Optical digital fragmentation measuring systems – inherent sources of error

Automated optical imaging systems of measuring fragmentation are increasingly being used in the mining, comminution, and materials handling industries. These methods have been well received in many of the industries involved. Considering that in many of these applications there are no alternative ways of sizing material, having even a rudimentary measurement of size distributions allows evaluations of explosive, blast design, detonator performance, crusher and milling performance, and material degradation due to transport. Optical methods have inherent limitations, which reflect on accuracy, precision, and reproducibility of measurement results. This stems from the fact that there are myriads of variables, which affect the outcome of the measurements. Errors start with the imaging process, which may distort the reality because of the scale of observation, may induce sampling bias, and may simply not return a good representation of the reality. More errors are introduced in the digital processing stage, where blocks may be miss-identified. The reconstruction of three-dimensional distributions from measured twodimensional distributions of partially-overlapped, fines-censured images can introduce further errors. Understanding of these limitations provides the key to the successful use of optical image systems. Understanding of what can and cannot be measured and the relative accuracy’s makes these systems useful. Defining realistic acceptable levels of error is also necessary. This paper seeks to clarify these issues, and to quantify them from laboratory studies wherever possible.