LimsPulse
Cosmetics testing

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)
Illustration of In vitro SPF analyser (UV transmittance analyser)
LEARN & PRACTICE

Test your SPF analyser knowledge

Multiple-choice questions

Pick an answer to see whether you are right and why.

  1. Q1. What wavelength range does an in vitro SPF analyser normally scan?

    basic

  2. Q2. Why is the product spread on a roughened PMMA plate rather than a smooth quartz cell?

    basic

  3. Q3. Why does an SPF analyser use an integrating sphere or diffuser?

    intermediate

  4. Q4. A film transmits 5% of the light at 305 nm. What is its absorbance at that wavelength?

    intermediate

  5. Q5. Which weighting spectrum is used to calculate SPF from the transmittance curve?

    intermediate

  6. Q6. What does a critical wavelength of 375 nm tell you about a sunscreen?

    advanced

  7. Q7. In vitro SPF results are much more variable than expected between plates. What is the most likely cause?

    advanced

Viva questions

Think of your answer first, then open it to compare.

  1. 1. Explain in words how SPF is calculated from a transmittance spectrum.

    At each wavelength the sunburn effectiveness (erythemal action spectrum) is multiplied by the sunlight intensity (reference solar spectrum). Adding these up gives the sunburn-weighted dose without protection. Multiplying each term also by the film's transmittance and adding gives the dose that gets through. SPF is the first sum divided by the second: if one-twentieth of the weighted dose gets through, SPF is 20.

  2. 2. Why does ISO 24443 irradiate the plate before calculating UVA-PF?

    Some UV filters break down in sunlight, so a fresh film overstates protection. Exposing the plate to a defined UV dose (based on the initial UVA-PF) and re-measuring means the final UVA-PF reflects a product that has been used in the sun, including any loss from poor photostability.

  3. 3. Can an in vitro SPF result replace the SPF on the label?

    Not automatically. Label SPF has traditionally been determined in vivo (ISO 24444) and many regulators still expect that. ISO 23675 describes an in vitro SPF method designed for this purpose, but labs must check what their market accepts. Routine in vitro results are used for screening, batch consistency and UVA claims.

In LimsPulse · Calibration Management

Keep every SPF analyser in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.

See how