To keep it simple, the phosphor is the chemical blend inside the lamp. The glass tube of a lamp is actually clear. What makes it a milky white is a blend of chemicals that are coated on the inside of the bulb, then bonded to the glass. The choice and mixture of chemical is what determines if the bulb is an office light, a blacklight, one of our UV lights or whatever. Many different types of chemicals are used in the different bulbs. A bulb might only have one or two chemicals (phosphors) inside, or many. 3 to 7 is common.

When we say "phosphor", we understand that most of these no longer contain phosphorous but the name has stuck. Phosphorus is a chemical element and the first mineral known to be "phosphorescent", hence the name.

Typically, a particular chemical phosphor will cause one or two "spikes" on the spectrum. That means, it will only put out light in certain wavelengths. For broadband UV lamps like the Universal UV or SG-1, this means we have to carefully blend multiple chemicals in exact ratios to produce a more "sun-like" spectrum.