The skin produces its own natural oils called Sebum. These help provide a waterproofing layer to the skin, reducing water loss and drying, and also have some fungicidal properties. While Sebum is good for the skin, too much can be a problem and is linked with a tendency to form acne.
A number of methods exist to look at Sebum and oils on the SC. Some are complex and require extraction of the oils for analysis. However some can measure the oil level directly by watching how the oil soaks into a plastic film, making it transparent. This is the same affect that occurs when oil gets on greaseproof paper when cooking making it become see through. Techniques such as the Sebumeter, measure this transparency and uses it to deduce the oiliness of the skin.
As with most skin methods, no two devices are ever precisely the same in how they measure the water, and as such it is good practice to use the same device throughout a study, and to always do change from the original condition, and also to measure untreated sites of skin, to determine whether there has been any day to day change in the skin itself.
The Sebumeter can be used to measure things like how efficacious a cleanser is at removing the oil from the skin, it can also be used to measure the deposition of oils onto skin from a product, as well as how long they last there. These types of tests are used to develop “effectively cleanses your skin”, or “removes more dirt that competitor” types of claims.
There are watchouts when measuring skin oiliness. The devices are sensitive to how they are placed in contact with the skin, so the operator needs to be well trained, and familiar with the device being used. As they rely on contact with the skin, the presence of hair will cause them to give low values for hydration. As with all skin measures, the panelist need to be acclimatised in the room where the measurements are done so that they become used to the temperature. It also picks up signals from oils in products, so you need to take that into account when using products on the skin.
However even considering these watchouts it is a highly precise tool for measuring levels of oils on skin and is a valuable technical method.
I use a Sebumeter for my work on oil assessment, both for measurement of Sebum on skin, and for cleanser testing, and measuring deposition on skin from products.