I am looking into creating an illuminated string of leds where the individual LEDs are not visible, but that a shape is illuminated. The shape would be a various sized rings the biggest being about 1ft in circumference. Here are some example pictures of what I am thinking of. Now, a caveat is that it has to be the same on both sides. That being that the top and the bottom both have to be illuminated. I am just asking what sorts of LEDs and materials that I should be looking into to achieve such a result.
What you are looking for are flat, wide-angled SMD LEDs. It will be easier to diffuse the light that way and will allow you to place the diffuser closer to the LED. You need to make some sort of diffuser to have the look you are going for. This can be made out of plastics or silicon. Since I am guessing this will be a hobby project, you will probably want to use plastic. You need to have a matte finish on one side of the plastic to diffuse the light. Cutting a piece of acrylic or polycarbonate, then sanding one side should work. I feel like that will give you a good start. There are many ways that you can diffuse the light, the best solution would be up to you though and your application.