INTRODUCTION
The sun is necessary for a healthy vitamin D synthesis and an overall wellbeing; however, if enjoyed uncontrolled it is potentially harmful leading to sunburn, skin aging, and most important to skin cancer.[1–3] The incidence of skin cancer is still growing every year.[4] Thus, a better sun protection strategy is necessary.
An important part of the sun protection strategy is the sunscreen application. There are various sunscreens with different UV‐filters, formulations and properties. Until now, for all these sunscreens the sun protection factor (SPF) is determined according to the ISO 24444[5] standard, which refers to an invasive method. The method is based on an erythema induction on protected and unprotected healthy skin, respectively. The minimal erythematous dose (MED) is determined, which is the amount of UV irradiation leading to minimal sunburn. The ratio of the MED on unprotected and protected skin from at least 10 subjects of skin types (ST) I to III is used to determine the SPF. Although this is considered as the reference method to define SPFs, the invasiveness of the method calls for noninvasive alternatives.[6]
ABSTRACT
The current method for determining the sun protection factor (SPF) requires erythema formation. Noninvasive alternatives have recently been suggested by several groups. Our group previously developed a functional sensor based on diffuse reflectance measurements with one UVB LED, which was previously evaluated on pig ear skin.
Here we present the results of a systematic in vivo study using 12 sunscreens on 10 volunteers (skin types [ST] I‐III). The relationship of the UVB‐LED reflectance of unprotected skin and melanin index was determined for each ST.
The spatial variation of the reflectance signal of different positions was analyzed and seems to be mainly influenced by sample inhomogeneity except for high‐protection factors (PFs) where signal levels are close to detection noise.
Despite the low‐signal levels, a correlation of the measured LED‐based UVB PF with SPF reference values from test institutes with R2 = 0.57 is obtained, suggesting a strong relationship of SPF and LED‐based UVB‐PF. Measured PFs tend to be lower for increasing skin pigmentation. The sensor design seems to be suitable for investigations where a fast measurement of relative changes of PFs, such as due to inhomogeneous application, bathing and sweating, is of interest.