Why white shirt then to fluorescence when under UV light

I think most of you have ever been into dark room that contain UV lamp and you notice those white colour T-shirt will fluorescence. This is because modern detergents contain phosphors which convert UV light into white light. So when you wear your T-shirt under the sunlight your T-shirt will looks whiter than white which seem that your T-shirt looks clean. As the phosphor will convert the UV light from the sun to emit white light.

So next time you can try buying 2 brand new T-shirt, one washed it with detergent contain phosphor another just leave it unwash. Then put both T-shirt in a dark room with UV lamp, you will notice the T-shirt that has been washed will glow better than the one which is unwash.