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I'm not sure that's fair. It is bulb shape and it emits light. The source of the light happens to be the sun. This is kind of like saying that tungsten lamps aren't lightbulbs because they don't work when not supplied with electricity. They're truly acting as a substitute by spreading light around a darkened area.

We're just arguing semantics here.

They actually -are- a substitute for the lightbulb they would otherwise have to be using during the daytime. It's just not a substitute for the lightbulb at night.

May not be true of what passes for slums in the US, but in other countries the next house in a slum begins where the current one ends. With hardly any windows to let natural light in, the indoors are quite dark even during the day. So yes, while it might not work at night unless there is a strong source of light such as streetlights nearby, it certainly does offer visual comfort during the day.

> But it's still not a substitute for a light bulb unless it works in the dark.

Of course it isn't, but it solves 50% of the problem.