A different kind of measurement
The two headline determinations behind a sun protection label are not variants of one method. The sun protection factor is a biological measurement on people, described in what an in vivo SPF test involves. The ultraviolet A determination is an optical measurement on a plate.
That difference matters when reading a label, because it means the two figures carry different kinds of uncertainty and are affected by different things. It also explains why the ultraviolet A determination can produce a full spectral curve while the sun protection factor produces a single ratio.
The plate
The film is applied to a substrate with a controlled surface roughness, most commonly a moulded plate designed to approximate the topography of skin at the relevant scale. The roughness is not decorative. A film spread on smooth glass is far more uniform than a film spread on skin, and a uniform film performs better than a real one, so a smooth substrate would flatter the product.
Substrate choice has been one of the persistent difficulties in this area of measurement, because results depend on it and different substrates give different answers. Standardising the substrate is a large part of what the method does.
Quantity and spreading
A controlled quantity of product per unit area is applied and spread by a specified technique, then allowed to settle. The relationship between quantity and result is as steep here as it is in the in vivo test, so the quantity is weighed and the spreading is timed.
The parallel with the in vivo method is deliberate. If the two determinations were made at different film thicknesses, the ratio between them would be meaningless, and the whole point of the ultraviolet A criterion is that it is expressed as a proportion of the labelled sun protection factor.
Reading the spectrum
The plate is placed in a spectrophotometer fitted with an integrating sphere, and ultraviolet transmittance is measured across the wavelength range at defined intervals. What comes out is a curve: the proportion of radiation transmitted at each wavelength.
Two quantities are derived from that curve, and both appear in the criteria behind the circled mark.
The ultraviolet A protection factor is calculated by weighting the transmittance across the ultraviolet A range and expressing the result as a protection factor, in the same form as the sun protection factor so that the two can be compared as a ratio.
The critical wavelength is the wavelength below which ninety per cent of the total ultraviolet absorbance falls. It describes the shape of the curve rather than its height, and a higher value means the absorbance reaches further into the long wavelength end.
| Sun protection factor | UVA protection factor | |
|---|---|---|
| Measured on | Human volunteers | A roughened plate |
| Endpoint | Visible reddening | Spectral transmittance |
| Source | Solar simulator | Spectrophotometer, with a separate irradiation step |
| Photostability accounted for | Implicitly, within the exposure | Explicitly, by pre-irradiation |
| Output | A single ratio | A protection factor and a critical wavelength |
| Shown on pack | As a rounded down integer | As a symbol, if the thresholds are met |
Framework of this publication. It describes how published rules and guidance fit together and is not a measurement, a survey or a study.
Pre-irradiation, which has no equivalent in the SPF test
The step that distinguishes this determination from a simple transmittance reading is that the film is irradiated with a controlled ultraviolet dose partway through, and the transmittance is measured again afterwards.
The reason is photostability. Ultraviolet filters are not inert under ultraviolet light. Some degrade, some change their absorbance profile, and the effect can be significant in the ultraviolet A range specifically. A determination made only on a fresh film would report the protection of a product that has not yet been used.
Building the irradiation step into the method means the reported ultraviolet A protection factor reflects a film that has already taken a dose. It is one of the more thoughtful features of the standard, and it is invisible on the pack.
Why the in vitro result is scaled to the in vivo one
A detail that catches people out is that the in vitro ultraviolet A determination is not read in isolation. The method includes a step that adjusts the measured curve so that the in vitro determination of the sun protection factor for the same film agrees with the in vivo result.
This is a calibration, and it exists because absolute values from a plate do not correspond exactly to values from skin. Scaling the curve so that the two agree in the range where an in vivo figure exists makes the ultraviolet A figure comparable to the sun protection factor, which is what the ratio criterion requires.
The practical implication is that the ultraviolet A figure depends on the in vivo figure. It is not an independent measurement, and a change to one propagates to the other.
From the figure to the mark
The criteria behind the circled ultraviolet A mark are that the ultraviolet A protection factor is at least a third of the labelled sun protection factor and that the critical wavelength is at least 370 nanometres. Both come from the European Commission recommendation on sunscreen efficacy claims.
Notice what happens to the information at this point. Two continuous quantities, each with an uncertainty, are compared against two thresholds, and the output is a single symbol with no degrees. Everything about how comfortably the thresholds were met is discarded, which is why the mark cannot be read as a level. That is discussed in the UVA circle and the star rating.
Why this determination is in vitro when the other one is not
It is a fair question why one determination uses people and the other uses a plate, and the answer is about what each quantity is defined against.
The sun protection factor is defined against a biological endpoint, the reddening of skin, so it has to be measured on skin. Ultraviolet A protection has no equivalent visible endpoint that can be produced reliably and read consistently at doses that would be acceptable to deliver. Pigment darkening responses have been used as endpoints in some methods, and they carry their own difficulties of assessment and of volunteer burden.
An optical determination sidesteps the problem entirely. Transmittance across a spectrum is a physical property of a film, it can be measured precisely, and it can be measured at every wavelength rather than at a single threshold. The trade off is that a plate is not skin, which the roughened substrate and the calibration step are both attempts to address.
The result is a label carrying two figures produced by fundamentally different kinds of measurement, presented adjacently and read as though they were the same sort of thing. That is one of the quieter reasons a sun protection pack is harder to interpret than it looks.
What the method does not tell you
It does not tell you how the film behaves on real skin, which is rougher, more variable and more mobile than any plate. It does not tell you how the product behaves after water, sweat or abrasion, which are separate determinations. It does not tell you anything about tolerance, and it does not tell you anything about how much product a person will apply.
What it does do well is characterise the optical performance of a film across the ultraviolet spectrum, including after a dose, which is exactly the property the mark is meant to certify.
What to take from this
A roughened plate, a weighed film, a spectral transmittance curve, a pre-irradiation step and a calibration against the in vivo figure. The output is two numbers compared against two thresholds and reduced to one symbol.