Wavelength and transmittance verification
Checks that the SPF analyser reads wavelengths correctly with a holmium oxide filter and reads transmittance correctly with certified neutral-density (or UV-absorbing) filters, then confirms the whole system with a reference sunscreen.
Why this is checked
SPF and UVA-PF are calculated by weighting transmittance at each wavelength with steep action spectra. A wavelength shift of a nanometre or two in the UVB region, or a small transmittance error where the film lets little light through, changes the result noticeably. This check proves both axes of the spectrum are right before results are reported.
Typically every 6–12 months, after a lamp change, repair or relocation, and whenever the reference sunscreen trends out of range, as defined in your SOP. A quick filter check before each series is good practice.
Learn how the SPF analyser worksStandards & tools
- Holmium oxide glass wavelength filter with a traceable certificate (peaks in the UV such as near 279, 287, 334 and 361 nm)
- Certified neutral-density or UV transmittance filters covering low to medium transmittance (for example about 10%, 30% and 50% T)
- Clean blank PMMA plate (glycerine-coated, as for routine blanks)
- Reference sunscreen with an expected value range, as given by the method (for example the ISO 24443 reference product)
Prerequisites
- Lamp warmed up for the time stated in the manual
- Filters clean, undamaged and within certification date
- Room at normal laboratory temperature, instrument free from vibration
Step by step
- 1Run a baseline (100% T) with an empty beam or blank plate as the manual describes.
- 2Insert the holmium oxide filter and scan 270–400 nm in 1 nm steps (or finer if available).
- 3Find the wavelength of each certified absorbance peak and record it.
- 4Calculate the error for each peak (measured − certified).
- 5Insert each transmittance filter in turn and record %T at the certified wavelengths (for example 300, 350 and 380 nm).
- 6Calculate the transmittance error for each filter and wavelength.
- 7Apply a reference sunscreen to plates as per the routine method and measure it; compare with its expected range.
- 8Record all results, filter serial numbers and certificate dates.
Example observation table
| Check | Certified value | Measured | Error | Tolerance (typical) | Result |
|---|---|---|---|---|---|
| Holmium peak 1 | 279.3 nm | 279.6 nm | +0.3 nm | ± 1 nm | Pass |
| Holmium peak 2 | 287.6 nm | 287.4 nm | −0.2 nm | ± 1 nm | Pass |
| Holmium peak 3 | 333.8 nm | 334.1 nm | +0.3 nm | ± 1 nm | Pass |
| Holmium peak 4 | 360.8 nm | 361.2 nm | +0.4 nm | ± 1 nm | Pass |
| Filter A at 300 nm | 10.2 %T | 10.4 %T | +0.2 %T | As per SOP / manufacturer | Pass |
| Filter B at 350 nm | 30.5 %T | 30.1 %T | −0.4 %T | As per SOP / manufacturer | Pass |
| Filter C at 380 nm | 50.1 %T | 50.6 %T | +0.5 %T | As per SOP / manufacturer | Pass |
How to calculate the result
- 1
Holmium peak 4: 361.2 − 360.8 = +0.4 nm, within ± 1 nm
- 2
Filter B at 350 nm: 30.1 − 30.5 = −0.4 %T
- 3
As absorbance, filter B reads −log₁₀(0.301) = 0.521 against a certified −log₁₀(0.305) = 0.516
Pass if
- Wavelength accuracy typically within ± 1 nm of the certified peak values, or the manufacturer's specification
- Transmittance accuracy within the manufacturer's specification or your SOP limit (often about ± 1–2 %T absolute in the working range)
- Reference sunscreen within the expected range stated in the method
What to do
- Label the instrument out of use for reportable testing.
- Clean the optics and filters and repeat once to rule out a handling error.
- If still failing, call the service engineer for realignment or lamp replacement.
- Re-verify after repair and assess any results reported since the last good check.
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.