Self-Healing LCD


Hi all you fellow calculator hounds,

An intersting anecdote: I sold a new 20s to a co-worker about 9 months ago---he had a singapore made 20s, and though it worked fine, there was a black "spot" in the lcd, along one edge.

Well, by and by, one day he pulls it out of the slipcase, and it's gone! The LCD is back to normal.

Anyone else ever seen this before?




I had already see this one on a watch, but not on a calculator. Maybe I can keep some hope for my Xpander which has the same problem.



I wonder if this could be a thin film interference effect. If you have two parallel reflecting surfaces very close together (with a gap somewhat less than 1/2 the wavelength of light apart), incident light can be reflected with destructive interference, yielding little or no reflected light. There would be internal reflection from the top surface and external reflection from the bottom. A 180 degree phase change takes place from the external reflection (as I recall), causing destructive interference in the two reflected rays, yielding a dark area. Perhaps there was a problem in the physical spacing of layers in the display, and, for some reason, the components got moved backed to their proper spacing.

Hope this explanation is clear. Anyway, it's a possibility.



Hi Larry,

Nice theory.

However I do not think that theory will hold, for a very simple reason. If you had this gap, you would necessarily also have a vanishing edge of the gap where the void disappears. Therefore, you would see the fringing effect, including rainbow colors, as the constructve interference sets in going through each 1/2 wavelength portion narrowing down from say a 2 wavelengths thick (800 nm) void. At 1/4 wavelength and less, you get total transmission/ no reflection, and at half wavelength you get total reflection (I think this is reverse of what you said but it is only the priciple of the thing we care about here and it is easy enough to derive it).

So the point is, I did not see the fringing effect, threfore I don't think it was a void.

Actually maybe if the whole void were less than 1/4 wavelength for all visible light (even violet) then you would not see the fringing.

So maybe you are correct!

Fun thought experiment---thanks!





Yes, you're right: colored fringes would be expected to be visible around the outside of the area/point of actual or near contact, where the interference effects would be wavelength-dependent.

I think I had something like this on one of my TIs. I'll have to keep my eyes open for it.



Hi Bill. My 9s had a black spot show up on the LCD shortly after I received it. At first, the black spot looked like an LCD defect. I was ready to return the calculator to HP when I noticed that the black spot starting moving around. At that point I assumed that the black spot was not an LCD defect but instead was a black speck of foreign material. Finally, out of frustration I shook the calculator vigorously and the black spot disappeared and has not returned.

I hope that this helps.



Hi Bill,

it is unbelievable, but I experienced exactly the same phenomenon on exactly the same machine. It took about six years of moderate use until the black spot suddenly disappeared. I did not ask what the reason was, I was simply happy with it...


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