Abstract: A take-up strip is applied to the element to be investigated, (for example the skin or the hair of a living subject), for a given time and with a given pressure; the side of this strip intended to come into contact with the element to be investigated is matt; the quantity of light reflected by the matt side of the strip is measured before and after impregnation with the greasy product: the quantity of the greasy product located on the surface of the element investigated is derived from the reflection measurement. The detection means is constituted by a phototransistor whose output signal is processed by an analog/digital converter associated with a programmable computer; the strip is disposed on a displaceable support. The reflection reading all along the strip gives a brightness curve in accordance with the location on the strip. For the hair, this curve reflects the greasy aspect from the root to its tip.