Fluorescent lamps require time to come to full output. For the sake of consistency, if you do not turn your light box on and leave it on, if you are going to be making more than one print, you may want to standardize your startup procedures so that you are always starting with lamps at about the same startup temp. The light output also varies over the life of the lamp but over the midlife period, which for fluorescents is quite long, the output is fairly consistent.