Mine operators have long had to keep a close eye on the buildup of coal dust in underground mines. Too much of the combustible dust could lead to an explosion. A group of researchers from the National Institute for Occupational Safety and Health in Pittsburgh has begun work on a handheld optical probe that could provide immediate readings on the amount of coal dust present in a mine. A light-emitting diode in the probe emits near-infrared radiation. When the meter is inserted into the dust mixture, the infrared radiation reflects off the dust's surface and back to a silicon photodiode in the optical module.