Lot's of amelanistic animals including snakes, are often photo-phobic. That's because the eye lacks pigments needed to block light from entering the eye. So, when the iris closes to prevent excessive light from entering the eye, it has limited effectiveness. Now, imagine you had bright light shined into your eyes but your pupil didn't react by closing. With no pigments in the iris to block light, that's essentially what's happening. Their pupil is closing like it should but it doesn't do any good because there's no pigment to block the light. It's like if your eyelids were transparent, closing them wouldn't do anything to block light. Likewise, their iris closing doesn't do any good because light can still pass through it, Due to lack of melanin it's essentially transparent so it's like trying to close a transparent door to block light.