White Ring on the Avionic LCD Screen

Colesterol medication maby?

That's not much information to use to determine the problem, but I know of two common causes of this type of phenomenon, excessive localized heating of the LCD and excessive localized pressure on the LCD.

Excessive localized heating can be caused by a backlight that is not properly heat-sunk to the chassis. This is more common with fluorescent backlights (since the cathode regions get so hot) than LED backlights, but it can happen with either.

Excessive localized pressure is due to a poor design of the gasket that supports the LCD between the keypanel/chassis and the backlight. As the screws that hold everything together are tightened the LCD gets squeezed, affecting the internal cell gap. This may not show up right away, but once the unit has seen some vibration the phenomenon appears, especially if the gasket allows the LCD to vibrate at its resonant frequency.

Does the phenomenon go away (or persist) if the backlight is dimmed all the way down or switched to night mode? Does it go away (or persist) if the power is switched off? Does it go away (or persist) if the unit is disassembled?

I can't really see the problem. I'm sure you're not complaining about the very solid white ring with tickmarks around the perimeter, are you? I mean, to someone not familiar with what the display should look like, that looks like it is some meaningful / necessary part of the instrument.

Then I see what appear to be reflections in the glass.

So, please clarify the problem.

I think he's talking about the washed-out area in the lower right-hand corner of the LCD. The worst of it appears to extend for about an inch along the bottom edge of the LCD and about 2 inches upward on the right hand edge of the LCD, but a thinner and less-washed-out zone extends further along the bottom and higher along the right side.

In addition to the possible causes I mentioned above, another possible cause is differential stress put on the LCD due to the lamination process used to strengthen it for avionics use. The adhesive bond between the cover glass and the LCD perhaps did not cure evenly, warping ("tin-can"ning) the LCD and leaving a stress line around the edge of the LCD.

[quote]

That's not much information to use to determine the problem, but I know of two common causes of this type of phenomenon, excessive localized heating of the LCD and excessive localized pressure on the LCD.

Excessive localized heating can be caused by a backlight that is not properly heat-sunk to the chassis. This is more common with fluorescent backlights (since the cathode regions get so hot) than LED backlights, but it can happen with either.

Excessive localized pressure is due to a poor design of the gasket that supports the LCD between the keypanel/chassis and the backlight. As the screws that hold everything together are tightened the LCD gets squeezed, affecting the internal cell gap. This may not show up right away, but once the unit has seen some vibration the phenomenon appears, especially if the gasket allows the LCD to vibrate at its resonant frequency.

(1) Does the phenomenon go away (or persist) if the backlight is dimmed all the way down or switched to night mode? (2) Does it go away (or persist) if the power is switched off? (3) Does it go away (or persist) if the unit is disassembled? [/quote]

[quote]

In addition to the possible causes I mentioned above, another possible cause is differential stress put on the LCD due to the lamination process used to strengthen it for avionics use. The adhesive bond between the cover glass and the LCD perhaps did not cure evenly, warping ("tin-can"ning) the LCD and leaving a stress line around the edge of the LCD. [/quote]

the answer for the first two question is yes.

I am in agreement with your opinion, except when my friend engineer did the investigation, she did not have a chance to open up the damaged unit.

Perhaps, based on your answer, Can I conclude that the possible cause is design problem which cause heat dissipation problem and unnecessary pressure applied to the LCD screen?

Can I also assume based on your answer, the process done to ruggedised the LCD for avionics use was not done properly which cause the peel-off occurs between cover glass (which is actually a plastic based multicoated filter to remove reflection) and the LCD. Because of the high humidity level in the environment and vast different operating temperature (60˚C noon ~ 24˚C night inside the cockpit), the fine wet dust filling the gap and remain there forever.?

You used a plastic layer as the cover glass? That's not such a good idea since plastic has material properties that are quite different from the LCD. The plastic will absorb water and will expand/contract more as the temperature changes -- and both of these effects can cause stresses in the LCD as the humidity and temperature change. Also the plastic isn't stiff like glass, so you don't get much protection from vibration.

I can't see the fine dust you mention. If it's really dust, you can delaminate the cover layer, clean the LCD and laminate a new (I recommend glass) cover onto the LCD. My worry here is that it's not dust, it's deterioration of the sub-pixels in the LCD (deterioration of the color filter and ITO coating inside the LCD) that can look like dust. If it's sub-pixel deterioration (look closely using a microscope) then the LCD is now junk.

I am thinking that you need to go back to your repair facility to investigate this. Having worked in a repair facility for equipment similar, it would almost have to be improper torque spec's allowing the lcd screen to be 'pinched' during take off and landing that may be causing this issue. You don't say what kind of an aircraft this is, so I am assuming it is not a mil-spec repair facility that would be ensuring the proper repair. The cover glass that you state is a plastic multilayer filter should be a seperate piece, and the cover glass should be glass. You just don't get the proper torque spec's if you are using a piece of plastic unless you are using a higher grade of lexan.

If this is also happening with new from the manufacturer, I would ask them the same questions. If it is put together properly with the proper materials that screen should last through your MTBF without degradation.

Indeed, I am about to bring the unit back to my workbench for further study. Previous study shown that the filter in front of LCD is made of plastic. The unit is fitted onto a heli, manufactured by Smith Industries and it is military aircraft.

Bringing back to unit to the manufacturer is a little bit reluctant, the user simply do not like it.