In vitro SPF analyser (UV transmittance analyser)
Measures how much ultraviolet light passes through a thin film of sunscreen spread on a roughened PMMA plate, then calculates in vitro SPF, UVA protection factor and critical wavelength from the transmittance spectrum.
- Cosmetics & personal care
- Sunscreen manufacturers
- UV filter suppliers
- Contract testing labs
- Textiles (UV protection)
What is SPF analyser?
Sunscreen labels make claims about sun protection factor (SPF) and UVA protection. The reference method for the SPF number on a label has long been an in vivo test on human volunteers (ISO 24444), which is slow, expensive and raises ethical questions. An in vitro SPF analyser gives a fast laboratory estimate by measuring how the product film absorbs UV light between 290 and 400 nm.
The instrument is essentially a UV spectrophotometer built for diffuse, scattering samples. A xenon flash or deuterium lamp illuminates a PMMA plate that carries a thin, evenly spread film of product (typically about 1.3 mg/cm² for ISO 24443). An integrating sphere or diffuser collects light that passes through, including scattered light, and a spectrometer records transmittance at each wavelength. Software then weights the transmittance with standard action spectra and a reference sun spectrum to give SPF and UVA values.
Cosmetics labs use in vitro SPF analysers for formulation screening, batch-to-batch consistency, UVA protection (ISO 24443 UVA-PF and critical wavelength, which are accepted for UVA claims in many markets) and photostability studies. The newer ISO 23675 describes an in vitro SPF method intended to replace human testing, but in many markets the label SPF still rests on in vivo data, so always check what your regulator accepts.
Used for
- UVA protection factor (UVA-PF) and UVA-PF/SPF ratio to ISO 24443
- Critical wavelength (broad-spectrum claim, typically ≥ 370 nm)
- In vitro SPF estimate for formulation screening and batch release
- In vitro SPF to ISO 23675 where the lab is equipped and the method is accepted
- Photostability: comparing spectra before and after a defined UV dose
- UV transmittance of packaging films, fabrics and day creams with UV filters
How it works
- 01
Reference blank
A PMMA plate coated with a thin layer of glycerine or a similar non-absorbing liquid is measured to set 100% transmittance.
- 02
Apply product
The product is weighed onto the plate at the specified dose and spread evenly with a fingertip (gloved, pre-saturated) or robot.
- 03
Dry
The film is left to settle, typically about 15–30 minutes in the dark at room temperature.
- 04
Scan
Transmittance is measured from 290 to 400 nm in 1 nm steps at several spots on each plate.
- 05
Irradiate (optional)
For UVA-PF the plate is exposed to a defined dose from a solar simulator and scanned again.
- 06
Calculate
Software weights transmittance with action spectra and the reference sun spectrum to give SPF, UVA-PF and critical wavelength.
Advantages
- Fast: results in minutes rather than weeks of volunteer testing
- No human exposure, so suitable for early development and routine QC
- Gives the full UV spectrum, so UVA balance and critical wavelength are seen at once
- Good for comparing batches or formulations under identical conditions
- Can include a controlled UV pre-irradiation step to test photostability
Limitations
- Result depends strongly on film application: amount, spreading technique and plate roughness
- In vitro SPF correlates only roughly with in vivo SPF unless a validated method (such as ISO 23675) is followed exactly
- Plate-to-plate variability is high; several plates and scans are always needed
- Water-resistance claims still need specific testing
- Fluorescent or strongly scattering products can bias transmittance readings
1 calibration procedure
Methods that use SPF analyser
Keep every SPF analyser in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.