Alternative SPF Methods

Uli Osterwalder
CEO Sun Protection Facilitator GmbH

In the first Sweep the Sun column we looked at the revised ISO 24444:2019, the Gold standard in SPF-testing. Practically since the dawn of SPF testing, which began in the 1960s as an outdoor method under the sun, there has always been a desire for an alternative method that does not involve humans. Over the years, as SPF levels became higher and higher and the variation between different laboratories increased, the desire to find reliable alternative methods grew. More recently there was also growing concern to expose subjects to potentially harmful radiation. Over the years, several non-invasive methods have evolved (Figure 1):

Figure 1: Overview of Alternative SPF Methods
  • A first ISO standardized in vitro method for the determination of UVA protection (UVAPF ISO 24443) is already in use 1. An ISO standard for the in vitro determination of SPF is currently under development (ISO/CD 23675) 2. It is based on the protection per- formance of a sunscreen product against erythema effective UV radiation, calculated from the measured in vitro transmittance and weighted with the erythema action spectrum and with the “standard” output spectrum of a UV solar simulator used for SPF testing.
  • Hybrid diffuse reflectance spectroscopy (HDRS) is a combined in vivo/in vitro method. The diffuse reflection of radiation by human skin covered with the investigated sunscreen product is combined with an in vitro transmission measurement of the same product on a roughened PMMA plate. The in vivo part leads to UVA-PF (320– 400 nm), considering the interactions of the sunscreen with the skin. UVB absorption (290– 320 nm) is measured in vitro, as the latter cannot be determined in vivo by reflectance spectroscopy. This method does not cause radiation-induced erythematous skin reactions3,4. The HDRS method has already undergone interlaboratory variability studies and is currently available as a draft for the ISO technical committee TC 217 (Cos- metics) 5. This method has been used by Stiftung Warentest to verify SPF claims on commercial products since 2020 6. CE recommends using these methods 7. Similar methods have also been developed by Cole et al. in the United States and Lademann et al. in Berlin 8,9
  • Instead of experimental measurements, the transmission through a solar control film can also be calculated in-silico. The calculation is based on the spectra of the filters used, including their possible degradation, the quantitative composition and a model of the product film on the skin. Simulations are not only useful for the development of new sunscreen formulations, but also contribute to the understanding of sunscreens in general. Simulation tools are freely available on the Internet 10, 11. In-silico SPF calculations provide realistic, rather conservative results. These calculations can also be used to check the plausibility of experimentally determined SPF values or for regulatory control of the market. However, knowledge of the quantitative composition of the filter is a prerequisite. The latter is not disclosed in coun- tries where sunscreens are regulated as cosmetics and must therefore be determined beforehand 12. Widespread and routine application of alternative methods for product labeling has not yet occurred in the market. An ALT-SPF consortium initiated by the ISO/TC 217 technical committee is currently working on comprehensive solutions to test alternative determination methods for their suitability to characterize the performance of sunscreen products 13.
  • Recent improvements in the availability of UV-sensitive cameras have opened up the opportunity of directly investigating and monitoring sunscreen product application using actual sunscreen formulations available to the public 14–16. The method is suitable to assessing sunscreen product distribution after spreading and to differentiate product based on film morphology and final in vivo behavior. Moreover, it allows to impressively demonstrate the consumer how well/complete he/she has applied the product. The latter is the key to informing consumers about product ap- plication and distribution and thus contributes to the overall pro- tective effect of the sunscreen product.
Figure 2: Toolbox of SPF-Methods, tentative accuracy and costs

In sunscreen performance characterization, we are now at a crucial point to substantially improve protection information with potentially suitable in-vitro, in-vitro/in-vivo and in-silico methods 17. ISO 24444:2019 is setting the bench mark in accuracy and costs (Figure 2). The validation process going on for the two ISO committee drafts ISO/CD 23675 and ISO/CD 23698 will determine to what extend the gold standard can be replaced. In any case it is wise to always conduct an in-silico calculation in the development phase and also as a “reality check”. This may sound bizarre, doing a “reality check” with a virtual tool, but it can really help to detect some formulations from “fantasy land”. The parameter “SPF” divided by “% UV filter in the sunscreen” is an indicator that can spot such “fantasy products”. In the past this value “SPF / %-UV Filter” was around 1, meaning that an SPF 20 contained 20% UV filter. With modern UV filters, formulators can make it 2 or even 3, i.e. achieving SPF 50+ with 20% UV Filter. But sometimes one can find values as high as 6. Theoretically such very highly efficient formulations are possible but rather unlikely. In such cases the SPF value should better be double checked in order to avoid problems in the future. We will talk more about SPF in upcoming columns. Stay tuned.


