SPF is printed as a number that implies a straightforward scale. It comes from a specific laboratory test, and both the test conditions and the endpoint chosen shape what the figure means.
The endpoint is visible redness
Testing exposes patches of skin to controlled doses of ultraviolet light and identifies the smallest dose that produces uniform redness some hours later.
That measurement is taken on unprotected skin and again on skin coated with the product, and the ratio between the two doses is the sun protection factor.
Redness is driven principally by UVB, so the number describes protection against the wavelengths that cause sunburn rather than the full ultraviolet range.
UVA protection is rated separately
UVA penetrates more deeply and contributes to photoageing and to skin cancer risk while producing little immediate redness.
Because the SPF endpoint is redness, a product could score highly while offering weak UVA protection, which is why separate ratings and broad spectrum labelling exist.
Different regions use different UVA marking systems, so the symbols on a package vary with where it was sold.
The application thickness is standardised and generous
Tests apply a fixed quantity per unit area, a layer thicker than most people apply in ordinary use.
Protection does not decline in proportion to the amount applied; it falls off more sharply than that, so a thin layer delivers considerably less than the labelled figure.
This is the largest single reason for the gap between laboratory performance and real-world protection.
The scale is not linear
An SPF of thirty is not twice the protection of fifteen. The relationship concerns the fraction of ultraviolet transmitted, and that fraction shrinks with diminishing returns.
Moving from low to moderate factors changes transmission substantially; moving from high to very high changes it slightly.
Higher numbers still carry a practical margin, because they leave more protection remaining when application is imperfect.
Time-based interpretations are unreliable
The number is sometimes read as a multiplier on safe exposure time. That reading assumes constant ultraviolet intensity, which changes with time of day, season, latitude, altitude and cloud.
It also assumes the film stays intact, while sweat, water and rubbing remove it, which is why reapplication instructions exist independently of the factor.
Anyone with a history of skin cancer, or taking medication that increases light sensitivity, should get advice specific to their situation rather than relying on a label.