"White" LEDs typically comprise of a blue LED with a peak at approximately 445nm which is coated with a broadband phosphor centered in the green section of the spectrum. Part of the blue emitted by the primary LED is absorbed and re-emitted at lower energy by the phosphor. This combination of blue primary and green / red secondary emission appears white to the human eye. Variants are sometimes described as cool white, neutral white or warm white reflecting respectively increased red output from the phosphor. For fluoresecence applications both the primary and secondary emission bands can be used, but the longer (green) band will need a relatively broad excitation filter.