Different brands may use slightly different phosphors and filters, and they may produce somewhat different light. It's just like the situation with standard cool white lamps - you can pick out different brands in a fixture. For consistency, it's best to keep all the tubes the same - same manufacturer, same kind, and same age. BLB bulbs may be a bit less effective than BL lamps because the paper is sensitive to the blue light produced by the BL lamp as well as UV, but the blue is filtered out by the BLB filter. You can get a rough idea of the evenness of the light by moving a fluorescent target around under the lights. Ordinary paper with UV brighteners will work.