If you want to show that your product is increasing hair thickness, or volume, it is useful to perform hair diameter measurements.  Additionally, since hair swells differently in water after various damaging treatments, hair swelling tests are also useful for measuring hair damage prevention and repair.

Hair diameter measurements involve shining a laser onto the fiber.  A detector behind the fiber measures the size of the shadow created by the laser light and uses this to calculate the hair diameter.  Hair diameter is thus measured without any contact with the instrument.  The instrument can also measure hair ellipticity.

TRI Publications

Evans (2018)