## 1 Introduction  
Zone refining is an effective way to produce high purity metals by controlling the impurities' distribution in crystalline materials. It works by repeatedly moving one or a series of molten zones very slowly along a solid bulk. This process, like any other methodologies based on the principle of fractional crystallization, can be applied to refine those metals, in which the distribution coefficient (\(k\)) of the impurities is smaller or bigger than unity. That is the ratio of the concentration of an impurity in the solid phase to that in the liquid phase. The smaller the \(k\), the more probable it is to remove that impurity from the base metal through zone refining even up to ultra-purity levels. This process was firstly introduced by [PERSON] for the purification of Germanium in 1952 [1], and since then, a large amount of investigation has been conducted with the focus on the optimization of experimental parameters for different materials. These optimized parameters generally include the effective distribution coefficient (\(k_{eff}\)), zone length, zone movement velocity and the number of passes. Particularly, zone length optimization has been studied extensively due to its significant influence on time duration, yield, as well as final purity [2]. [PERSON] et al. [3] showed that it is not necessary to keep the zone length constant from one pass to the next, i.e., refining efficiency can be improved by using a longer zone at the early passes, followed by a shorter zone at the later passes. [PERSON] [4] suggested to use a longer zone in the first part of the ingot, followed by a shorter zone in the second part. Through numerical and modeling analysis, [PERSON] and [PERSON] [5] introduced a variable zone size (VZS) technique, by which the zone length is always changing in relation to the position of the freezing (liquid to solid) interface as the heater moves. This technique was rather concentrated to move the center of the impurity further and not really employed to optimize the zone length. However, it is difficult to instantly control the zone length at all times practically, and until now, no experimental research has been conducted to control the zone length along the zone refined ingot. [PERSON] et al. [6] have investigated the optimal zone length in a single zone pass and found out that it increases with the increase in the distribution coefficient (\(k\)) and decreases with the increasing number of zone passes. In addition, [PERSON] et al. [2] concluded, also via numerical modeling, that using a zone length of \(0.2L\) (\(L\) is the whole length of the bar) for the initial three passes, followed by \(0.1L\) for the fourth to sixth passes and \(0.05L\) for other passes could deliver satisfactory results. Zone length, however, is influenced by many experimental parameters, like heating power, zone movement velocity, crucible material, charging material and the way of cooling, all of which make the precise regulation of the zone length remarkably challenging [7]. Especially working with materials with high thermal conductivity for the charging material and/or crucible, [8] leads to an even more difficult achievement of a stable zone length. In order to compensate these problems, many researchers [9; 10; 11; 12] attempted to use simulative or modeling methods to predict the zone length in correlation with experimental parameters, with regard to the heat transfer process in the whole system. However, the values of these parameters corresponding to one certain zone length differ significantly from another one [13]. Therefore, it is motivated to measure and even control the zone length in real time with some special techniques directly during the refining process. This work is to demonstrate a new methodology to detect, as well as to control the zone length in a horizontal zone refining system by installation and implementation of an infrared (IR) camera.