Limited Usefulness of Artificial Light Sources

To the Editor.—  We and other readers of the Archives learned much from Dr Urbach's reply to our recent communication¹ in which he precisely out-lined the technical properties and limitations of the xenon arc solarsimulator. However, we addressed ourselves to the clinical report of Mathews-Roth et al² and to their statement that they were concerned about "a lack of more quantitative correlation" between results obtained with sunlight and the solar simulator for patients with erythropoietic protoporphyria (EPP). We wondered why Mathews-Roth et al employed the solar simulator to evaluate the protective action of β-carotene against long ultraviolet and visible light. Our confusion was somewhat resolved when Dr Pathak (verbal communication, December, 1974) informed us that the instrument had been modified to provide more light energy in the long ultraviolet and visible light regions. The question of whether the modified solar simulator was used appropriately is one