During the past few years the differences in values of the erythema dose reported in the medical literature have varied so much that one of us (Belisario) began in 1937 to investigate the reasons for these discrepancies. This work led finally to an experimental program which resulted in the formulation of a standard threshold erythema dose which can be recognized by workers throughout the world with greater accuracy than any value employed in the past.
Although begun in Australia, the investigation finally included personal visits to many dermatologic centers in Great Britain and in the United States and correspondence with members of many more. In order to correlate properly the erythema values in use in the three countries, it was considered necessary to talk to the dermatologists personally, see their machines and tubes in action and observe at first hand the type and degree of reactions that were called erythema