The 50/150 lux recommendations were established by Thomson on the basis of visibility. We see most of what there is to see in this range, but we see better and differently with brighter light. Thus best lighting will always involve opinion, and not just the conservator's. What the conservator must provide are specific rates of fading, not vague, often false generalizations. Lightfastness data have been available for decades, but the conservation profession has ignored it. The Canadian Conservation Institute has reviewed this literature, and developed a slide rule to calculate fading, once the international lightfastness rating is known. Summaries on historic colorants are under way for use with the slide rule. Less technical guidelines are being drafted based on Feller's broad but specific categories: fugitive, intermediate, durable, and permanent.