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)
WORKING PRINCIPLE

How SPF analyser works

A thin product film on a roughened PMMA plate is illuminated with UV light from 290 to 400 nm. The transmitted light, including scattered light collected by an integrating sphere, is compared with light through a blank plate to give transmittance T(λ). SPF is the ratio of the erythemally weighted UV dose without the film to that with the film; UVA-PF uses a UVA-specific action spectrum. The critical wavelength is where 90% of the area under the absorbance curve (290–400 nm) has been reached.

FLOW

The path of a sample

  1. 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.

  2. 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.

  3. 03

    Dry

    The film is left to settle, typically about 15–30 minutes in the dark at room temperature.

  4. 04

    Scan

    Transmittance is measured from 290 to 400 nm in 1 nm steps at several spots on each plate.

  5. 05

    Irradiate (optional)

    For UVA-PF the plate is exposed to a defined dose from a solar simulator and scanned again.

  6. 06

    Calculate

    Software weights transmittance with action spectra and the reference sun spectrum to give SPF, UVA-PF and critical wavelength.

COMPONENTS

Main parts and what they do

1

UV light source

Pulsed xenon or deuterium/xenon lamp giving continuous output from 290 to 400 nm.

2

Monochromator / spectrometer

Separates wavelengths (typically 1 nm resolution) and measures intensity at each.

3

Integrating sphere or diffuser

Collects transmitted and scattered light so that scattering films are measured correctly.

4

Sample stage

Holds the PMMA plate; often motorised to scan several positions per plate.

5

PMMA plates

Moulded or sandblasted plates of defined roughness that carry the product film.

6

Solar simulator (separate or built-in)

Delivers a controlled UV dose for photostability pre-irradiation in UVA-PF testing.

7

Software

Stores spectra, applies action spectra and sun spectra, calculates SPF, UVA-PF, ratios and critical wavelength, and keeps an audit trail.

KEY CONCEPTS

Terms every analyst should know

Transmittance and absorbance

T(λ) is the fraction of light that passes through the film. Absorbance A(λ) = −log₁₀ T(λ). A film with T = 0.05 (5%) has A = 1.30.

Erythemal action spectrum

A standard curve (CIE) that weights each UV wavelength by how strongly it causes sunburn. UVB around 300 nm is weighted far more than UVA.

Reference sun spectrum

A standard spectral irradiance of midday sunlight used so that results from different labs and lamps are comparable.

SPF calculation

SPF = Σ E(λ)·S(λ) ÷ Σ E(λ)·S(λ)·T(λ). If the weighted dose reaching the skin is cut to 1/20, SPF = 20.

UVA-PF

Calculated like SPF but with the persistent pigment darkening (PPD) action spectrum and a UVA source spectrum, 320–400 nm. ISO 24443 scales the curve to the in vivo SPF first.

Critical wavelength

The wavelength below which 90% of the integrated absorbance from 290 nm lies. A higher value means more even protection across UVB and UVA.

PMMA plates

Polymethyl methacrylate plates with a controlled surface roughness that mimics skin texture. Roughness strongly affects how the film spreads.

In LimsPulse · Calibration Management

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

See how