The eyes of many animals have pupils that expand and contract in response to ambient lighting conditions. This is a minor detail, but it can help create an immersive fursuit viewing experience.
There are many light sensors available. The only challenge is placing a sensor where it has a good view of the surroundings but is not visible. One potential location is on the upper surface of the muzzle just behind the nose. The sensor could hide in the nose-fur border, and the fur would be short enough to not block light.
Clear plastic hemispheres with paint on their flat surfaces can be used to make static eyes. Instead of paint, we can use a small LCD or OLED display to show a dynamic eye. To demonstrate that this is a viable concept, we used Synfig Studio to create a simple animation of an eye. We played the animation on a smartphone and placed a plastic hemisphere on the screen. Viewed straight-on, the hemisphere makes the eye reflective and adds some variation to the iris color. As the viewing angle moves away from the axis, the display becomes eclipsed by a view of the opposite side of the hemisphere.
Small LCD and OLED displays in suitable size ranges are widely available. Our next prototype will use an LCD module instead of a smartphone screen.
Traditional LCD and OLED displays have some disadvantages for use in eyes. They must be constantly powered, and if they are not working they make the eyes look empty and soulless. A potential alternative is Electronic Paper Displays (EPDs). EPDs reflect light like paper (and like real eyes). They only use power when changing what they show, and they can show an image indefinitely without power. However, EPDs take hundreds of milliseconds or seconds to refresh. For eyes that adjust gradually to ambient light, this limitation is not a major problem. Current EPDs can only display black and red on a white background, so they can only be used for red eyes. When full-color EPDs become available, they may be a good choice for eye display.
Transparent OLED displays could be used to display eyes while still allowing the wearer to see through them. 4D Systems previously sold a transparent OLED display module, but it is no longer available.
There may be a simpler solution that does not involve electronic displays.