ABSTRACT
Introduction 854
Basic Principles of In Vitro Measurements of Sunscreen Protection 854
Experience with In Vitro Measurements of Sunscreen Protection 856
Early In Vitro Measurements of Sunscreen SPF 856
The O’Neill Step Film Model 858
SPF Predictions Using the Step Film Model 860
Substrates for In Vitro Measurements of Sunscreen Protection 860
Critical Wavelength 861
Simultaneous In Vitro Measurement of SPF and Photostability 862
UV-A Index 865
Ring Test of In Vitro SPF, PPDPF, and UV-A Index Measurements 866
European Ring Test of In Vitro SPF Measurements 869
Discussion 873
Product SPF Values 873
Product Application and Substrate Considerations 873
UV Source 875
Beam Geometry 876
Photostability 876
Applications of Sunscreen Transmittance Spectra 877
Conclusions 877
References 877
INTRODUCTION
The ideal index of sunscreen protection would predict product performance in
understandable terms for all consumers over the full range of possible conditions
of use. Such an index would be measurable with reliability and precision for all
product forms and compositions, and its measurement would be rapid and inex-
pensive and would not require testing on humans or animals. Unfortunately, there
is no such index, at present, and the chronic effects of sunlight are not understood
sufficiently to permit rigorous definition of the need for sunscreen protection
beyond prevention of sunburn.
