UVB, which is medium-wavelength ultraviolet light (280-320 nm), can cause sunburn and erythema on the skin. Sun Protection Factor (SPF) is a measure of a sunscreen's ability to prevent sunburn caused by ultraviolet (UV) B radiation. It tells you how much longer the skin with a sunscreen will be protected from sunburn than the skin without sunscreen. For example, an SPF 30 sunscreen theoretically provides 30 times more protection than no sunscreen.

How to Calculate the SPF?
The SPF value is determined by applying 0.4 ounces or 2 mg/cm2 of sunscreen to the skin, then shining artificial UVB light on the skin, measuring the time to minimum erythema on the protected and unprotected skin, and determining the SPF value.

What does the SPF value represent?
Common sunscreen products on the market usually have three values: SPF 15, SPF 30 and SPF 50.

SPF 15: This is the fact that this sunscreen has a limit on erythema for 15 × 15 = 225 minutes. In other words, if used correctly, SPF15 sunscreen can prevent 1/15 of UVB rays from reaching the skin, which means it can block (1 - 1/15) × 100% = 93.3% of UVB.

SPF 30: This means that this sunscreen product can prevent the appearance of erythema for 15 × 30 = 450 minutes, and can block (1 - 1/30) × 100% = 96.6% of UVB.

SPF 50: Similarly, it can prevent the appearance of erythema for 15 × 50 = 750 minutes, and can block 98% of UVB.

Testing Methods of SPF
In Vitro Methods
Spectrophotometric Method: This is the technique used to determine how the sunscreen reflects UV light at different wavelengths. Using the absorption spectrum, scientists can calculate the SPF (for reference only, not the exact value).
Colorimetric Testing: Here, the amount of UV light that penetrates the sunscreen is measured with a chemical indicator. Color change will vary based on the SPF.
In Vivo Methods
Result of apply Sunscreen lotion at arm with difference SPF level.

Minimal Erythema Dose (MED) Test: This is the gold standard for SPF testing. It involves exposing human volunteers to simulated amounts of UV light (with and without sunscreen). The SPF is calculated by comparing the amount of UV radiation required to produce a minimal sunburn with and without the sunscreen.
Photoacoustic Spectroscopy: Noninvasive testing for how much heat the skin absorbs by exposing it to ultraviolet light. From the heat generated with and without sunscreen, the SPF can be estimated.

The ingredient in sunscreen, how much you use it, and the formulation of the product can affect a product's SPF. Alfa Chemistry has listed out a few typical ingredients and how they may affect SPF for you.

• Organic UV filters: They filter UV radiation using chemicals like oxybenzone, avobenzone, octinoxate, etc. They can significantly increase SPF. For instance, 1 % oxybenzone is projected to provide an SPF addition of 1.5.
• Inorganic UV filters: They are UV absorbing and reflecting materials such as titanium dioxide and zinc oxide. They also boost SPF, especially if made into nanoparticles. For instance, 1% titanium dioxide will give 1.5 SPF addition and 1% micronized titanium dioxide will give 2.5 SPF addition.