Coal dust is produced in coal mines during normal mining operations and by the movement of men and machines. 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 flat 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/cm/sup 2/ for coal and 10 mg/cm/sup 2/ for rock dust. This is well within the hazard range with an accuracy of about +or-5%. Results of the laboratory and experimental mine testing of a second portable meter are reported. >