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I think you're thinking of a different watch. The F-91W has a green LED[1] that's notoriously terrible (not to mention lopsided).

[1] https://www.instructables.com/id/Watch-LED-Light-Mod/

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Ah yes indeed, I had the F-105W. I thought the F-91W also had this EL film, because it's also labeled "Illuminator" on the big picture at the top of the article, which was casio's branding for this technology at the time. But it doesn't have the blue branding. And I think that pic at the top isn't actually an F-91W.

The EL light was super, very bright and very well distributed (because the whole backlight film would light up). I'm surprised they managed to put one in such a small watch as EL film needs fairly high voltages, you could hear the switching coil whine when it was on :P Yet it still lasted a good while on battery.

The F105W is what you graduate to after you realize the F-91W is essentially useless is the dark. I've worn both since the early 90s. The band always breaks before the battery or watch itself dies, but i still get a few years before the band breaks.

The somewhat more complicated and modern Casio watch I had also had this EL whine. Now I finally found out why it occurs, so thanks for that! The silent F-91W was almost pleasant in comparison despite its uneven and (IMO) not very pretty lighting.

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