In one of my flight of fancy I was pondering how a inexpensive dilating pupil could be made for an animatronic eye, and I think I have a solution.
1. Firstly make the half eyeball but do not paint the iris on it. Mark where you want the iris to be, and the center of the pupil.
2. Drill a hole through the eye at the center of the pupil - the hole being the minimum diameter of the pupil. Bevel the edges so that it is a slight funnel shape.
3. On a disk of thin silicone paint the iris and pupil. The pupil should be the maximum diameter you want.
4. Behind the silicone and at the center of the pupil glue a thread. Make sure it is secure and that the fixture does not distort the visible side of the pupil.
5. Carefully feed the thread through the hole in the eyeball and then glue the outer edge of the iris to the eyeball.
1. At rest the silicone is undistorted and so you have the maximum pupil expansion.
2. As tension is put on the thread it draws the center of the thin silicone disk into the funnel.
3. This causes the iris to be pulled in towards the funnel thus giving the impression of the pupil dilating.
4. When pressure on the thread is reduced it causes the silicone to relax and thus withdraw from the funnel and make the pupil look larger.
Very sorry - I suck at drawing, and can visualize things easily.. Bit I hope this give people some idea's of a way to do this. One should only nee a small servo to enact this