It is really difficult to get consistent optical properties using additive manufacturing techniques. There’s a reason optics (lenses, etc) are basically universally made from uniform pieces ground down and polished smooth, and compound lenses are avoided unless absolutely necessary and to make them optically clear enough requires exceptionally complex and expensive methods. With typical additive manufacturing, you are making something that is basically hundreds or thousands of compound lenses stacked onto each other, and the optics are always going to be pretty awful no matter how much care you put into the process. There is no easy answer, except to not use additive manufacturing for this. For optical properties, you really want to stick to a single shot of consistent material as much as possible, to minimize internal refraction (which happens at every material surface transition if it’s not perfect, which it won’t be). Cutting material away is fine, you’re getting rid of the old surface transition and creating a new one, you always have to interface with the outside air at some point, and that’s the minimum number of refraction layers you’re going to get. Adding material to it creates another layer of internal refraction for the light, making many of those is very not good.

While there are people working hard to make additive manufacturing methods that can do this as well as possible, and in a few cases they’ve gotten quite impressively good at it, they’re still starting from a compromised beginning, the deck is stacked against them and it really is a challenge. If at all possible, don’t use additive manufacturing for this. It’s the wrong process for the job. The traditional approaches of molding or cutting or machining or polishing to create the shape you want, is the right way to do it. If you really need something unique you may have few good choices, but if you can get something off the shelf, it’ll save you a world of headache. This is only something worth doing for the challenge of it, and you should go in with the expectation of failure, and prepared for joy if you succeed, and I’ll be happy to know that you’ve proven me wrong.