Coal dust is produced in coal mines during normal mining operations and by the movement of men and machines. This fine float coal dust, typically less then 74 pm, entrained in the ventiliating air is carried into the return airway, where it settles on exposed surfaces. In the event of an explosion this float dust can be re-entrained into the air, where it can propagate an explosion. The degree of combustion hazard associated with float coal dust is related to the surface density of the deposited coal dust, and the entry cross-sectional area. The operation of a prototype optical dust deposition meter used to measure the mass loading density of stratified coal and rock dust layers, to assess the float coal dust hazard, is described. The meter determines the surface loading density of a dust layer by measuring its optical reflectivity. The theory of the prototype unit's operation is developed and compared with experiment. The studies showed that the prototype unit is capable of measuring coal dust and rock dust surface loading densities of up to 7 mg/cm2 for coal and 10 mg/cm2 for rock dust. This is well within the hazard range, with an accuracy of about + 5 pct. The results of the laboratory and experimental mine testing of a second portable meter is also reported.