SPF equation

Article by: Guest Author

Article by -Nitesh Rajput – Cosmetic scientist

Sunscreen, also commonly known as sunblock, sun screen, suntan lotion, sunburn cream, sun cream or block out, is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun’s ultraviolet (UV) radiation on the skin exposed to sunlight and thus helps protect against sunburn.

Depending on the mode of action sunscreens can be classified into physical sunscreens (i.e., those that reflect the sunlight) or organic sunscreens (i.e., those that absorb the UV light)

What is SPF

SPF, also known as sun protection factor, is determined by how long you expose yourself to the sun before you start to burn, but in reality it is mainly a measure of UV A-B protection.

How to measure SPF

The SPF can be measured by applying sunscreen to the skin of a volunteer and measuring how long it takes before sunburn occurs when exposed to an artificial sunlight source. In the US, such an in vivo test is required by the FDA. It can also be measured in vitro with the help of a specially designed spectrometer. In this case, the actual transmittance of the sunscreen is measured, along with the degradation of the product due to being exposed to sunlight. The transmittance of the sunscreen must be measured over all wavelengths in the UV-B range (290–320 nm), along with a table of how effective various wavelengths are in causing sunburn (the erythemal action spectrum) and the actual intensity spectrum of sunlight. Such in vitro measurements agree very well with in vivo measurements.

Numerous methods have been devised for evaluation of UVA and UVB protection. The most reliable spectrophotochemical methods eliminate the subjective nature of grading erythema.

SPF math

Mathematically, the SPF is calculated from measured data as

SPF equation

E(\lambda) is the solar irradiance spectrum,
A(\lambda) the erythemal action spectrum, and
\mathrm{MPF}(\lambda) the monochromatic protection factor, all functions of the wavelength \lambda.

The MPF is roughly the inverse of the transmittance at a given wavelength.
The MPF is roughly the inverse of the transmittance at a given wavelength.

The above means that the SPF is not simply the inverse of the transmittance in the UV-B region. If that were true, then applying two layers of SPF 5 sunscreen would be equivalent to SPF 25 (5 times 5). The actual combined SPF is always lower than the square of the single-layer SPF.

Difference b/w Sunscreen and Sunblock

Both sunblock and sunscreen will protect your skin from UVA and UVB light (aging rays and burning rays), they use different chemicals and those chemicals work in different ways, which is why different terms are used.

Sunblock works like a mirrored shield. It contains minute particles of reflective material. They reflect the sun’s rays away from your skin blocking the damaging rays from ever reaching at all.
The ingredients included in sunblocks include Titanium dioxide and zinc oxide. (This includes Powder Sunscreen and Mineral Makeup.)

A sunscreen works in a way more like a sponge: where the sunblock reflects the beams away from your skin, the sunscreen soaks them up before they can reach you. Those rays that escape have been altered in wavelength, so that they are no longer in frequencies that endanger the skin. The ingredients that are most often found in sunscreens include PABA, Benzophenones, Cinnamates, Salicylates,

Sunscreen ingredients and there limitations

Here are the primary sunscreen ingredients used around the world.

p-Aminobenzoic acid -15 % (banned in US)
Cinoxate -3 %
Benzophenone-8 -3 %
Benzophenone-3 – Up to 10 %
Benzophenone-9 -Up to 10 %
Octyl methoxycinnamate – up to 20 %
Octyl salicylate – Up to 10 %
Sulisobenzone – Up to 10 %
Trolamine salicylate- Up to 12 %
Avobenzone – Up to 10 %
Titanium dioxide – Up to 25 %
Zinc oxide -Up to 25 %
(% vary according to regions and countries)



  1. Elisa

    Thanks for the article! Do you know the best way to find out if a sunscreen formulation is infringing some patent ? Because i have seen that it is a common problem choosing the % of filters.. and there are million patents.. is there a website or some place to check?

    1. Perry Romanowski

      That’s a good question. I don’t see how there can be much patenting in this area because the active ingredients allowed (at least in the US) are determined by the FDA.

      1. Kelly

        There are patents on several combinations of sun filters. J&J, Merck, and L’Oreal are a few sunscreen patent holders that I can think of offhand. To ensure that a formulation is not infringing, most companies will have a review with patent lawyers prior early on in the development process.

        1. Eli

          Thanks Perry! I was generalizing but i work in Europe and it’s kind of stressing trying to make a formulation with all the restrictions that i have been told.. Exactly what Kelly says, the combination of filters.. Avobenzone cannot be used with Zinc Oxide and DBT can be legally used only if we use at least small amounts of EHMC .. and all this “rules” i can’t seem to find the patents online so easy.., i spend hours searching for this patents, that are also long to read .. i was wondering if it was an easier way to check this restrictions..

  2. Bhagwat Ingle

    Generally, most of substances get oxidized in presence of light (sun light) , is it happens
    with these ingredients listed above?

    1. Nitesh

      Oxidize might be a combination of various factors including pH, humidity, temperature, preservatives and mainly other additives. many factors were there to responsible for oxidation. at time choosing sunscreen these all parameter must have to consider at time design of formula.

Leave a Reply

Your email address will not be published. Required fields are marked *