of Pulse-Width-Modulation with CCFLs to achieve dimming, while a few use only Amplitude-Modulation of the backlight current. The advantage of PWM is the ability to vary brightness by a large factor, but can also lead to transformer humming and "visible effects" (see [2]).

References:
[1] http://www.eetindia.co.in/ART_8800544788_1800010_AN_2b3b3807.HTM
[2] http://www.ergpower.com/pdf30/dimming_options.pdf

Both displays I have seen that do not show the color effect appear to have smaller brightness ranges than other displays, so maybe they use only AM dimming? Could the PWM dimming be causing what I am seeing? If so, how would I go about finding a monitor with only AM dimming?
 

Walker

Limp Gawd
Joined
Oct 24, 2004
Messages
355
We recently got a couple of ZR24W's at work and remembering this thread I went to check for this issue with a vertical white line in the center of a black background. Yep, I can see the exact same effect that I can see on my plasma except it is far less noticeable on the LCD. I also tried it on a couple of newer Samsung TNs, not sure of the exact model right now and sure enough they have the same green/blue flashes as well. The effect seems to be completely gone when using the displays at full brightness. My old PVA doesn't do this but it apparently controls the brightness through the matrix so you may be onto something it's the backlight that is causing it.


Here's a good explanation:
http://www.hdtvtest.co.uk/news/plasma-phosphor-trail-2007040133.htm

It doesn't happen with LCDs as they are inherently monocrome (i.e. blocks or passes white light). The color filter is completely passive. For plamas, to produce white, three phosphor types are "excited" similarly to a CRT.

I thought that's what it was because I can see those green trails on my plasma as explained in the article but although similar, this seems to be a totally different issue because it happens on static images and I can see it on some LCDs as well, just like the OP.
 
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