The critical wavelength Xc, proposed by Diffey et al., is calculated by determining the wavelength at which the integral of the thin-film absorbance curve reaches 90% of the integral from 290 to 400 nm . The "ideal" sunscreen would absorb all UV wavelengths equally and would have a critical wavelength of 389 nm. The critical wavelength is a measure of the extent to which a sunscreen product is "ideal" in this respect. A sunscreen with a critical wavelength of 370 nm or higher is considered to be a "broad-spectrum" product , although that claim is not permitted under the current Sunscreen Monograph (see later, Sec. 6).
Figure 5 gives the critical wavelengths for the sunscreen products shown in
SPF45 SPF4 i SPF30 Broad Spectrum
Figure 3. Critical wavelengths are 362, 372 and 375 nm, for the SPF 45, SPF 4, and SPF 30 broad-spectrum sunscreen products, respectively. These curve illustrate that a product with a critical wavelength too short to be considered "broad spectrum" (viz. <370 nm) can still have a high SPF, that a low SPF product can have a critical wavelength above 370 nm and be considered "broad spectrum," and that a photostable product containing Avobenzone generally has a critical wavelength above 370 nm.
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