Okay, so that whole thing with the "DOAS AU experiment slides 1 & 2" was all about my nitpick-y little brain being nitpick-y about technical details. To my mind, it made no sense for Danny, Ember, and Cujo to grow pupils on earth. Why would they, especially if it was blindingly bright-- pupils absorb light. the bigger they are, the more light they absorb. It's VERY dark in the ghost world, and ghost have enhanced vision in the dark of the human world. so, my alternate explanation/anatomical feature is that ghost already HAVE pupils, and no iris, or at least, a very small, non-visible iris.
The ghost's "iris" is actually their entire, very large pupil. Now, pupils are actually holes in the surface of the eye through which light passes through to the receptors, and being that Ghosts are made up mostly of energy, this is why their eyes usually match their energy color--because behind their eyes is a wall of specialized energy meant to process light.
Now that we have discussed pupils and my theory, let's get down to the two basic ghost eye types, their advantages, and their disadvantages. The eye types are named after the two most prominent examples from the DP universe: Danny (D), and Vlad (V).
D-Type (Danny-type) eyes much resemble normal human eyes in shape and size. Ghosts who have D-type eyes have stronger night vision, as their eyes absorb much more light than V-type eyes. Since their eyes naturally absorb more light, their Pupil (visible as a pseudo-iris) is smaller, but can expand and contract the same as any eye type depending on the level of light. Ghosts with D-type eyes are much more suited to ghost world's dark atmosphere.
V-type (Vlad-Type) Eyes are pupils/pseudo-irises that do not absorb light nearly as well as D-Type eyes, and so they are expanded to their max to absorb as much light as possible from their surroundings, leading to their eyes appearing one solid color. V-type eyes can fair well in the ghost zone as their natural habitat, but do not have as acute night vision as D-type eyes and can struggle to see in pitch black rooms without the aid of energy. V-type eyes are much more suited to the very bright human world, or artificially lighted interiors, and when in such areas, their pseudo-iris will shrink down to about the size of a normal human iris or that of a D-type Iris, allowing them to see much better and have less strain on the eyes. For possessors of V-type eyes, living their whole life in dimly-lit (by ghost standard) areas can expect sight issues down the line, unless corrective lenses or energy is used to improve their light-absorbing efficiency.
Enhancing Sight using Energy: Many ghosts can, with enough training, enhance their sight by focusing energy into their eyes. The human theory that eyes emit Light is in these instances, is true for ghosts. They can release their own spectral energy in their surroundings to highlight their environment, visible only to that particular ghost or those familiar with their energy.
Again, I would like to once again state that this theory/headcanon is NOT canon to Pearl84's Dawning of a Sun. Once i get an idea in my head my mind runs with it like there's no tomorrow, thus the very long descriptions and details. I hope you like my theory on ghost anatomy Pearl!