Photosensitivity is also known as ‘sun allergy’ and is an immune system reaction that is triggered by sunlight; more particularly ultraviolet radiation. Some very sensitive people may also react to strong artificial light.
Photosensitivity reacts with different agents such as perfume and other cosmetics (including some sunscreens, ironically) as well as certain medications to react to light stimulus causing an allergic reaction. 
Symptoms of photosensitivity may include skin irritation, rashes or small white acne.
If you suspect that you have photosensitivity you would have to get it confirmed by a medical evaluation.
Lights of different types or from different sources would be shone on to the skin in small areas to see if reactions such as a rash or quick sunburn can occur.
Photo-patch tests can also be carried out to determine the presence or absence of photosensitivity.
If a person is photosensitive, they have to exercise care in avoiding light exposure. If going out in summer, make it early in the morning or late in the evening.
Use appropriate sun protection including sunscreens and protective clothing and head gear on a regular basis, even if you are going to be in the shade. Remember light can reflect off sand, concrete and snow as well, so summer is not the only time to take care.
