> What do you do about all the scattered light?

I think it's actually fine. Quick back-of-the-envelope calculation:

- The lens collects about 1 m^2 of sunlight.

- At the focal point, the concentration is way higher than sunlight.

- But when the light is scattered away from the focal point, it's distributed over an area of ~4 pi R^2.

- So from the point of view of a person looking out the window, R^2 would be much greater than 1 m^2, so the crucible would be less bright than the outdoor sun.

- The outdoor sun doesn't blind people unless they look directly at it, and humans are good at avoiding that.

So I think the crucible would appear as an annoying glare that people would instinctively look away from, but it probably wouldn't accidentally blind people.