You can get light and privacy both
See, when you speak of transparency, you're actually talking about 2 wildly separate things.
- Light can come through it --versus-
- An observable image can come through it
If you've ever used your car's defogger, you know how you can have one and not the other. There was plenty of light, but you couldn't see.
This answer brought to you by the word "translucent"
Transparent: Light comes through, clear image comes through (window)
Translucent: Light comes through, image does not (fogged up window)
Opaque: Light or image does not come through (anything solid-colored)
Transparent, translucent, translucent of opaque, and opaque. (Source)
Note how on the left side you cannot distinguish a tree from a sasquatch. Yet the light level is equal! Source
The shower curtain you have pictured is nearly opaque, a patchwork of 90% opaque and 10% transparent, so it greatly reduces the light in the shower. That is why you are having problems.
Translucent is what you want, because it lets through the light, but not the observable image. If that is not emotionally comfortable for you, that is a different problem, and we don't fix those here. In that case I recommend low voltage LED lighting fed from a power supply that's not in the shower proper, since it's easy and has no risk of electric shock.
Needless to say, translucent shower curtains are highly desirable, and therefore, plentiful. It's possible you may have been mistaking them for transparent ones; both exist.
Translucency can also be had in glass, one of two ways: By fogging the glass (a sandblaster will do the trick, though it will reduce light penetration somewhat). Or by having one side of the glass "pebbled" so it lets essentially all the light through, but impossibly distorts any possibility of an image coming through. You put the hazing/pebbling on the outside, since it picks up mineralization, and that's very hard to remove from a hazed or pebbled surface.
Another example of a pebbled surface is diffusers on fluorescent overhead "troffers" (the kind in drop ceilings where the whole square glows and you can't quite see the tubes).