First, I believe that State Law does not permit medical exemptions for tinting beyond what State Law allows. However, doctors here still write letters and sometimes it can help get people out of a ticket.
My dermatologist told me that with his letters, cops usually back off as long as it isn't so dark that they can't see anything at all. (i.e. 5 percent). I have read many encounters of other people in CA and found that some cops do let them go and some still write a ticket. I even found a few cases where the person got a ticket, showed the letter to a judge, and got the ticket thrown out.
A few days ago, my doctor said he would write me a letter for my tint and mail it to me. I'm curious to see how well they actually work if I ever do get caught. Nevertheless, I'm really glad to have that because it may just help me someday.
BTW, I actually have a skin disorder. I burn quickly in sun light and start to get sick after more than 5-10 minutes of direct contact. First I get nauseous and after about an hour, I start to get migraines. I'm going to be taking a bunch of tests soon so they can narrow me down to exactly what disorder I have and they are going to see what light spectrums I am most sensitive to... if not all.