Base building beds contain more UVB rays which are shorter than UVA rays. This means the rays penetrate less deeply into the skin, typically affecting the outer layers of the epidermis. Because of the relatively high level of UVB rays in regular tanning beds, they are more likely to result in a more reddish tan. 

Bronzing beds contain only 2% UVB rays or less. The UVA will bronze the melanin deep in your skin as well as the top layer of dead skin, developing a long-lasting tan that builds over time. UVA causes the skin to produce melanocytes. These melanocytes produce melanin--a brown pigment that makes people appear tan. The more melanin produced, the more tan a person appears.