Intense/bright sun can nearly LITERALLY paralyze my dad ANYTIME of day. Any idea what this reaction is?

deborahO, with global warming the sun is much brighter than it was when I was growing up. Rarely did I ever wear sunglasses until the past couple of years. If I look at the sun and then go inside, it takes a few minutes before my eyes can adjust to the inside environment [that is normal].

Has your Dad had any eye surgeries such as cataracts? Many people find that they can no longer deal with very bright light. My elderly parents couldn't. They got those eyeglasses that change to dark if in a bright setting.

As for passing out, sometimes being dehydrated can caused a variety of different health issues. Get Dad to drink more water and see if that can help.

Otherwise, as mentioned above, have Dad see an ophthalmologist.

I'd have him consult with an opthamologist. I have recurring extreme sensitivity to light, due to an eye condition. Mine would come out of nowhere and literally, I could not function outside.  I might have to wear TWO pair of sunglasses at once. It was quite painful.  I'd check to see what might be causing it with his doctor.  With treatment, mine was drastically improved.  

Have you had his vision tested? The change from indoor to bright light could be causing him temporary vision loss, try some of those very dark, wrap around sunglasses.