1. An instrument for measuring the comparative absorption of roentgen rays: The penetrometer gives a rough estimation of the ability of X-rays to penetrate bodily tissues.

A penetrometer measures the penetrating power of a beam of electromagnetic radiation.

2. A device for measuring the power of something; such as, an X-ray to pass through semisolids or other materials: Penetrometers compare the absorption of roentgen rays in various metals; especially, silver, lead, and aluminum.