Retrofit and upgrade kits convert existing fixtures to LED or more energy-efficient types of fluorescent bulbs to reduce operating costs. They allow the base or housing of the existing fixture to remain in place so that the fixture doesn't have to be completely removed, replaced, and rewired. Retrofit kits fit specific types of fixtures such as troffers, strip lights, can lights, hazloc lights, and vapor-tight lights. Light engines and light bars are a simple type of retrofit that fits fixtures of a compatible size and shape.