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J. Cosmet. Sci., 67, 101–108 (March/April 2016) 101 A new in vitro method to determine sun protection factor XIN QU, XIAOMIN ZHAO, and ZHIHUA CHEN, ASI Shanghai Technical Center, Ashland Inc., Shanghai, China 200233. Accepted for publication April 17, 2016. Synopsis A new in vitro sun protection factor (SPF) test method to determine the effi cacy of sun care products is proposed and evaluated. Based on ultraviolet (UV) dose cumulate response protocols, the new method employs Gafchromic EBT3 fi lm, a self-developing dosimetry fi lm originally created for applications in radiotherapy. Unlike the current standardized method, a UV spectrophotometer is not required. In vitro SPF values of 15 commercial products were measured with EBT3 fi lm and compared with labeled in vivo SPF values. It is apparent that the new method is accurate, cost-effective, and reproducible. These results can be seen across sun care product lines, including sunscreen lotions, blemish balm (BB) creams, foundations, and sprays with organic and/or inorganic UV fi lters. INTRODUCTION Determining the effi ciency of sun care products is normally assessed in accordance with a long-established, standardized in vivo sun protection factor (SPF) test (1). This test uti- lizes minimal erythema dose (MED), defi ned as the minimum dose or time of radiation that produces skin erythema, to determine SPF, the dose (MED), or time of UV radiation required to cause sunburn/erythema on skin with sunscreen on, as a multiple of the dose or time required without sunscreen (equation 1). For example, if unprotected skin has a dose to erythema (40 J) and protected skin has dose to erythema (640 J), then SPF = 640/40 = 16. An SPF equal to 1 affords no UV protection. Dosage with sunscreen SPF= Dosage without sunscreen MED MED (1) The more commonly used standard nowadays, the in vitro SPF test method, dates back to the early 1990s and is based on the measurement of spectral transmittance of sunscreen applied to a transparent substrate material (2). Specifi cally, transmittance of sunscreen is the ratio of spectral transmittance through a defi ned amount of sunscreen applied. The measured spectral power distribution is then weighted by the defi ned solar spectrum and Address all correspondence to emascari@resourceadvantage.com.