Figure 15 shows the normalized reflectance of the limestone and marble dust mixed with Pittsburgh coal dust of different sizes. The coal size is identified by the percent by weight minus 200 mesh (75 μm). The data points show the mixtures at the explosibility limits, which correspond to the yellow-green boundaries for the CDEM (Sapko and Verakis, 2006). As can be seen, the normalized reflectance remains relatively constant for both sizes of coal dust, but the normalized reflectance at the explosibility limit is higher for the limestone than for the marble. Therefore, mines that inert with marble dust will require a different normalized reflectance value at the explosibility limit to be programmed into the CDEM for the correct color output. The difference in the normalized reflectances for the two rock dusts is most likely due to a difference in the particle sizes of the limestone and marble. More study on the effects of rock dusts from different districts is needed to determine the appropriate color output.