Critical wavelength
Also called: Critical wavelength sunscreen, Critical wavelength test
Critical wavelength is a sunscreen test value used to judge how far its UV protection extends into the UVA range. In the U.S. FDA broad-spectrum test, 370 nm or higher qualifies as broad spectrum.
At a glance
- It is a lab measurement, not a number you usually see on the front of sunscreen packaging.
- In the FDA system, a mean critical wavelength of at least 370 nm is classified as broad-spectrum protection.
- It complements SPF because SPF mainly describes sunburn protection, while critical wavelength helps assess UVA coverage.
On this page
The short answer
Critical wavelength is a lab measurement used in sunscreen testing to judge how far a product's UV protection extends into the UVA range.
In the FDA broad-spectrum system, a sunscreen must show a critical wavelength of at least 370 nm to be labelled broad spectrum[1].
That sounds technical because it is technical. For everyday label reading, the practical point is simpler: SPF is not the whole sunscreen story.
What the number means
The U.S. regulation defines critical wavelength as the wavelength where the area under the sunscreen's absorbance curve reaches 90 percent of the total measured area from 290 to 400 nm[2].
In calmer words: the test looks at the spread of UV protection rather than the height of one protection number.
That matters because:
- UVB is closely tied to sunburn and SPF
- UVA reaches longer wavelengths and matters for pigmentation and photoageing
- a sunscreen can sound strong by SPF while still needing a broad-spectrum check
What to do with the term
Most shoppers do not need to calculate critical wavelength. Please do not turn sunscreen shopping into maths homework.
Use the term as background for one practical habit: choose a sunscreen that says broad spectrum, has an SPF level you can use consistently, and feels good enough that you apply enough of it. If the sunscreen is miserable to wear, the lab data cannot help from inside the drawer.
Mads's practical read
Critical wavelength is useful because it keeps sunscreen claims honest. It reminds us that protection is about coverage across UV, not a single impressive front-label number.
For daily life, keep it simple: broad spectrum, enough product, repeated use, and a texture you do not secretly hate.
Keep reading
Common questions
What does critical wavelength mean in sunscreen?
It is a lab value showing the wavelength where 90 percent of the sunscreen's measured UV absorbance area has been reached. It helps assess whether protection extends far enough into UVA.
Is critical wavelength the same as SPF?
No. SPF mainly reflects UVB sunburn protection. Critical wavelength is used in broad-spectrum testing to assess the spread of UV protection.
What critical wavelength counts as broad spectrum?
Under the FDA broad-spectrum test, a mean critical wavelength of 370 nm or greater is classified as broad-spectrum protection.
Citations
- FDA: Labeling and Effectiveness Testing: Sunscreen Drug Products for Over-The-Counter Human Use. - U.S. Food and Drug Administration
- 21 CFR § 201.327: Over-the-counter sunscreen drug products; required labeling based on effectiveness testing. - Electronic Code of Federal Regulations