Glossary
Glossary of sunscreen testing terms
The terms used in SPF and UVA studies, explained briefly and precisely. Guide to the new SPF methods
- Coefficient C
- Scaling factor in ISO 24443 that adjusts the in vitro absorbance curve so that its calculated SPF equals the in vivo SPF. It is valid between 0.6 and 1.6. See the method →
- Critical wavelength (λc)
- Wavelength at which 90 % of the area under the product absorbance curve between 290 and 400 nm is reached. A value of at least 370 nm indicates broad-spectrum protection and is one of the EU UVA logo criteria.
- Double Plate method
- In vitro SPF method standardised as ISO 23675:2024. The product is spread on moulded and sandblasted PMMA plates, treated as pairs, and the UV transmission is measured before and after irradiation. See the method →
- DRS (diffuse reflectance spectroscopy)
- Optical technique in which a fibre-optic probe illuminates the skin with UV light and measures the light remitted with and without the product. It is the in vivo part of HDRS. See the method →
- Erythema
- Reddening of the skin caused by UV exposure. It is the biological end point of the in vivo SPF method; the new ISO 23675 and ISO 23698 methods do not induce it.
- EU UVA logo
- Label indicating that UVA protection is adequate. Commission Recommendation 2006/647/EC requires a UVA-PF/SPF ratio of at least 1/3 and a critical wavelength of at least 370 nm. See the method →
- HDRS (hybrid diffuse reflectance spectroscopy)
- Method standardised as ISO 23698:2024. It combines a diffuse reflectance measurement on skin (UVA part) with a PMMA plate measurement (UVB part) to determine SPF, UVA-PF and critical wavelength without erythema. See the method →
- Hybridisation wavelength
- Wavelength (345 nm in ISO 23698) at which the in vivo and in vitro absorbance curves are joined to form the hybrid spectrum. See the method →
- ITA° (individual typology angle)
- Skin colour parameter calculated from colorimeter readings (L* and b*). ISO 24444 requires an ITA° of at least 28° on the back and a panel mean between 41° and 55°. See the method →
- MED (minimal erythema dose)
- Lowest UV dose that produces the first perceptible, unambiguous erythema with clearly defined borders. In ISO 24444 it is read 16–24 h after exposure on unprotected (MEDu) and protected (MEDp) skin; the individual SPF is MEDp/MEDu. See the method →
- Photodegradation (photostability)
- Loss of UV absorbance of a sunscreen film during UV exposure. In vitro methods account for it by measuring the film before and after irradiation.
- PMMA plates (moulded and sandblasted)
- Roughened polymethyl methacrylate plates that simulate the micro-relief of skin in in vitro tests. Moulded plates are rougher and give lower SPF readings; sandblasted plates are smoother and give higher readings. See the method →
- PPD (persistent pigment darkening)
- Darkening of the skin that persists after UVA exposure. Its action spectrum is used to weight the UVA-PF, and its minimal dose is the end point of the in vivo UVA method ISO 24442. See the method →
- Reference sunscreen
- Standard formulation (for example P2, P3, P5, P6 or P8) tested in parallel with the product in ISO 24444; its SPF must fall within defined limits for the study to be valid. See the method →
- Screening
- At ISD Research, a reduced study delivered without an ISO report, faster and lower cost, used to compare R&D prototypes and decide before a full study.
- Solar simulator
- Filtered xenon-arc UV source that reproduces the solar UV spectrum (290–400 nm) at high intensity, used to irradiate skin or plates under controlled conditions.
- SPF (sun protection factor)
- Ratio between the UV dose that produces a minimal erythema on skin protected by the product and on unprotected skin. It mainly reflects protection against UVB. Measured in vivo (ISO 24444), in vitro (ISO 23675) or with a hybrid method (ISO 23698). See the method →
- UVA-PF (UVA protection factor)
- Protection factor against UVA radiation, weighted with the persistent pigment darkening (PPD) action spectrum. Measured in vitro (ISO 24443), in vivo (ISO 24442) or with HDRS (ISO 23698). See the method →
- UVB and UVA
- Bands of ultraviolet radiation considered in sunscreen testing: UVB from 290 to 320 nm, mainly responsible for sunburn, and UVA from 320 to 400 nm, which penetrates deeper and causes pigmentation and photoageing.
- Water resistance (% WR)
- Proportion of SPF retained after standardised water immersion (ISO 16217), calculated according to ISO 18861. It supports “water resistant” and “very water resistant” claims. See the method →