  1. ISO 24443:2012 Determination of sunscreen UVA photoprotection in vitro, standard/46522.html, Viewed 5.5.22
  2. ISO/CD 23675 Cosmetics – Sun protection test Methods – In Vitro determination of Sun Protection Factor,, Viewed 5.5.22
  3. Ruvolo Junior E, Kollias N, Cole C. New noninvasive approach assessing in vivo sun protection factor (SPF) using diffuse reflectance spectroscopy (DRS) and in vitro transmission. Photodermatol Pho- toimmunol Photomed. 2014 Aug;30(4):202-11. doi: 10.1111/phpp.12105. Epub 2014 Feb 19. PMID: 24417335.
  4. Ruvolo E, Rohr M, Oliveira S, Nogueira L, Carvalho J, Cole C. Multi-laboratory study of hybrid dif- fuse reflectance spectroscopy to assess sunscreen SPF and UVA-PFs. Photodermatol Photoimmunol Photomed. 2021 Nov;37(6):511-520. doi: 10.1111/phpp.12703. Epub 2021 Aug 24. PMID: 34060681.
  5. ISO/CD 23698 Cosmetics Sun protection test methods- Measurement of the Sunscreen Efficacy by Diffuse Reflectance Spectroscopy, 76699.html, Viewed 5.5.22
  6. Stiftung  Warentest  (2020)  Stiftung  Warentest. Test-4722079-4722082/.Zugegriffen: 15.Dez.2021
  7. Cosmetics Europe Recommendation No. 26 on the use of alternative methods to ISO 24444:2019, 2022-03-23, Viewed 5.5.22.
  8. Cole C, Silverman J, BonitatibusM(2019) Evaluating sunscreen ultraviolet protection using a poly- chromatic diffuse reflectance device. Photodermatol Photoimmunol Photomed 35(6):436441.https://
  9. Throm, C.M.,, Wiora, G., Reble, C., Schleusener, J., Schanzer, S., Karrer, H., Kolbe, L., Khazaka, G., Meinke, M.C.,and Lademann,J., In vivo sun protection factor and UVA protection factor determina- tion using(hybrid) diffuse reflectance spectroscopymand a multi-lambda-LED light source, J Biopho- tonics, 14 (2021) e202000348.
  10. BASF sunscreen simulator, BASF SE, Ludwigshafen. 2010. Available from: sunscreen-simulator, accessed November 14, 2017.
  11. DSM  SUNSCREEN  OPTIMIZER.  Available  from:, viewed September 30, 2020.
  12. DIN EN 17156 (2017) Cosmetics – Analytical methods – LC/UV method for the identification and quantitative determination in cosmetic products of the 22 organic UV filters in use in the EU, Ger- man and English version, Viewed 5.5.22.
  13. Consortium ALT-SPF (2021) Call for interest: to evaluate alternative SPF methods. https://www.alt- Accessed 2022-09-27
  14. Pratt H, Hassanin K, Troughton LD, Czanner G, Zheng Y, McCormick AG, et al. UV imaging reveals facial areas that are prone to skin cancer are disproportionately missed during sunscreen applica- tion. PLoS One. 2017;12(10):e0185297. doi: 10.1371/journal.pone.0185297
  15. Crowther JM. Understanding sunscreen SPF performance using crosspolarized UVA reflectance pho- tography. Int J Cosmet Sci. 2018;40(2):127–33. doi: 10.1111/ ics.12443. Epub 2018 Jan 30. PMID: 29247508.
  16. de Gálvez MV, Aguilera J, Buendía EA, Sánchez-Roldán C, Herrera-Ceballos E. Time required for a standard sunscreen to become effective following application: a UV photography study. J Eur Acad Dermatol Venereol. 2018;32(4):e123–e124. doi: 10.1111/jdv.14626
  17. Osterwalder U, Surber C. Charakterisierung von Sonnenschutzleistung: Quo vadis? [Characteriza- tion of sun protection performance: Quo vadis?]. Hautarzt. 2022 Apr;73(4):276-282. German. doi: 10.1007/s00105-022-04958-x. Epub 2022 Mar 25. PMID: 35333933; PMCID: PMC8964537.

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