Sun-In - The Original Hair Lightener - is a brand name for a hair-lightening product created by the Chattem drug company. Similar products could also fall under this category. The product is a lemon juice-based spray that can be applied to hair before a period of time spent in sunlight, which then reacts and lightens the user's hair color.
Sun-In has been around since the 1970s and is still popular today, particularly among people with light brown or blonde hair who want to add some highlights or brighten up their current color. The product is easy to use and can be done at home without the need for a professional hair stylist.