Since last three years I have this problem. When exposed to sun, even though its not too hot, it leads to mild headache. By evening it gets severe followed by vomit-tings. There is uneasiness in chest and relief comes only after 2 or 3 vomits. Doctors give Pan-dsr or other such medicines, but I have no cure. One unani doctor diagnosed accumulation of sputum in brain. After taking his medicine lot of sputum (cough) eased into throat from nose and chest. Then there was relief for a few months, it started again. I started the medicine again, but this time it still happens, not daily, but if exposed to sun or hot weather, the same thing start. I consulted ENT, Neuro and general physician, but no relief. Can any one advice me..

Sun allergy does exist, but it's skin reactions such as rashes and hives. It sounds like your headaches are triggering the vomiting, and that sounds like migraines. I wonder if you have another allergy to certain plants outside, not the sun. I also wonder if you have an allergy to the usual foods that cause migraines (google them) and it has no connection to the sun other than dehydration. Dehydration can give you headaches and vomiting and dizzyness and fainting. Obviously the cure for that is to drink more on sunny and hot days! And if you drink lots of tea or coffee, those don't count as liquids, because they dehydrate, being diuretics.
So... hydrate, avoid migraine foods, and answer back how it is going.
How is your son?

Migraines; I was reading about this yesterday, what a coincidence.

It could be migraines. It could be dehydration too. You see, there is a fluid between your skull and brain, if you don't drink lots of water (drinking juices including soda drinks will actually dehydrate your body; please don't drink them when you get a headache) this fluid will than reduce in amount. As a result, a bit of contact between the brain and skull will cause you to have a headache. I get massive headaches only rarely, so I do a little research every now and then. (The fluid keeps your brain from touching the skull, not getting enough water, and only water will cause this problem).

Maybe you should speak to different specialists, what is wrong with seeking more than one opinion. Some doctors don't diagnose right. We can't blame them because our brains are the most complicated things in this universe; it's no wonder why detecting problems like these can be very complex.