**3.3 Areas where further study is required**

Very correlated with the study of acoustic monitoring is the analysis of the arc light emission. It is used to control the metallic transfer. A low power laser helps to obtain greater strength in the detachment of the drop. Weglowski [47] demonstrate that light emission has a linear correlation with the current. By another hand, Shao et al. [48] affirm that the light signal indicates the arc radiation intensity and the arc radiation is proportional to the power released which has been a relevant indicator of the energy supplied into the weld.

To a lesser extent and with little representation in the scientific literature, there are works related to the quantification of the emission of fumes*.* Yamamoto et al*.* [49] conclude that a relation between heat content, fume emission rate, and molten metal transfer mode exists; consequently the fume emission rate per unit weight of consumed wire increases with the increase in heat content.

But Meneses et al. [50] proved that there is no correlation between the amount of spattering generated and fume generation rate, because the regularity of the transfer did not show influence on the morphology, size or composition of the fumes. Then the generation of fumes is not correlated with the stability of the process.

Finally, other techniques that can be developed are 3D computational modeling, simulation, spectroscopy, spectral analysis, and X-ray observation system.
