One thing they are missing is collimation. You might want the light rays to be parallel, to simulate the far away sun and to get sharp shadows. I tried to come up with a solution to do this in a flat package, but optics is not my strong suit.
Generally, you always have a trade-off here: If you want to get rid of the harsh point-like light sources, you have to add a diffuser, and then you get indirect light. But too diffused, and you get the feeling of sitting in a studio or in a bright cave. I think having a window or a skylight with parallel, non-diffused light and letting the reflections on the walls diffuse it is probably the most natural-looking solution.
Yeah but he uses a huge parabolic mirror, which he places in the next room. He mentions fresnel lenses, but they are expensive, have color fringes, and depending on the focal length the whole light will also not be very flat.
The holy grail would be something that has the size of a picture frame, and you can slap below the ceiling, to fake a roof window.
If you combine a good fresnel lens with some clever mechanics or a micro mirror array light source, I wonder if you could even make the angle move over the day? Using fresnel lenses, you could also split it up into multiple cells, to make cooling easier. But I guess in the end there is a reason why the commercial system costs $30.000 and takes more than a foot of thickness.
Generally, you always have a trade-off here: If you want to get rid of the harsh point-like light sources, you have to add a diffuser, and then you get indirect light. But too diffused, and you get the feeling of sitting in a studio or in a bright cave. I think having a window or a skylight with parallel, non-diffused light and letting the reflections on the walls diffuse it is probably the most natural-looking solution.