From: rseymour@reed.edu (Robert Seymour)
Newsgroups: comp.sys.mac.hardware
Subject: PowerBook Displays (Was Re: Grayscale ghosts on my new PB160 screen?)
Keywords: powerbook, lcd, display, ghosting, bleeding
Date: 17 Nov 92 01:09:06 GMT
Article-I.D.: reed.1992Nov17.010906.8118
Reply-To: rseymour@reed.edu
Organization: Reed College, Portland, OR

In article <41048@sdcc12.ucsd.edu> stine@jeeves.ucsd.edu (Blaine Stine) writes:
> Has anyone else noticed that when you have the monitor on a new
> PowerBook set to 16 colors or gray that there is a dramatic
> ghost of the title bar that extends up to the top of the screen
> and down to about the middle forming a gray box. It also seems
> that the border lines of any window where the scroll boxes are
> also seem to form ghosts. One other thing is if you put the
> mouse cursor in the title bar a vertical line appears in the 
> ghost. It is such a pain that I'm not using the gray scale
> setting. I've tried to adjust the brightness/contrast to get
> rid of it but no way.This occurs on both of the PB160's I've 
> used. Anyone have any ideas?
> 
> Blaine Stine
> UCSD Biology

	The color of the pixels is set by the amount of voltage applied to the  
junction of electrodes at the pixel. The voltage is applied by a grid of row  
and column electrodes. The idea is that the voltage is applied only at the  
intersection of the electrodes. However, this is not always (or usually) the  
case. The voltage is indeed highest at the intersection, but it is also present  
above, below, and to the left and right of the intersection. 
	Thus, when an intersection has high voltage applied to it (dark pixel),  
the "leakage" of voltage along the electrodes causes adjacent pixels to darken  
due to a lower, but present voltage (this is the banding you observe, often  
called "bleeding").
	It's just an unfortunate consequence of the passive matrix tft  
(thin-film-transistor) lcds (liquid-crystal-displays) used in the PowerBook  
100, 140 (both mono), 160, Duo 210, and Duo 230 (these are 4bit greyscale  
[16-greys]). When the display is set to 16 greys it is more sensitive to  
voltage leakage (since it now has 15 voltage thresholds instead of one), so  
bleeding is more noticable. There isn't any way to fix it except for new  
display technology. 
	I had a PowerBook 140 issued to me by my employer over the summer, and  
the bleeding was much worse than on the PowerBook 160 (and it was atrociously  
slow, but that's another story). All you can do to help the problem is to play  
around with contrast and brightness, and to choose backgrounds, window borders,  
scroll bar patterns, etc. that hide the effect to some extent.
	The active matrix tft lcd on the PowerBook 170 and 180 is less  
susceptible to this problem since an individual transistor is placed at the  
intersection for each pixel. However, it is still there, so buyer beware.  
Anyone who is considering a PowerBook should look at the display for a long  
time and make sure they are making the right decision (not that there are any  
better alternatives...). 
	Also, N.B.: the active matrix displays on the PowerBook 170 and  
PowerBook 180 may have up to five permanently whited pixels on the display, and  
Apple will not exchange the display whether you were informed about this or not  
(the official policy is a display is good if there are five or less of these  
and they are not closer than an inch apart [a display is rejected if any pixels  
are permanently black]). I suggest looking at the unit you will buy if you are  
considering a 180 (though this means waiting until they are on shelves...).

--
Robert Seymour				rseymour@reed.edu
Departments of Physics and Philosophy
Artificial Life Project			Reed College
Reed Solar Energy Project (SolTrain)	Portland, OR