ISO 23698 · Hybrid · New
ISO 23698 HDRS: SPF, UVA-PF and CW on skin, without erythema
A hybrid method: a fibre-optic probe reads the UVA absorbance of the product on real skin, and a PMMA plate measurement adds the UVB part. One study gives SPF, UVA-PF and critical wavelength, with no sunburn and about 60 minutes per volunteer.
Advantages
Compared with in vivo ISO 24444
- In vivo measurement on real skin, without human visual reading
- Non-invasive: UV dose below 10 J/m², under 5 % of a MED; no erythema
- SPF, UVA-PF and critical wavelength from one test
- About 60 min per volunteer; no 24 h reading
- No expected SPF required; takes photodegradation into account
- Lower cost than in vivo ISO 24444
Limitations
- Does not report water resistance (as of today)
- Requires complex training and certification of technicians
- Can differ from ISO 24444 for specific formulas, mainly low-SPF and mineral products
Principle
A polychromatic diffuse reflectance system shines UV light (290–400 nm) on the skin through a fibre-optic probe and measures the remitted light with a detector matched to the persistent pigment darkening (PPD) action spectrum. The ratio between unprotected and product-treated skin, measured at nine sub-sites, gives the in vivo UVA-PF (UVA-PF DRS).
Because the skin absorbs strongly in the UVB, the UVB shape of the spectrum comes from a thin film of product on a PMMA plate, irradiated with a dose derived from the in vivo value. Both curves are combined at the hybridisation wavelength (345 nm) to form the hybrid spectrum, from which SPF, UVA-PF and critical wavelength are calculated.
How it is performed
In vivo: unprotected reading
After acclimatisation, the probe measures remitted light at nine sub-sites of each test area on the back.
In vivo: protected reading
2.00 mg/cm² of product is applied and dried for at least 15 min; the same sub-sites are measured again.
In vitro
The product is spread on moulded (1.3 mg/cm²) or sandblasted (1.2 mg/cm²) PMMA plates, measured before and after a UV dose of 1.2 × UVA-PF DRS.
Hybridisation and results
In vivo and in vitro curves are combined; SPF and UVA-PF are the geometric means over 10–20 valid subjects, with statistical acceptance criteria.
Related reading
How many volunteers does an HDRS screening need?EU context
HDRS reports SPF, UVA-PF and critical wavelength together, the parameters behind the EU UVA logo (UVA-PF/SPF ≥ 1/3 and critical wavelength ≥ 370 nm, Commission Recommendation 2006/647/EC).
As a method without erythema, it reduces the burden on volunteers, in line with the European move towards more ethical sunscreen testing.
At ISD Research
ISD Research performs HDRS in routine with certified technicians. We size each screening to the decision it has to support and report the uncertainty attached to the result.
Frequently asked questions
What is HDRS (ISO 23698)?
Hybrid Diffuse Reflectance Spectroscopy, standardised as ISO 23698:2024. It combines an in vivo diffuse reflectance measurement on skin with an in vitro PMMA plate measurement to determine SPF, UVA-PF and critical wavelength without inducing erythema.
Is HDRS equivalent to ISO 24444?
Published validation shows agreement on average, but results can differ for specific formulas, mainly low-SPF and mineral products. We measure that offset on your own portfolio before relying on screening results.
How long does an HDRS study take?
About 60 minutes per volunteer, in a single session. Results are usually delivered in about 2 weeks for a screening and 2–3 weeks for a full study with ISO report, subject to availability.
References
- ISO 23698:2024 (UNE-EN ISO 23698:2025). Cosmetics, Measurement of the sunscreen efficacy by diffuse reflectance spectroscopy.
- Ruvolo E et al. Int J Cosmet Sci 2025;47(S1):53–77. doi:10.1111/ics.13089
- Reble C et al. Int J Cosmet Sci 2025;47(Suppl 1):96–114. doi:10.1111/ics.70007
- ISD Research internal data, 55 ISO 23698 studies (2026), bootstrap resampling, 1–20 volunteers.
Indicative summary of the standard: it does not reproduce or replace it, nor the ISD Research SOP or the study protocol.
Other methods
Is this the right method for your product?
Tell us your product, claim and market. We will propose the method and option that fit.