If you want to show that your product is increasing hair thickness, or hair volume, it is useful to perform hair diameter measurements.  Hair swelling tests are also useful for measuring hair damage prevention and repair because damaged hair swells differently when wet when compared with non-damaged hair.

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