A procedure of producing a single crystal by zone melting, comprisingcrystallizing molten material on a seed crystal to form a growing single crystal; the rotation of the growing single crystal at one speed; the creation and maintenance of a melt zone lying on the growing single crystal by melting solid material in the presence of an induction heating coil; taking pictures with a camera comprising a part of the melting zone; determining a measurand by evaluating an image section of the captured images; and generating a warning signal in response to an event causing a disturbance of the growth of the single crystal and/or causing the cessation of the crystal growth as soon as the measured quantity passes a threshold set in this respect, the procedure further comprising either a) Determining a vibration amplitude of the fusion zone by evaluating an image section of the captured images comprising an upper part of the fusion zone including a part of a neck of the fusion zone and a lower part of the induction heating coil and generating the warning signal and/or causing the termination of the crystal growth, as soon as the vibration amplitude passes the threshold set with respect thereto, wherein for evaluating gray values of the image section of an image are compared with those of a preceding image, the amount of the difference of the gray values is summed up and normalized and interpreted as the vibration amplitude of the melting zone or b) determining the distance between a left-side edge of the growing single crystal and a reference point lying on an axis through the center of the seed crystal, and the distance between a right-side edge of the growing single crystal and the reference point by evaluating an image portion of the acquired images comprising a boundary region between the melting zone and the growing single crystal; and as soon as the amplitude of a variation of at least one of the distances passes the threshold set in this respect, generating the warning signal and/or causing the termination of the crystal growth within the scope of exhausting a discretionary margin consisting in allowing the single crystal to continue growing with a misorientation or in terminating the crystal growth. comprisingcrystallizing molten material on a seed crystal to form a growing single crystal; the rotation of the growing single crystal at one speed; the creation and maintenance of a melt zone lying on the growing single crystal by melting solid material in the presence of an induction heating coil; taking pictures with a camera comprising a part of the melting zone; determining a measurand by evaluating an image section of the captured images; and generating a warning signal in response to an event causing a disturbance of the growth of the single crystal and/or causing the cessation of the crystal growth as soon as the measured quantity passes a threshold set in this respect, the procedure further comprising either a) Determining a vibration amplitude of the fusion zone by evaluating an image section of the captured images comprising an upper part of the fusion zone including a part of a neck of the fusion zone and a lower part of the induction heating coil and generating the warning signal and/or causing the termination of the crystal growth, as soon as the vibration amplitude passes the threshold set with respect thereto, wherein for evaluating gray values of the image section of an image are compared with those of a preceding image, the amount of the difference of the gray values is summed up and normalized and interpreted as the vibration amplitude of the melting zone or b) determining the distance between a left-side edge of the growing single crystal and a reference point lying on an axis through the center of the seed crystal, and the distance between a right-side edge of the growing single crystal and the reference point by evaluating an image portion of the acquired images comprising a boundary region between the melting zone and the growing single crystal; and a) Determining a vibration amplitude of the fusion zone by evaluating an image section of the captured images comprising an upper part of the fusion zone including a part of a neck of the fusion zone and a lower part of the induction heating coil and generating the warning signal and/or causing the termination of the crystal growth, as soon as the vibration amplitude passes the threshold set with respect thereto, wherein for evaluating gray values of the image section of an image are compared with those of a preceding image, the amount of the difference of the gray values is summed up and normalized and interpreted as the vibration amplitude of the melting zone or b) determining the distance between a left-side edge of the growing single crystal and a reference point lying on an axis through the center of the seed crystal, and the distance between a right-side edge of the growing single crystal and the reference point by evaluating an image portion of the acquired images comprising a boundary region between the melting zone and the growing single crystal; and as soon as the amplitude of a variation of at least one of the distances passes the threshold set in this respect, generating the warning signal and/or causing the termination of the crystal growth within the scope of exhausting a discretionary margin consisting in allowing the single crystal to continue growing with a misorientation or in terminating the crystal growth.