And you specifically have an F91W? In pitch black, it maybe illuminates 2/3 of the screen for me. I can't imagine using it as a flashlight! But mine is newer, maybe the older ones have better lights.

It has to be extremely dark though, or maybe mine has a worse than normal backlight. Other models are much better, it's just the F-91W.

The fact that Casio is well aware that the backlight is just terrible, but haven't fixed it in close to 40 years is hilarious and just makes me love it even more.